
[Federal Register: June 29, 2010 (Volume 75, Number 124)]
[Proposed Rules]               
[Page 37311-37339]
From the Federal Register Online via GPO Access [wais.access.gpo.gov]
[DOCID:fr29jn10-20]                         

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Proposed Rules
                                                Federal Register
________________________________________________________________________

This section of the FEDERAL REGISTER contains notices to the public of 
the proposed issuance of rules and regulations. The purpose of these 
notices is to give interested persons an opportunity to participate in 
the rule making prior to the adoption of the final rules.

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[[Page 37311]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 25 and 33

[Docket No. FAA-2010-0636; Notice No. 10-10]
RIN 2120-AJ34

 
Airplane and Engine Certification Requirements in Supercooled 
Large Drop, Mixed Phase, and Ice Crystal Icing Conditions

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of Proposed Rulemaking (NPRM).

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SUMMARY: The Federal Aviation Administration proposes to amend the 
airworthiness standards applicable to certain transport category 
airplanes certified for flight in icing conditions and the icing 
airworthiness standards applicable to certain aircraft engines. The 
proposed regulations would improve safety by addressing supercooled 
large drop icing conditions for transport category airplanes most 
affected by these icing conditions, mixed phase and ice crystal 
conditions for all transport category airplanes, and supercooled large 
drop, mixed phase, and ice crystal icing conditions for all turbine 
engines. These proposed regulations are the result of information 
gathered from a review of icing accidents and incidents.

DATES: Send your comments on or before August 30, 2010.

ADDRESSES: You may send comments identified by Docket Number FAA-2010-
0636 using any of the following methods:
     Federal eRulemaking Portal: Go to http://
www.regulations.gov and follow the online instructions for sending your 
comments electronically.
     Mail: Send comments to Docket Operations, M-30; U.S. 
Department of Transportation, 1200 New Jersey Avenue, SE., Room W12-
140, West Building Ground Floor, Washington, DC 20590-0001.
     Hand Delivery or Courier: Bring comments to Docket 
Operations in Room W12-140 of the West Building Ground Floor at 1200 
New Jersey Avenue, SE., Washington, DC, between 9 a.m. and 5 p.m., 
Monday through Friday, except Federal holidays.
     Fax: Fax comments to Docket Operations at 202-493-2251.
    For more information on the rulemaking process, see the 
SUPPLEMENTARY INFORMATION section of this document.
    Privacy: The FAA will post all comments we receive, without change, 
to http://www.regulations.gov, including any personal information you 
provide. Using the search function of our docket Web site, anyone can 
find and read the electronic form of all comments received into any of 
our dockets, including the name of the individual sending the comment 
(or signing the comment for an association, business, labor union, 
etc.). You may review DOT's complete Privacy Act Statement in the 
Federal Register published on April 11, 2000 (65 FR 19477-78) or you 
may visit http://DocketsInfo.dot.gov.
    Docket: To read background documents or comments received, go to 
http://www.regulations.gov at any time and follow the online 
instructions for accessing the docket. Or, go to Docket Operations in 
Room W12-140 of the West Building Ground Floor at 1200 New Jersey 
Avenue, SE., Washington, DC, between 9 a.m. and 5 p.m., Monday through 
Friday, except Federal holidays.

FOR FURTHER INFORMATION CONTACT: For part 25 technical questions 
contact Robert Hettman, FAA, Propulsion/Mechanical Systems Branch, ANM-
112, Transport Airplane Directorate, Aircraft Certification Service, 
1601 Lind Avenue, SW., Renton, WA 98057-3356; telephone (425) 227-2683; 
facsimile (425) 227-1320, e-mail robert.hettman@faa.gov.
    For part 33 technical questions contact John Fisher, FAA, 
Rulemaking and Policy Branch, ANE-111, Engine and Propeller Directorate 
Standards Staff, Aircraft Certification Service, 12 New England 
Executive Park, Burlington, MA 01803; telephone (781) 238-7149, 
facsimile (781) 238-7199, e-mail john.fisher@faa.gov.
    For part 25 legal questions contact Douglas Anderson, FAA, Office 
of the Regional Counsel, ANM-7, Northwest Mountain Region, 1601 Lind 
Avenue, SW., Renton, WA 98057-3356; telephone (425) 227-2166; facsimile 
(425) 227-1007, e-mail douglas.anderson@faa.gov.
    For part 33 legal questions contact Vince Bennett, FAA, Office of 
the Regional Counsel, ANE-007, New England Region, 12 New England 
Executive Park, Burlington, MA 01803; telephone (781) 238-7044; 
facsimile (781) 238-7055, e-mail vincent.bennett@faa.gov.

SUPPLEMENTARY INFORMATION: Later in this preamble under the Additional 
Information section, the FAA discusses how you can comment on this 
proposal and how the agency will handle your comments. Included in this 
discussion is related information about the docket, privacy, and the 
handling of proprietary or confidential business information. The FAA 
also discusses how you can get a copy of this proposal and related 
rulemaking documents.

Authority for This Rulemaking

    The FAA's authority to issue rules on aviation safety is found in 
Title 49 of the United States Code. Subtitle I, section 106 describes 
the authority of the FAA Administrator. Subtitle VII, Aviation 
Programs, describes in more detail the scope of the agency's authority.
    This rulemaking is proposed under the authority described in 
subtitle VII, part A, subpart III, section 44701, ``General 
requirements.'' Under that section, the FAA is charged with promoting 
safe flight of civil aircraft in air commerce by prescribing minimum 
standards required in the interest of safety for the design and 
performance of aircraft; regulations and minimum standards in the 
interest of safety for inspecting, servicing, and overhauling aircraft; 
and regulations for other practices, methods, and procedures the 
Administrator finds necessary for safety in air commerce. This 
regulation is within the scope of that authority because it would 
prescribe--
     New safety standards for the design and performance of 
certain transport category airplanes and aircraft engines; and
     New safety requirements that are necessary for the design, 
production, and operation of those airplanes, and for other practices, 
methods, and

[[Page 37312]]

procedures relating to those airplanes and engines.

Summary of the Proposal

    The FAA proposes to revise certain regulations in Title 14, Code of 
Federal Regulations (14 CFR) part 25 (Airworthiness Standards: 
Transport Category Airplanes) and part 33 (Airworthiness Standards: 
Aircraft Engines) related to the certification of transport category 
airplanes and turbine aircraft engines in icing conditions. We also 
propose to create new regulations: Sec.  25.1324--Angle of attack 
systems; Sec.  25.1420 SLD icing conditions; part 25, appendix O (SLD 
icing conditions); part 33, appendix C (this will be intentionally left 
blank as a placeholder); and part 33, appendix D (Mixed phase and ice 
crystal icing conditions). To improve the safety of transport category 
airplanes operating in SLD, mixed phase, and ice crystal icing 
conditions, the proposed regulations would:
     Expand the certification icing environment to include 
freezing rain and freezing drizzle.
     Require airplanes most affected by SLD icing conditions to 
meet certain safety standards in the expanded certification icing 
environment, including additional airplane performance and handling 
qualities requirements.
     Expand the engine and engine installation certification, 
and some airplane component certification regulations (for example, 
angle of attack and airspeed indicating systems), to include freezing 
rain, freezing drizzle, ice crystal, and mixed phase icing conditions. 
For certain cases, a subset of these icing conditions is proposed.
    The benefits and costs are summarized below. The estimated benefits 
are $405.6 million ($99.5 million present value). The total estimated 
costs are $71.0 million ($54.0 million present value). On an annualized 
basis, for the time period 2012-2064, the benefits are $7.0 million, 
and the costs are $3.8 million.

----------------------------------------------------------------------------------------------------------------
                                              Nominal benefits                     PV benefits
----------------------------------------------------------------------------------------------------------------
                                                    Benefits
----------------------------------------------------------------------------------------------------------------
Smaller & Medium Airplanes.................       $249,580,915  $69,994,259
Larger Airplanes...........................        156,004,884  29,498,469
    Total Benefits.........................        405,585,799  99,492,728
¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤
                                                                (7.0 million annually)
----------------------------------------------------------------------------------------------------------------
                                                      Costs
----------------------------------------------------------------------------------------------------------------
                                                Nominal cost    PV cost
----------------------------------------------------------------------------------------------------------------
Engine Cert Cost...........................          7,936,000  6,931,610
Engine Capital Cost........................          6,000,000  5,240,632
                                            --------------------------------------------------------------------
    Total Engine...........................         13,936,000  12,172,242
================================================================================================================
Smaller Airplane Certification Cost........         24,999,039  21,835,129
New Larger Airplane Certification Cost.....          3,154,600  2,755,350
Derivative Larger Airplane Certification            10,438,800  9,117,652
 Cost.
Hardware Costs.............................         10,390,000  5,842,024
Fuel Burn All..............................          8,046,676  2,261,941
================================================================================================================
    Total Costs............................         70,965,115  53,984,338
----------------------------------------------------------------------------------------------------------------
                                                                ($3.8 million annually)
----------------------------------------------------------------------------------------------------------------

Background

    In the 1990s, the FAA became aware that the types of icing 
conditions considered during the certification of transport category 
airplanes and turbine aircraft engines needed to be expanded to 
increase the level of safety during flight in icing. The FAA determined 
that the revised icing certification standards should include 
supercooled large drops (SLD), mixed phase, and ice crystals.\1\
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    \1\ Appendix 1 of this preamble contains definitions of certain 
terms used in this notice of proposed rulemaking (NPRM).
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    Safety concerns about the adequacy of the icing certification 
standards were brought to the forefront of public and governmental 
attention by a 1994 accident in Roselawn, Indiana, involving an Avions 
de Transport Regional ATR 72 series airplane. The FAA, Aerospatiale, 
the French Direction G[eacute]n[eacute]ral de l'Aviation Civile, Bureau 
Enquete Accident, the National Aeronautics and Space Administration, 
the National Transportation Safety Board (NTSB), and others conducted 
an extensive investigation of this accident. These investigations led 
to the conclusion that freezing drizzle conditions created a ridge of 
ice on the wing's upper surface aft of the deicing boots and forward of 
the ailerons. It was further concluded that this ridge of ice 
contributed to an uncommanded roll of the airplane. Based on its 
investigation, the NTSB recommended changes to the icing certification 
requirements.
    The certification requirements for icing conditions are specified 
in part 25, appendix C. The atmospheric condition (freezing drizzle) 
that contributed to the Roselawn accident is currently outside the 
icing envelope for certifying transport category airplanes. The term 
``icing envelope'' is used within part 25, appendix C, and this NPRM to 
refer to the environmental icing conditions within which the airplane 
must be shown to be able to safely operate. The term ``transport 
category airplanes'' is used throughout this rulemaking document to 
include all airplanes type certificated to part 25 regulations.
    Another atmospheric icing condition that is currently outside the 
icing envelope is freezing rain. The FAA has not required airplane 
manufacturers to show that airplanes can operate safely in freezing 
drizzle or freezing rain conditions. These conditions constitute an 
icing environment known as supercooled large drops (SLDs).
    As a result of this accident and consistent with related NTSB

[[Page 37313]]

recommendations \2\ the FAA tasked the Aviation Rulemaking Advisory 
Committee (ARAC),\3\ through its Ice Protection Harmonization Working 
Group (IPHWG), to do the following:
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    \2\ NTSB recommendations A-96-54 and A-96-56; available in the 
Docket and on the Internet at: http://www.ntsb.gov/Recs/letters/
1996/A96_48_69.pdf.
    \3\ Published in the Federal Register, December 8, 1997 (62 FR 
64621).
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     Define an icing environment that includes SLDs.
     Consider the need to define a mixed phase icing 
environment (supercooled liquid and ice crystals).
     Devise requirements to assess the ability of an airplane 
to either safely operate without restrictions in SLD and mixed phase 
conditions or safely operate until it can exit these conditions.
     Study the effects icing requirement changes could have on 
Sec. Sec.  25.773, Pilot compartment view; 25.1323, Airspeed indicating 
system; and 25.1325, Static pressure systems.
     Consider the need for a regulation on ice protection for 
angle of attack probes.
    This proposed rule is based on the ARAC's recommendations to the 
FAA. Terms used in this notice of proposed rulemaking (NPRM) are 
defined in Appendix 1 of this preamble.

A. Existing Regulations for Flight in Icing Conditions

    Currently, the certification regulations applicable to transport 
category airplanes for flight in icing conditions require that: ``The 
airplane must be able to operate safely in the continuous maximum and 
intermittent maximum icing conditions of appendix C.'' \4\ The 
certification regulations also require minimum performance and handling 
qualities in these icing conditions and methods to detect airframe 
icing and to activate and operate ice protection systems.\5\ Icing 
regulations applicable to engines are in Sec. Sec.  33.68 and 33.77. 
Operating regulations in parts 91 (General Operating and Flight Rules) 
and 135 (Operating Requirements: Commuter and On Demand Operations) 
address limitations in icing conditions for airplanes operated under 
these parts.\6\ Part 121 (Operating Requirements: Domestic, Flag and 
Supplemental Operations) addresses operations in icing conditions that 
might adversely affect safety and requires installing certain types of 
ice protection equipment and wing illumination equipment.\7\
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    \4\ 14 CFR 25.1419, Ice Protection.
    \5\ For a complete discussion of the regulations see Amendment 
25-121 (72 FR 44665, August 8, 2007), and Amendment 25-129 (74 FR 
38328, August 3, 2009).
    \6\ 14 CFR 91.527, Operating in icing conditions; and Sec.  
135.227, Icing conditions: Operating limitations.
    \7\ 14 CFR 121.629(a), Operation in icing conditions and Sec.  
121.341, Equipment for operations in icing conditions.
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    Some of the part 25 and 33 regulations specify that the affected 
equipment must be able to operate in some or all of the icing 
conditions defined in part 25, appendix C. Other regulations within 
these parts do not specify the icing conditions that must be considered 
for airplane certification, but, historically, airplane certification 
programs have only considered icing conditions that are defined in 
appendix C.
    Appendix C addresses continuous maximum and intermittent maximum 
icing conditions within stratiform and cumuliform clouds ranging from 
sea level up to 30,000 feet. Appendix C defines icing cloud 
characteristics in terms of mean effective drop diameters, liquid water 
content, temperature, horizontal and vertical extent, and altitude. 
Icing conditions that contain drops with mean effective diameters that 
are larger than the cloud mean effective drop diameters defined in 
appendix C are typically referred to as freezing drizzle or freezing 
rain. Icing conditions containing freezing drizzle and freezing rain 
are not currently considered when certifying an airplane's ice 
protection systems. Because the larger diameter drops typically impinge 
farther aft on the airfoil, exposure to these conditions can result in 
ice accretions aft of the ice protection area, which can negatively 
affect airplane performance and handling qualities.
    Likewise, mixed phase (supercooled liquid and ice crystals) and 
100% ice crystal icing conditions are not currently considered when 
certifying an airplane's ice protection systems. Exposing engines and 
externally mounted probes to these conditions could result in hazardous 
ice accumulations within the engine that may result in engine damage, 
power loss, and loss of or misleading airspeed indications. The 
certification regulations for transport category airplanes and engines 
do not address the safe operation of airplanes in SLD, mixed phase, or 
ice crystal icing conditions and the operating rules do not 
specifically prohibit operations in these conditions.

B. National Transportation Safety Board Safety Recommendations

    The NTSB issued NTSB Safety Recommendation Numbers A-96-54 \8\ and 
A-96-56 \9\ as a result of the Roselawn accident previously discussed. 
This rulemaking activity partially addresses the NTSB recommendations 
because there are separate rulemaking activities associated with 
revisions to 14 CFR part 23 regulations for small airplanes and 14 CFR 
part 121 operational regulations. The NTSB recommendations are as 
follows:
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    \8\ NTSB recommendation A-96-54; available in the Docket and on 
the Internet at: http://www.ntsb.gov/Recs/letters/1996/A96_48_
69.pdf.
    \9\ NTSB recommendation A-96-56; available in the Docket and on 
the Internet at: http://www.ntsb.gov/Recs/letters/1996/A96_48_
69.pdf.
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1. A-96-54
    Revise the icing criteria published in 14 Code of Federal 
Regulations (CFR), parts 23 and 25, in light of both recent research 
into aircraft ice accretion under varying conditions of liquid water 
content, drop size distribution, and temperature, and recent 
developments in both the design and use of aircraft. Also, expand the 
appendix C icing certification envelope to include freezing drizzle/
freezing rain and mixed water/ice crystal conditions, as necessary. 
(Class II, Priority Action) (A-96-54) (Supersedes A-81-116 and--118)
2. A-96-56
    Revise the icing certification testing regulation to ensure that 
airplanes are properly tested for all conditions in which they are 
authorized to operate, or are otherwise shown to be capable of safe 
flight into such conditions. If safe operations can not be demonstrated 
by the manufacturer, operational limitations should be imposed to 
prohibit flight in such conditions and flightcrews should be provided 
with the means to positively determine when they are in icing 
conditions that exceed the limits for aircraft certification. (Class 
II, Priority Action) (A-96-56)

C. Related Rulemaking Activity

    The ARAC's Ice Protection Harmonization Working Group (IPHWG) 
submitted additional part 121 icing rulemaking recommendations to the 
FAA that may lead to future rulemaking, but do not directly impact this 
NPRM. Those recommendations would improve airplane safety when 
operating in icing conditions. The recommendations would:
     Address when ice protection systems must be activated.

[[Page 37314]]

     Require some airplanes to exit all icing conditions after 
encountering large drop icing conditions conducive to ice accretions 
aft of the airframe's protected area.

D. Advisory Material

    The proposed new AC and revisions to existing ACs would provide 
guidance material for one acceptable means, but not the only means, of 
demonstrating compliance with the proposed regulations contained in 
this NPRM. The guidance provided in these documents is directed at 
airplane manufacturers, modifiers, foreign regulatory authorities, and 
FAA transport airplane type certification engineers, flight test 
pilots, and their designees. The proposed ACs will be posted on the 
``Aircraft Certification Draft Documents Open for Comment'' Web site, 
http://www.faa.gov/aircraft/draft_docs, after this NPRM is published 
in the Federal Register
    For advisory material related to this NPRM, the FAA is:
     Developing a new AC 25-xx, Compliance of Transport 
Category Airplanes with Certification Requirements for Flight in Icing 
Conditions.
     Revising AC 20-147, Turbojet, Turboprop, and Turbofan 
Engine Induction System Icing and Ice Ingestion.
     Revising AC 25-25, Performance and Handling 
Characteristics in the Icing Conditions Specified in Part 25, Appendix 
C.
     Revising AC 25.629-1A, Aeroelastic Stability 
Substantiation of Transport Category Airplanes.
     Revising AC 25.1329-1B, Approval of Flight Guidance 
Systems.

General Discussion of the Proposal

    The FAA proposes to revise certain regulations in parts 25 and 33 
related to the certification of transport category airplanes and 
turbine aircraft engines in icing conditions.
    We also propose to create a new: Sec.  25.1324--Angle of attack 
systems; Sec.  25.1420--Supercooled large drop icing conditions; part 
25, appendix O (supercooled large drop icing conditions; part 33, 
appendix C (intentionally left blank); and part 33, appendix D (Mixed 
phase and ice crystal icing conditions). Part 33, appendix C, is 
intentionally left blank and retained as a placeholder for non-icing 
related regulations so that part 33, appendix C, would not be confused 
with the icing conditions defined in part 25, appendix C.
    To improve the safety of transport category airplanes operating in 
SLD, mixed phase, and ice crystal icing conditions, the proposed 
regulations would:
     Expand the certification icing environment to include 
freezing rain and freezing drizzle.
     Require airplanes most affected by SLD icing conditions 
(transport category airplanes with a maximum takeoff weight less than 
60,000 pounds or with reversible flight controls) to meet certain 
safety standards in the expanded certification icing environment, 
including additional airplane performance and handling qualities 
requirements.
     Expand the engine and engine installation certification, 
and some airplane component certification regulations (for example, 
angle of attack and airspeed indicating systems) to include freezing 
rain, freezing drizzle, ice crystal, and mixed phase icing conditions. 
For certain cases, a subset of these icing conditions is proposed.

A. Safety Concern

    The ARAC's IPHWG reviewed icing events involving transport category 
airplanes and found accidents and incidents that are believed to have 
occurred in icing conditions that are not addressed by the current 
regulations. The icing conditions resulted in flightcrews losing 
control of their aircraft and, in some cases, engine power loss. The 
review found hull losses and fatalities associated with SLD conditions, 
but not for ice crystal and mixed phase conditions. However, there have 
been 14 documented cases of ice crystal and mixed phase engine power 
loss events between 1988 through 2009. Of those events, there were 13 
occurrences of multi-engine power loss events. Fifty percent of those 
events were defined as ``aircraft level events,'' since they occurred 
on multiple engines installed on the same airplane. Two of these 
aircraft level events resulted in diversions.
    The incident history also indicates that flightcrews have 
experienced temporary loss of or misleading airspeed indications in 
icing. Airspeed indications on transport category airplanes are derived 
from the difference between two air pressures--the total pressure, as 
measured by a pitot tube mounted somewhere on the fuselage, and the 
ambient or static pressure, as measured by a static port. The static 
port may be flush mounted on the airplane fuselage or co-located on the 
pitot tube. When the static and pitot systems are co-located, the 
configuration is referred to as a pitot-static tube. Static ports are 
not prone to collecting ice crystals, either because of their flush 
mounted locations or their overall shape.
    Due to the way pitot or pitot-static tubes are usually mounted, 
they are prone to collecting ice crystals. Encountering high 
concentrations of ice crystals may lead to blocked pitot or pitot-
static tubes because the energy necessary to melt the ice crystals can 
exceed the tubes' design requirements. Pitot or pitot-static tube 
blockage can lead to errors in measuring airspeed. The regulatory 
changes which add ice crystal conditions for airspeed indicating 
systems are intended to apply to either a pitot tube or pitot-static 
tube configuration.
    The IPHWG did not identify any events due to ice accumulations on 
probes that are used to measure angle of attack, or other angle of 
attack sensors. However, the IPHWG determined there are angle of attack 
probe designs that are susceptible to mixed phase conditions.
    The IPHWG concluded that the current regulations do not adequately 
address SLD, mixed phase, and ice crystal conditions. The concerns 
regarding mixed phase and ice crystal conditions were limited to 
engines, propulsion installations, airspeed indications, and angle of 
attack systems. The FAA concurs with the IPHWG's conclusions.

B. Prior FAA Actions To Address the Safety Concern

    The FAA has issued airworthiness directives (ADs) to address the 
unsafe conditions associated with operating certain airplanes in severe 
icing conditions, which can include SLD icing conditions. These ADs are 
applicable to airplanes equipped with both reversible flight controls 
in the roll axis and pneumatic deicing boots. The ADs require the 
flightcrews to exit icing when visual cues are observed that indicate 
the conditions exceed the capabilities of the ice protection equipment. 
In addition, for new certifications of airplanes equipped with 
unpowered roll axis controls and pneumatic deicing boots, the airplanes 
are evaluated to ensure the roll control forces are acceptable if the 
airplane operates in certain SLD conditions. However, the scope of 
these actions is limited because they do not address all transport 
category airplanes and do not address the underlying safety concern of 
the unknown performance and handling qualities safety margins for 
airplanes and engines operating in freezing drizzle, freezing rain, 
mixed phase, and ice crystal conditions. The IPHWG concluded there is a 
need to improve the regulations to ensure safe operation

[[Page 37315]]

of airplanes and engines in these conditions.

C. Alternatives to Rulemaking

    Before proposing new rulemaking, the FAA considers alternative ways 
to solve the safety issue under consideration. Following is a brief 
discussion of two of the alternatives we considered during 
deliberations on this proposed rule.
1. Alternative 1: Terminal Area Radar and Sensors
    The IPHWG considered the use of terminal area radar and ground-
based sensors to identify areas of SLDs so they can be avoided, rather 
than require certification for operations in SLD. Equipment for 
detecting and characterizing icing conditions in holding areas is being 
developed. However, the equipment would have limited coverage area. For 
areas not covered by terminal area radar and ground-based sensors, 
airborne radars and sensors are being developed that would identify SLD 
conditions in sufficient time for avoidance. These ground-based and 
airborne systems are not mature enough to provide sufficient protection 
for all flight operations affected by SLD. Even if the equipment was 
mature, rulemaking would still be necessary to establish safety margins 
for inadvertent flight into such conditions and to provide an option 
for applicants to substantiate that the airplane is capable of safe 
operation in SLD conditions.
2. Alternative 2: Icing Diagnostic and Predictive Weather Tools
    The IPHWG considered the use of icing diagnostic and predictive 
weather tools to avoid SLD rather than certify an airplane to operate 
in SLD conditions. Tools have been developed that can provide 
information on icing and SLD potential, but may not report all 
occurrences of SLD. These experimental tools are available on the 
Internet and can be used to provide flight planning information 
guidance for avoidance of SLD conditions. However, rulemaking would 
still be necessary to establish safety margins for inadvertent flight 
into such conditions and to provide an option for applicants to 
substantiate that the airplane is capable of safe operation in SLD 
conditions.

Discussion of the Proposed Regulatory Requirements

Appendix O to Part 25

    The proposed appendix O is structured like part 25, appendix C, one 
part defining icing conditions and one defining ice accretions. 
Appendix O, part I, would define SLD icing conditions and part II would 
define the ice accretions that a manufacturer must consider when 
designing an airplane.

Supercooled Large Drop Icing Conditions

    Proposed Sec.  25.1420 would add safety requirements that must be 
met in SLD icing conditions for certain transport category airplanes to 
be certified for flight in icing conditions. This change would require 
evaluating the operation of these airplanes in the SLD icing 
environment; developing a means to differentiate between different SLD 
icing conditions, if necessary; and developing procedures to exit all 
icing conditions.
    The proposed regulation would require consideration of the SLD 
icing conditions (freezing drizzle and freezing rain) defined in a 
proposed new part 25, appendix O, part I, in addition to the existing 
part 25, appendix C, icing conditions. Proposed appendix O would 
include drop sizes larger than those considered by current icing 
regulations. These larger drops impinge and freeze farther aft on 
airplane surfaces than the drops defined in appendix C and may affect 
the airplane's performance, handling qualities, flutter 
characteristics, and engine and systems operations. The appendix O 
icing conditions, if adopted, may affect the design of airplane ice 
protection systems.
    The SLD icing conditions described in the proposed appendix O would 
be those in which the airplane must be able to either safely exit 
following the detection of any or specifically identified appendix O 
icing conditions, or safely operate without restrictions. Specifically, 
the proposed Sec.  25.1420 would allow three options:
     Detect appendix O conditions and then operate safely while 
exiting all icing conditions (Sec.  25.1420(a)(1)).
     Safely operate in a selected portion of appendix O 
conditions, detect when the airplane is operating in conditions that 
exceed the selected portion, and then operate safely while exiting all 
icing conditions (Sec.  25.1420(a)(2)).
     Operate safely in all of the appendix O conditions (Sec.  
25.1420(a)(3)).
    As discussed below in the section titled ``Differences from the 
ARAC Recommendations,'' the proposed Sec.  25.1420 would apply to 
airplanes with either: (1) a takeoff maximum gross weight of less than 
60,000 pounds, or (2) reversible flight controls.
    To establish that an airplane could operate safely in the proposed 
appendix O conditions described above, proposed Sec.  25.1420(b) would 
require both analysis and one test, or more as found necessary, to 
establish that the ice protection for the various components of the 
airplane is adequate. The words ``as found necessary'' would be applied 
in the same way as they are applied in Sec.  25.1419(b). During the 
certification process, the applicant would demonstrate compliance with 
the rule using a combination of analyses and test(s). The applicant's 
means of compliance would consist of analyses and the amount and types 
of testing it finds necessary to demonstrate compliance with the 
regulation. The applicant would choose to use one or more of the tests 
identified in paragraphs Sec.  25.1420(b)(1) through (b)(5). Although 
the applicant may choose the means of compliance, it is ultimately the 
FAA that determines whether the applicant has performed sufficient 
test(s) and analyses to substantiate compliance with the regulation. 
Similarly, the words ``as necessary,'' which appear in Sec.  
25.1420(b)(3) and (b)(5), would result in the applicant choosing the 
means of compliance that is needed to support the analysis, but the FAA 
would make a finding whether the means of compliance is acceptable. If 
an applicant has adequate data a similarity analysis may be used in 
lieu of the testing required by Sec.  25.1420(b). For an airplane 
certified to operate in at least a portion of proposed appendix O icing 
conditions, proposed Sec.  25.1420(c) would extend the requirements of 
Sec.  25.1419(e), (f), (g), and (h) \10\ to include activation and 
operation of airframe ice protection systems in the appendix O icing 
conditions for which the airplane is certified. Proposed Sec.  
25.1420(c) would not apply to airplanes certified to proposed Sec.  
25.1420(a)(1) because proposed Sec.  25.1420(a)(1) would require a 
method to identify and safely exit all appendix O conditions.
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    \10\ These requirements were recently adopted in Amendment 25-
129 (74 FR 38328, August 3, 2009). Generally, that amendment 
requires methods to detect airframe icing and to activate and 
operate ice protection systems.
---------------------------------------------------------------------------

    The proposed appendix O defines SLD conditions. It was developed by 
the ARAC IPHWG, which included meteorologists and icing research 
specialists from industry, FAA/FAA Tech Center, Meteorological Services 
of Canada, National Aeronautics and Space Administration (NASA), and 
Transport Canada/Transport Development Center. The IPHWG collected and 
analyzed airborne measurements of pertinent SLD variables, developed an 
engineering standard to be used in aircraft certification, and 
recommended that

[[Page 37316]]

standard to the FAA. The FAA concurs with the recommendation.
    The SLD conditions defined in appendix O, part I, include freezing 
drizzle and freezing rain conditions. The freezing drizzle and freezing 
rain environments are further divided into conditions in which the drop 
median volume diameters are either less than or greater than the 40 
microns. Appendix O consists of measured data that was divided into 
drop distributions within these four icing conditions. These 
distributions were averaged to produce the representative distributions 
for each condition.
    The distributions of drop sizes are defined as part of appendix O. 
The need to include the distributions comes from the larger amount of 
mass in the larger drop diameters of appendix O. The water mass of the 
larger drops affects the amount of water that impinges on airplane 
components, the drop impingement, icing limits, and the ice buildup 
shape.
    Appendix O provides a liquid water content scale factor that would 
be used to adjust the liquid water content for freezing drizzle and 
freezing rain. The scale factor is based on the liquid water contents 
of continuous freezing drizzle and freezing rain conditions decreasing 
with increasing horizontal extents.

Performance and Handling Qualities

    The ice accretion definitions in proposed appendix O, part II, and 
the proposed revisions to the performance and handling qualities 
requirements for flight in icing conditions are similar to those 
required for flight in appendix C icing conditions. The proposals 
address the three options allowed by proposed Sec.  25.1420(a). 
Proposed appendix O, part II, would contain definitions of the ice 
accretions appropriate to each phase of flight. The proposed appendix 
O, part II(b), would define the ice accretions used to show compliance 
with the performance and handling qualities requirements for any 
portion of appendix O in which the airplane is not certified to 
operate. The proposed appendix O, part II(c), would define the ice 
accretions for any portion of appendix O in which the airplane is 
certified to operate.
    Proposed appendix O, part II(d), would define the ice accretion in 
appendix O conditions before the airframe ice protection system is 
activated and is performing its intended function to reduce or 
eliminate ice accretions on protected surfaces. This ice accretion 
would be used in showing compliance with the controllability and stall 
warning margin requirements of Sec. Sec.  25.143(j) and 25.207(h), 
respectively, that apply before the airframe ice protection system has 
been activated and is performing its intended function. Even if the 
airplane is certified to operate only in a portion of the appendix O 
icing conditions, the ice accretion used to show compliance with 
Sec. Sec.  25.143(j) and 25.207(h) must consider all appendix O icing 
conditions since the initial entry into icing conditions may be into 
appendix O icing conditions in which the airplane is not certified to 
operate.
    To reduce the number of ice accretions needed to show compliance 
with Sec.  25.21(g), the proposed appendix O, part II(e), would allow 
the option of using an ice accretion defined for one flight phase for 
any other flight phase if it is shown to be more critical than the ice 
accretion defined for that other flight phase.
    Existing Sec.  25.21(g)(1) \11\ requires that the performance and 
handling qualities requirements of part 25, subpart B, with certain 
exceptions,\12\ be met in appendix C icing conditions.\13\ Proposed 
Sec.  25.21(g)(3) would identify the performance and handling qualities 
requirements that must be met to ensure that an airplane certified to 
either the proposed Sec.  25.1420(a)(1) or (a)(2) could safely exit 
icing if the icing conditions of proposed appendix O, for which 
certification is not sought, are encountered. Such an airplane would 
not be approved to take off in proposed appendix O icing conditions and 
would only need to be able to detect and safely exit those icing 
conditions encountered en route. Therefore, it is proposed that, in 
addition to the exceptions identified in the existing Sec.  
25.21(g)(1), such an airplane would not need to meet certain 
requirements \14\ for appendix O icing conditions.
---------------------------------------------------------------------------

    \11\ 14 CFR 25.21(g)(1) is proposed to be redesignated as Sec.  
25.21(g)(2).
    \12\ The exceptions listed in this requirement are Sec. Sec.  
25.121(a), 25.123(c), 25.143(b)(1) and (b)(2), 25.149, 25.201(c)(2), 
25.207(c) and (d), 25.239, and 25.251(b) through (e).
    \13\ For a complete discussion of these requirements, see 
Amendment 25-121 (72 FR 44665, August 8, 2007).
    \14\ 14 CFR 25.105, 25.107, 25.109, 25.111, 25.113, 25.121, and 
25.123.
---------------------------------------------------------------------------

    With one exception, for an airplane certified under proposed Sec.  
25.1420(a)(1) or (a)(2), the same handling qualities requirements that 
must currently be met for flight in appendix C icing conditions are 
proposed for flight in appendix O icing conditions for which 
certification is not sought. That exception is Sec.  25.143(c)(1), 
which addresses controllability following engine failure during takeoff 
at V2. Compliance with that rule would not be necessary 
since the airplane would not be approved for takeoff in appendix O 
icing conditions. No justification for a relaxation of other handling 
qualities requirements could be identified.
    The requirements for safe operation in all or any portion of 
proposed appendix O icing conditions under proposed Sec.  25.21(g)(4) 
are similar to those currently required for appendix C icing 
conditions. With one exception, the list of part 25, subpart B 
requirements that currently do not have to be met for flight in 
appendix C icing conditions would not have to be met in proposed 
appendix O icing conditions. The exception is that compliance with 
Sec.  25.121(a), Climb: One-engine-inoperative would be required for 
appendix O icing conditions because, unlike for appendix C icing 
conditions, the FAA cannot justify an assumption that the ice accretion 
in this flight phase can be assumed insignificant. In practice, it is 
expected that some applicants may use an operating limitation to 
prohibit takeoff in appendix O icing conditions. Otherwise, the same 
rationales behind the requirements are used for both appendix C and 
appendix O icing conditions. For continued operation in appendix O 
icing conditions, there should effectively be no degradation in 
handling qualities, and any degradation in performance should be no 
greater than that allowed by the regulations for appendix C icing 
conditions.

Component Requirements for All Part 25 Transport Category Airplanes

    In certification programs, both the airplane as a whole and its 
individual components are evaluated for flight in icing conditions. 
There are several rules in part 25 \15\ that contain icing related 
requirements for specific components. We propose to revise those rules 
to ensure the airplane can safely operate in the new icing conditions 
established in this proposed rule.
---------------------------------------------------------------------------

    \15\ 14 CFR 25.773, 25.929, 25.1093, 25.1323, and 25.1325.
---------------------------------------------------------------------------

    Section 25.1419 requires that an airplane be able to safely operate 
in all of the conditions specified in appendix C, whereas the proposed 
Sec.  25.1420 would not require an airplane to safely operate in all of 
the appendix O icing conditions. Proposed Sec.  25.1420(a)(1) and 
(a)(2) only require an airplane to be capable of safely exiting icing 
conditions after encountering an appendix O icing condition for which 
that airplane will not be certified. The existing regulations for pilot 
compartment view, airspeed indication

[[Page 37317]]

system, and static pressure system \16\ contain requirements for 
operation in icing conditions. These sections would be revised to add 
requirements for operation in appendix O icing conditions. Section 
25.1323, Airspeed indicating system, would also be revised to include 
and define mixed phase and ice crystal conditions. New proposed Sec.  
25.1324 includes an icing requirement for angle of attack systems. This 
would be similar to the icing requirements for airspeed indication 
systems. The proposed section would require the angle of attack system 
to be heated to prevent malfunction in appendices C and O icing 
conditions and in the mixed phase and ice crystal conditions defined in 
Sec.  25.1323.
---------------------------------------------------------------------------

    \16\ 14 CFR 25.773, 25.1323, and 25.1325.
---------------------------------------------------------------------------

    In the proposed revisions to the requirements for pilot compartment 
view, airspeed indication system, and static pressure system,\17\ and 
the new proposed requirements for angle of attack systems, an airplane 
certified in accordance with Sec.  25.1420(a)(1) or (a)(2) would not be 
required to be evaluated for all of appendix O. For airplanes certified 
in accordance with Sec.  25.1420(a)(1), the icing conditions that the 
airplane is certified to safely exit following detection must be 
considered. For airplanes certified in accordance with Sec.  
25.1420(a)(2), the icing conditions that the airplane is certified to 
safely operate in, and to safely exit following detection, must be 
considered. For airplanes certified in accordance with Sec.  
25.1420(a)(3) and for airplanes not subject to Sec.  25.1420, all icing 
conditions must be considered. Airplanes not certified for flight in 
icing need not consider appendix O.
---------------------------------------------------------------------------

    \17\ Ibid.
---------------------------------------------------------------------------

    The engine induction system icing section (Sec.  25.1093) and 
propeller deicing section (Sec.  25.929) contain requirements for 
operation in icing conditions. As a conservative approach to ensure 
safe operation of an airplane in an inadvertent encounter with icing, 
the existing language in Sec.  25.1093 contains requirements for 
operation in icing conditions, even for an airplane that is not 
approved for flight in icing. Since proposed appendix O defines icing 
conditions that also may be inadvertently encountered, Sec.  25.1093 
would be revised to reference appendix O in its entirety. This would 
maintain the FAA's conservative approach for this section. Section 
25.929 (propeller deicing) would also be revised to reference appendix 
O in its entirety.
    Sections 25.929 and 25.1323 generically reference icing instead of 
specifically mentioning appendix C. Historically, the icing conditions 
specified in appendix C have been applied to these rules. For clarity, 
we are revising Sec. Sec.  25.929 and 25.1323 so they specifically 
reference appendix C, as well as appendix O. The proposed revisions to 
icing regulations for pilot compartment view, propellers, engine 
induction system icing protection, airspeed indication system, static 
pressure system, and angle of attack system would be applicable to all 
transport category airplanes to ensure safe operation during operations 
in icing conditions.
    The proposed revisions to Sec.  25.903 would retain the existing 
regulations and add new subparagraphs to be consistent with the 
proposed part 33 changes in Sec.  33.68. These revisions would allow 
for approving new aircraft type certification programs with engines 
certified to earlier amendment levels. The proposed revisions would 
make it clear that the proposed part 33 changes would not be 
retroactively imposed on an already type certified engine design, 
unless service history indicated that an unsafe condition was present.
    The proposed revision to Sec.  25.929 clarifies the meaning of the 
words ``for airplanes intended for use where icing may be expected.'' 
The intent has been for the rule to be applicable to airplanes 
certified for flight in icing.

Engine and Engine Installation Requirements

    The proposed revisions to Sec. Sec.  25.1093, 33.68, and 33.77 
would change the icing environmental requirements used to evaluate 
engine protection and operation in icing conditions. The reason for 
these changes is that the incident history of some airplanes has shown 
that the current icing environmental requirements are inadequate. The 
effect of the change would be to require an evaluation of safe 
operation in the revised icing environment. The proposed revision to 
Sec.  25.1093 restructures paragraph (b) and adds a new Table 1--Icing 
Conditions for Ground Tests. The proposed rules would require engines 
and engine installations to operate safely throughout the SLD 
conditions defined in proposed new part 25, appendix O, and the newly 
defined mixed phase and ice crystal conditions defined in proposed new 
part 33, appendix D.\18\ The proposed appendix D was developed by the 
ARAC Engine Harmonization Working Group and the Power Plant 
Installation Harmonization Working Group, which included meteorologists 
and icing research specialists from industry, FAA/FAA Tech Center, 
Meteorological Services of Canada, National Aeronautics and Space 
Administration (NASA), and Transport Canada/Transport Development 
Center. The ARAC recommended appendix D and the FAA concurs with the 
recommendation.
---------------------------------------------------------------------------

    \18\ See FAA report DOT/FAA/AR-09/13, Technical Compendium from 
Meetings of the Engine Harmonization Working Group, March 2009 for 
details on appendix D and its development.
---------------------------------------------------------------------------

    The proposed revision to Sec.  25.1521 would retain the existing 
regulations and add a new subparagraph that would require an additional 
operating limitation for turbine engine installations during ground 
operation in icing conditions defined in Sec.  25.1093(b)(2). That 
operating limitation would address the maximum time interval between 
any engine run-ups from idle and the minimum ambient temperature 
associated with that run-up interval. This limitation is necessary 
because we do not currently have any specific requirements for run-up 
procedures for engine ground operation in icing conditions. The engine 
run-up procedure, including the maximum time interval between run-ups 
from idle, run-up power setting, duration at power, and the minimum 
ambient temperature demonstrated for that run-up interval proposed in 
Sec.  25.1521, would be included in the Airplane Flight Manual in 
accordance with existing Sec.  25.1581(a)(1) and Sec.  25.1583(b)(1).
    The engine run-up procedure from ground idle to a moderate power or 
thrust setting is necessary to shed ice build-up on the fan blades 
before the quantity of ice reaches a level that could adversely affect 
engine operation if ice is shed into the engine. The proposed revision 
to Sec.  25.1521 would not require additional testing. The ice shedding 
demonstration may be included as part of the Sec.  33.68 engine icing 
testing.

Operating Limitations

    The proposed revision to Sec.  25.1533 would establish an operating 
limitation applicable to airplanes that are not certified in accordance 
with proposed Sec.  25.1420(a)(1) or (a)(2). The flightcrews of these 
airplanes would be required to exit all icing conditions if they 
encounter appendix O icing conditions that the airplane has not been 
certified to operate in.

Expansion of Proposed Icing Requirements

    The proposed regulations \19\ for the airspeed indicating system 
and angle of

[[Page 37318]]

attack system would address the operation of those systems in specific 
mixed phase and ice crystal conditions, as defined in proposed Appendix 
O. During the drafting of this NPRM the FAA became aware of airspeed 
indicating system malfunctions in environmental conditions that may not 
be addressed by these proposed regulations. The FAA is reviewing the 
malfunctions and is considering the need to change the proposed mixed 
phase and ice crystal parameters to include freezing rain. The maximum 
mixed phase and ice crystal parameters that we are considering are 
those defined in the proposed part 33, appendix D. The freezing rain 
parameters that we are considering are based on standards some 
manufacturers have used for airdata probes. The maximum freezing rain 
parameters that we are considering are:
---------------------------------------------------------------------------

    \19\ 14 CFR 25.1323, and 25.1324.



----------------------------------------------------------------------------------------------------------------
Static air temperature                          Altitude range               Liquid                      Droplet
                                                                              water                          MVD
                                                                            content  Horizontal extent
----------------------------------------------------------------------------------------------------------------
([deg]C)                                          (ft)             (m)       (g/m3)    (km)   (nmiles)  ([micro]
                                                                                                              m)
----------------------------------------------------------------------------------------------------------------
-2 to 0...............................     0 to 10 000       0 to 3000            1     100         50      1000
                                                                                  6       5          3      2000
                                                                                 15       1        0.5      2000
----------------------------------------------------------------------------------------------------------------

    We consider the mixed phase and ice crystal parameters defined in 
the proposed part 33, appendix D, plus the freezing rain parameters 
defined above to be adequate to prevent potential airspeed indicating 
system malfunctions in these newly defined environmental conditions. We 
request technical and economic comments on whether the proposed 
airspeed indicating system and angle of attack system regulations 
should include these expanded parameters. Based on comments we receive, 
we may add these parameters to the final rule.

Differences From the ARAC Recommendations

    The IPHWG recommended changes to parts 25 and 33 to ensure the safe 
operation of airplanes and engines in icing conditions. The FAA concurs 
with the recommendations, but has determined it is necessary to revise 
to which airplanes the new airplane icing certification requirements in 
the proposed Sec.  25.1420 would apply. The proposed Sec.  25.1420 in 
this NPRM would apply to airplanes with either: (1) a takeoff maximum 
gross weight of less than 60,000 lbs (27,000 kg), or (2) reversible 
flight controls. An airplane with reversible flight controls in any 
axis (pitch, roll, or yaw), even if these flight controls are 
aerodynamically boosted and/or power-assisted, would be considered to 
have reversible flight controls under this proposed rule. An airplane 
with flight controls that are irreversible under normal operating 
conditions, but are reversible following a failure, would not be 
considered to have reversible flight controls under this proposed rule. 
Reversible, aerodynamically boosted, and power-assisted flight controls 
are defined in Appendix 1 to the preamble of this NPRM. The ADs 
described above in section B. ``Prior FAA Actions to address the Safety 
Concern'' are only applicable to airplanes equipped with both 
reversible flight controls in the roll axis and pneumatic deicing 
boots.
    A group of IPHWG members (Boeing, Airbus, and Embraer, supported by 
Cessna) held a minority position in their belief that the applicability 
of the proposed Sec.  25.1420 should exclude airplanes with certain 
design features. Their rationale for the position is that large 
transport airplanes still in production have not experienced any 
accidents or serious incidents as a result of flying in SLD icing 
conditions. These manufacturers proposed that airplanes having all 
three of the following design features should be excluded from 
compliance with Sec.  25.1420:
    (1) Gross weight in excess of 60,000 lbs (27,000 kg);
    (2) Irreversible powered flight controls; and
    (3) Wing leading-edge high-lift devices.
    These manufacturers included the gross weight criterion in this 
list, in part, because size has a direct bearing on an airplane's 
susceptibility to the adverse effects of ice accretion. The size of an 
airplane determines the sensitivity of its flight characteristics to 
ice thickness and roughness. The relative effect of a given ice height 
(or ice roughness height) decreases as airplane size increases.
    The irreversible powered flight controls design feature was chosen, 
in part, because using irreversible powered flight controls reduces an 
airplane's susceptibility to SLD conditions. The concern that SLD 
accretions can produce hinge moment or other anomalous control force/
trim effects is not applicable to those systems.
    The wing leading-edge high-lift devices design feature was chosen, 
in part, because, for wings without ice contamination, those devices 
provide a considerable increase in the maximum lift coefficient (CLmax) 
compared to fixed leading edges. When wings equipped with those devices 
are contaminated with ice, they have smaller relative CLmax losses due 
to ice accretion than wings with fixed leading edges.
    The IPHWG majority (Air Line Pilots Association, International 
(ALPA), Civil Aviation Authority for the United Kingdom (CAA/UK), FAA/
FAA Tech Center, Meteorological Services of Canada, National 
Aeronautics and Space Administration (NASA), SAAB, Transport Canada/
Transport Development Center) did not accept the exclusion of airplanes 
with the three aforementioned design features because one cannot 
predict with confidence that the past service experience of airplanes 
with these specific design features will be applicable to future 
designs. The IPHWG majority recommended applying the new SLD airplane 
certification requirements proposed in the new Sec.  25.1420 to all 
future transport category airplane type designs.
    The IPHWG majority opposed limiting the applicability of the rule 
based on airplane gross weight, in part, because the ratio of wing and 
control surface sizes to airplane weight varies between airplane 
designs. Therefore, airplane takeoff weight is not a consistent 
indicator of lifting and control surface size or chord, which are the 
important parameters affecting sensitivity to a given ice accretion.
    Excluding airplanes with irreversible flight controls was opposed, 
in part, because hinge moment and other anomalous control forces are 
not the only concern in SLD icing conditions. An irreversible control 
surface may not be deflected by the SLD accumulation but the 
aerodynamic efficiency of the control is likely to be degraded by the 
presence of SLD icing in front of the control surface.
    Excluding airplanes with wing leading edge high-lift devices was 
opposed, in part, because there are many different designs for such 
devices, which may not all be equally effective

[[Page 37319]]

in mitigating the negative effects of SLD ice accretions. The designs 
for those devices include:
     Slats that may be slotted or sealed to the basic wing 
leading edge, over or under deflected, with deflection and slotting 
that may be automated as a function of stall warning or airplane angle 
of attack;
     Krueger flaps that may be slotted or sealed to the wing 
leading edge, flexed to optimum curvature or conformed to the wing's 
leading edge lower surface; and
     Vortilons or some other vortex creating devices.
    In addition, for transport category airplanes with leading edge 
high-lift devices, the spanwise extent of ice protection varies from 
100 percent for some early turbo-jet airplane slats, to the span of two 
slats for later airplane designs, to none for Krueger flaps. The 
variations in the designs lead to varying degrees of aerodynamic 
benefit. Without defining the specific performance benefits associated 
with the above designs, the potential safety margins for SLD conditions 
cannot be determined.
    The complete minority and majority positions are discussed in the 
working group report, which is available in the public docket.\20\
---------------------------------------------------------------------------

    \20\ The complete IPHWG working group report is available on the 
Internet at http://regulations.gov. A copy will also be placed in 
the docket (FAA-2010-0636).
---------------------------------------------------------------------------

    In order to propose a rule with the estimated costs commensurate 
with the estimated benefits, the FAA determined the applicability of 
the proposed rule should be limited based on service histories of 
certified airplanes, and the assumption that similar future designs 
will continue to not experience the safety problems addressed by this 
proposal. Therefore, the FAA decided to revise the IPHWG rulemaking 
recommendation by incorporating, in part, the IPHWG minority position 
to exclude airplanes with certain design features.
    The FAA continues to agree with the IPHWG majority position that 
the presence (or conversely, the absence) of leading edge high lift 
devices should not be used as a basis for determining the applicability 
of the proposed Sec.  25.1420. There is insufficient data to conclude 
either that every type of leading edge high lift device, or that a 
specific leading edge high lift device design will affect (positively 
or negatively) an airplane's ability to operate in SLD atmospheric 
icing conditions. Also, leading edge high lift devices are only 
deployed in certain phases of flight (for example, takeoff and 
landing), and their deployment may differ for different flap 
configurations. For example, a leading edge slat may be sealed in one 
flap configuration, but slotted (that is, with a gap opened up between 
the trailing edge of the slat and the wing) in others. Therefore, the 
applicability of the proposed Sec.  25.1420 is not affected by the 
presence or absence of leading edge high lift devices.
    We request comment on whether this proposed rule, if adopted, 
should be applied to airplanes larger than 60,000 pounds MTOW or 
airplanes with other design features whose presence or absence would 
result in the airplane being susceptible to safety problems while 
operating in the SLD icing conditions defined in the proposed appendix 
O, as well as the economic analysis associated with these 
decisions.\21\
---------------------------------------------------------------------------

    \21\ A copy of the Initial Regulatory Evaluation (dated October 
5, 2009) can be found in the docket (FAA-2010-0636).
---------------------------------------------------------------------------

    This NPRM also differs from the ARAC recommendation by proposing a 
revision to Sec.  25.1533 for airplanes not certified to operate in all 
of the SLD atmospheric icing conditions specified in the proposed new 
appendix O (that is, airplanes certified in accordance with proposed 
Sec.  25.1420(a)(1) or (a)(2)). The proposal would establish an 
operating limitation that requires the flightcrews to exit all icing 
conditions if they encounter appendix O icing conditions in which the 
airplane has not been certified to operate.
    Another difference between this NPRM and the ARAC recommendation 
concerns an ARAC recommendation to establish separate stall warning 
margin and controllability requirements using the ice accretion 
associated with detection of appendix O icing conditions that require 
exiting all icing conditions. For airplanes that require exiting all 
icing conditions after encountering certain appendix O icing 
conditions, the ARAC recommended (and the FAA proposes in this NPRM) 
stall warning margin and controllability requirements that must be met 
with the ice accretion existing at the time the airplane exits all 
icing conditions. The ARAC was concerned that some future airplanes 
would be incapable of complying with these recommended requirements 
without including some means to increase the stall warning margin and 
airplane controllability upon detection of appendix O icing conditions. 
The ARAC recommended applying less stringent stall warning and 
controllability requirements with the ice accretion existing at the 
time appendix O icing conditions are detected, before the means to 
increase the stall warning margin and airplane controllability becomes 
effective.
    The FAA considers these ARAC recommended requirements to add 
significant complexity to the proposed rule to address an issue that 
may not arise. The FAA considers it unlikely that future airplane 
designs will include means to increase the stall warning margin and 
airplane controllability upon detection of appendix O icing conditions 
in addition to the means that are incorporated in many current 
transport category airplane designs to change the stall warning device 
activation point upon activation of the ice protection system. 
Therefore, these ARAC recommendations are not included in this NPRM. If 
needed, the FAA can issue special conditions, in accordance with Sec.  
21.16, to provide adequate safety standards in the unlikely event that 
such design features are included in a future transport category 
airplane.
    Another difference between this NPRM and the ARAC recommendation 
concerns the requirements for pilot compartment view, airspeed 
indication system, angle of attack system and static pressure 
system.\22\ For these rules the ARAC recommendation would have required 
airplanes certified in accordance with Sec.  25.1420(a)(1) or (a)(2) to 
consider all appendix O icing conditions. However, the ARAC recommended 
advisory circular material allowed these airplanes to consider less 
than the full appendix O icing conditions. The FAA is not proposing 
that these airplanes must meet the performance and handling qualities 
requirements for all of the icing conditions specified in appendix O. 
Therefore, for pilot compartment view, airspeed indication system, 
angle of attack system and static pressure system,\23\ the agency 
concurs that it would only be necessary to show compliance under the 
applicable conditions in appendix O.
---------------------------------------------------------------------------

    \22\ 14 CFR 25.773, 25.1323, 25.1324, and 25.1325.
    \23\ Ibid.
---------------------------------------------------------------------------

Discussion of Working Group Non-Consensus Issues

    One goal of the ARAC process is to have a working group achieve 
consensus on all of the recommendations. The IPHWG did not unanimously 
agree on the following issues:
    1. Whether it is necessary to flight test in natural SLD icing 
conditions.
    2. Whether airplanes with certain design features should be exempt 
from the recommendation for Sec.  25.1420.

[[Page 37320]]

    3. Whether it is acceptable to certificate an airplane to a portion 
of appendix O, as proposed in the recommendation for Sec.  
25.1420(a)(2).
    4. Whether certain icing related accidents might have been 
prevented if an accident airplane had complied with the recommendations 
in the IPHWG report.
    A detailed discussion of the IPHWG's minority and majority opinions 
on these issues is included in the working group report. A copy of the 
working group report is in the public docket.\24\
---------------------------------------------------------------------------

    \24\ The complete IPHWG working group report is available on the 
Internet at http://regulations.gov. The docket number is FAA-2010-
0636.
---------------------------------------------------------------------------

    The FAA predominantly concurred with the ARAC's recommendations, 
but determined it was necessary to revise the applicability of the 
recommendation for Sec.  25.1420, as discussed previously.

Paperwork Reduction Act

    The Paperwork Reduction Act of 1995 (44 U.S.C. 3507(d)) requires 
that the FAA consider the impact of paperwork and other information 
collection burdens imposed on the public. The information collection 
requirements associated with this NPRM have been previously approved by 
the Office of Management and Budget (OMB) under the provisions of the 
Paperwork Reduction Act of 1995 (44 U.S.C. 3507(d)) and have been 
assigned OMB Control Number 2120-0018.

International Compatibility

    In keeping with U.S. obligations under the Convention on 
International Civil Aviation, it is FAA policy to comply with 
International Civil Aviation Organization (ICAO) Standards and 
Recommended Practices to the maximum extent practicable. The FAA has 
determined that there are no ICAO Standards and Recommended Practices 
that correspond to these proposed regulations.

European Aviation Safety Agency

    The European Aviation Safety Agency (EASA) was established by the 
European Community to develop standards to ensure safety and 
environmental protection, oversee uniform application of those 
standards, and promote them internationally. EASA formally became 
responsible for certification of aircraft, engines, parts, and 
appliances on September 28, 2003. EASA has a project similar to SLD on 
its rulemaking inventory and our intent is to harmonize these 
regulations.

Regulatory Evaluation, Regulatory Flexibility Determination, 
International Trade Analysis, and Unfunded Mandates

    Changes to Federal regulations must undergo several economic 
analyses. First, Executive Order 12866 directs that each Federal agency 
shall propose or adopt a regulation only upon a reasoned determination 
that the benefits of the intended regulation justify its costs. Second, 
the Regulatory Flexibility Act of 1980 (Pub. L. 96-354) requires 
agencies to analyze the economic impact of regulatory changes on small 
entities. Third, the Trade Agreements Act (Pub. L. 96-39) prohibits 
agencies from setting standards that create unnecessary obstacles to 
the foreign commerce of the United States. In developing U.S. 
standards, this Trade Act requires agencies to consider international 
standards and, where appropriate, that they be the basis of U.S. 
standards. Fourth, the Unfunded Mandates Reform Act of 1995 (Pub. L. 
104-4) requires agencies to prepare a written assessment of the costs, 
benefits, and other effects of proposed or final rules that include a 
Federal mandate likely to result in the expenditure by State, local, or 
tribal governments, in the aggregate, or by the private sector, of $100 
million or more annually (adjusted for inflation with base year of 
1995). This portion of the preamble summarizes the FAA's analysis of 
the economic impacts of this proposed rule. We suggest readers seeking 
greater detail read the full regulatory evaluation, a copy of which we 
have placed in the docket for this rulemaking.
    In conducting these analyses, FAA has determined that this proposed 
rule: (1) Has benefits that justify its costs; (2) is not an 
economically ``significant regulatory action'' as defined in section 
3(f) of Executive Order 12866; (3) is ``significant'' as defined in 
DOT's Regulatory Policies and Procedures; (4) would not have a 
significant economic impact on a substantial number of small entities; 
(5) would not create unnecessary obstacles to the foreign commerce of 
the United States; and (6) would not impose an unfunded mandate on 
State, local, or tribal governments, or on the private sector by 
exceeding the threshold identified above. These analyses are summarized 
below.

Total Benefits and Costs of This Rule

    This NPRM would amend the airworthiness standards applicable to 
certain transport category airplanes certified for flight in icing 
conditions and the icing airworthiness standards applicable to certain 
aircraft engines. The affected fleet and categories of benefits and 
costs are customized to the requirements contained in this proposal. 
So, depending on the category and type of airplane, the benefits and 
costs are analyzed over different time periods. It is important for the 
reader to focus on present value benefits and costs. The total 
estimated benefits are $405.6 million ($99.5 million present value). 
The total estimated costs are $71.0 million ($54.0 million present 
value). On an annualized basis, for the time period 2012-2064, the 
benefits are $7.0 million, and the costs are $3.8 million. Therefore, 
the benefits of the proposed rule justify the costs, and the proposed 
rule is cost beneficial.

Persons Potentially Affected by This Rule

     Part 25 airplane manufacturers.
     Engine manufacturers.
     Operators of Affected Equipment.

Assumptions

     Discount rate--7%.
     Costs and benefits are expressed in 2009 dollars and that 
both costs and benefits start to occur in 2011. We conservatively 
assume that all certifications are approved one year after the rule is 
codified (2011), and that production/deliveries begin to occur the 
following year (2012). Airplane deliveries continue to accumulate until 
the airplane is out of production and then begin to retire in the 25th 
year of service. We have customized different fleet types (smaller, 
medium, larger) based upon the actual historical production cycles and 
deliveries. The varying periods are based on all the historical data 
that we have available. The production cycles for smaller airplanes are 
shorter than the production cycles of larger airplanes, thus the 
differing time periods.
     Value of an Averted Fatality--$6.0 million.
     Fuel Cost per gallon--$1.92.

Benefits of This Proposed Rule

    The industry, with the FAA, analyzed the SLD events for part 25 
certified airplanes. We evaluated the events for applicability and 
preventability in context with the requirements contained in this 
proposed rule.
    First, we develop an annual risk of a catastrophic SLD event per 
aircraft and assume a uniform annual likelihood. Next, we multiply the 
total annual affected aircraft by the annual risk per aircraft. Lastly, 
we multiply the total annual risk by the estimated cost of an average 
SLD event. When summed over time, the total estimated benefits are

[[Page 37321]]

$405.6 million ($99.5 million present value).

Costs of This Proposed Rule

    The total estimated costs are $71.0 million ($54.0 million present 
value). We obtained the basis of our cost estimates from the industry. 
The manufacturers used accompanying advisory circulars (AC) describing 
acceptable means for showing compliance. The compliance costs are 
analyzed in context of the part 25 and part 33 certification 
requirements.
    The FAA originally asked ARAC to estimate other operational costs 
beyond the additional hardware and fuel consumption costs. The 
additional hardware costs would be for SLD ice detectors that 
manufacturers would install to be in compliance with the proposed 
requirements. The additional hardware costs would be accompanied by 
additional fuel consumption costs from the accompanying weight changes 
due to the SLD ice detectors. Accordingly, ARAC provided this data to 
the FAA. However, as we neared completion of our cost analysis for 
these requirements, we queried individual operators and they informed 
us that they were already in compliance and there were no additional 
operational costs beyond fuel and hardware.
    As summarized below, the cost categories in the regulatory 
evaluation incorporate both certification and operational costs. We 
analyze each cost category separately. The cost categories in this 
evaluation are the same as those provided by industry to comply with 
the requirements contained in this proposal. For this analysis, the 
estimated costs were:

------------------------------------------------------------------------
                                       Nominal Cost         PV Cost
------------------------------------------------------------------------
Engine Cert Cost..................         $7,936,000         $6,931,610
Engine Capital Cost...............          6,000,000          5,240,632
Total Engine......................         13,936,000         12,172,242
Small Aircraft Certification Cost.         24,999,039         21,835,129
New Large Aircraft Certification            3,154,600          2,755,350
 Cost.............................
Amended Type Certificate Large             10,438,800          9,117,652
 Airplane Certification Cost......
Hardware Costs....................         10,390,000          5,842,024
Fuel Burn All.....................          8,046,676          2,261,941
                                   -------------------------------------
    Total.........................         70,965,115         53,984,338
------------------------------------------------------------------------

Alternatives Considered

    Alternative 1--Make all sizes of aircraft applicable to the 
proposal. Not all the requirements in this proposal extend to larger 
transport category aircraft (those with a maximum takeoff weight 
greater than 60,000 pounds). Under this alternative, the proposed 
design requirements would extend to all transport category aircraft. 
This alternative was rejected because this alternative would add 
significant cost without a commensurate increase in benefits.
    Alternative 2--Limit the scope of applicability to small aircraft. 
Although this alternative would decrease the estimated cost, the FAA 
believes that medium airplanes have the same risk as small airplanes. 
The FAA does not want a significant proportion of the future fleet to 
be disproportionately at risk.

Regulatory Flexibility Determination

    The Regulatory Flexibility Act of 1980 (Pub. L. 96-354) (RFA) 
establishes ``as a principle of regulatory issuance that agencies shall 
endeavor, consistent with the objectives of the rule and of applicable 
statutes, to fit regulatory and informational requirements to the scale 
of the businesses, organizations, and governmental jurisdictions 
subject to regulation. To achieve this principle, agencies are required 
to solicit and consider flexible regulatory proposals and to explain 
the rationale for their actions to assure that such proposals are given 
serious consideration.'' The RFA covers a wide-range of small entities, 
including small businesses, not-for-profit organizations, and small 
governmental jurisdictions.
    Agencies must perform a review to determine whether a rule will 
have a significant economic impact on a substantial number of small 
entities. If the agency determines that it will, the agency must 
prepare a regulatory flexibility analysis as described in the RFA.
    However, if an agency determines that a rule is not expected to 
have a significant economic impact on a substantial number of small 
entities, section 605(b) of the RFA provides that the head of the 
agency may so certify and a regulatory flexibility analysis is not 
required. The certification must include a statement providing the 
factual basis for this determination, and the reasoning should be 
clear. Based on the analysis presented below, we determined there would 
not be a significant impact on a substantial number of small entities.

Airplane and Engine Manufacturers

    Aircraft and Engine Manufacturers would be affected by the 
requirements contained in this proposal.
    For aircraft manufacturers, we use the size standards from the 
Small Business Administration for Air Transportation and Aircraft 
Manufacturing specifying companies having less than 1,500 employees as 
small entities. The current United States part 25 airplane 
manufacturers include: Boeing, Cessna Aircraft, Gulfstream Aerospace, 
Learjet (owned by Bombardier), Lockheed Martin, McDonnell Douglas (a 
wholly-owned subsidiary of The Boeing Company), Raytheon Aircraft, and 
Sabreliner Corporation. Because all U.S. transport-aircraft category 
manufacturers have more than 1,500 employees, none are considered small 
entities.
    United States aircraft engine manufacturers include: General 
Electric, CFM International, Pratt & Whitney, International Aero 
Engines, Rolls-Royce Corporation, Honeywell, and Williams 
International. All but one exceeds the Small Business Administration 
small-entity criteria for aircraft engine manufacturers. Williams 
International is the only one of these manufacturers that is a U.S. 
small business. One small entity is not a substantial number.

Operators

    In addition to the certification cost incurred by manufacturers, 
operators would incur fuel costs due to the estimated additional impact 
of weight changes from equipment on affected airplanes. On average, an 
affected airplane would incur additional fuel costs of roughly $525 per 
year.
    Because this proposed rule would apply to airplanes that have yet 
to be designed, there would be no immediate cost to small entities. 
However, as of 2007, there are at least 54 small entity operators with 
1,500 or fewer employees who would qualify as small entities.

[[Page 37322]]

    According to the ``Airliner Price Guide,'' the average cost of a 
new aircraft that would incur such expenses is approximately $17 
million. The corresponding 3-year average total aircraft operating 
expenses on an affected per airplane basis was $758,000. The estimated 
additional cost of $525 would add only 0.07% to the total annual 
operating expenses. We do not consider this a significant economic 
impact.
    Because this proposed rule would not have a significant economic 
impact on a substantial number of airplane manufacturers, engine 
manufacturers or operators, the FAA certifies that this proposed rule 
would not have a significant economic impact on a substantial number of 
small entities. The FAA solicits comments regarding this determination.

International Trade Analysis

    The Trade Agreements Act of 1979 (Pub. L. 96-39), as amended by the 
Uruguay Round Agreements Act (Pub. L. 103-465), prohibits Federal 
agencies from establishing standards or engaging in related activities 
that create unnecessary obstacles to the foreign commerce of the United 
States. Pursuant to these Acts, the establishment of standards is not 
considered an unnecessary obstacle to the foreign commerce of the 
United States, so long as the standard has a legitimate domestic 
objective, such the protection of safety, and does not operate in a 
manner that excludes imports that meet this objective. The statute also 
requires consideration of international standards and, where 
appropriate, that they be the basis for U.S. standards.
    The FAA has assessed the potential effect of this proposed rule and 
determined that it would impose the same costs on domestic and 
international entities and thus has a neutral trade impact.

Unfunded Mandates Assessment

    Title II of the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-
4) requires each Federal agency to prepare a written statement 
assessing the effects of any Federal mandate in a proposed or final 
agency rule that may result in an expenditure of $100 million or more 
(in 1995 dollars) in any one year by State, local, and tribal 
governments, in the aggregate, or by the private sector; such a mandate 
is deemed to be a ``significant regulatory action.'' The FAA currently 
uses an inflation-adjusted value of $143.1 million in lieu of $100 
million. This proposed rule does not contain such a mandate; therefore, 
the requirements of Title II do not apply.

Executive Order 13132, Federalism

    The FAA has analyzed this proposed rule under the principles and 
criteria of Executive Order 13132, Federalism. We determined that this 
action would not have a substantial direct effect on the States, on the 
relationship between the national Government and the States, or on the 
distribution of power and responsibilities among the various levels of 
government, and, therefore, would not have federalism implications.

Regulations Affecting Intrastate Aviation in Alaska

    Section 1205 of the FAA Reauthorization Act of 1996 (110 Stat. 
3213) requires the Administrator, when modifying regulations in Title 
14 of the CFR in a manner affecting intrastate aviation in Alaska, to 
consider the extent to which Alaska is not served by transportation 
modes other than aviation, and to establish appropriate regulatory 
distinctions. Because this proposed rule would apply to the 
certification of future designs of transport category airplanes and 
their subsequent operation, it could, if adopted, affect intrastate 
aviation in Alaska. The FAA, therefore, specifically requests comments 
on whether there is justification for applying the proposed rule 
differently in intrastate operations in Alaska.

Environmental Analysis

    FAA Order 1050.1E identifies FAA actions that are categorically 
excluded from preparation of an environmental assessment or 
environmental impact statement under the National Environmental Policy 
Act in the absence of extraordinary circumstances. The FAA has 
determined this proposed rulemaking action qualifies for the 
categorical exclusion identified in paragraph 4(j) and involves no 
extraordinary circumstances.

Regulations That Significantly Affect Energy Supply, Distribution, or 
Use

    The FAA has analyzed this NPRM under Executive Order 13211, Actions 
Concerning Regulations that Significantly Affect Energy Supply, 
Distribution, or Use (May 18, 2001). We have determined that it is not 
a ``significant energy action'' under the executive order because, 
while it is a ``significant regulatory action,'' it is not likely to 
have a significant adverse effect on the supply, distribution, or use 
of energy.

Plain English

    Executive Order 12866 (58 FR 51735, Oct. 4, 1993) requires each 
agency to write regulations that are simple and easy to understand. We 
invite your comments on how to make these proposed regulations easier 
to understand, including answers to questions such as the following:
     Are the requirements in the proposed regulations clearly 
stated?
     Do the proposed regulations contain unnecessary technical 
language or jargon that interferes with their clarity?
     Would the regulations be easier to understand if they were 
divided into more (but shorter) sections?
     Is the description in the preamble helpful in 
understanding the proposed regulations?
    Please send your comments to the address specified in the ADDRESSES 
section of this preamble.

Additional Information

Comments Invited

    The FAA invites interested persons to participate in this 
rulemaking by submitting written comments, data, or views. We also 
invite comments relating to the economic, environmental, energy, or 
federalism impacts that might result from adopting the proposals in 
this document. The most helpful comments reference a specific portion 
of the proposal, explain the reason for any recommended change, and 
include supporting data. To ensure the docket does not contain 
duplicate comments, please send only one copy of written comments, or 
if you are filing comments electronically, please submit your comments 
only one time.
    We will file in the docket all comments we receive, as well as a 
report summarizing each substantive public contact with FAA personnel 
concerning this proposed rulemaking. Before acting on this proposal, we 
will consider all comments we receive on or before the closing date for 
comments. We will consider comments filed after the comment period has 
closed if it is possible to do so without incurring expense or delay. 
We may change this proposal in light of the comments we receive.

Proprietary or Confidential Business Information

    Do not file in the docket information that you consider to be 
proprietary or confidential business information. Send or deliver this 
information directly to the person identified in the FOR FURTHER 
INFORMATION CONTACT section of this document. You must mark the 
information that you consider

[[Page 37323]]

proprietary or confidential. If you send the information on a disk or 
CD ROM, mark the outside of the disk or CD ROM and also identify 
electronically within the disk or CD ROM the specific information that 
is proprietary or confidential.
    Under 14 CFR 11.35(b), when we are aware of proprietary information 
filed with a comment, we do not place it in the docket. We hold it in a 
separate file to which the public does not have access, and we place a 
note in the docket that we have received it. If we receive a request to 
examine or copy this information, we treat it as any other request 
under the Freedom of Information Act (5 U.S.C. 552). We process such a 
request under the DOT procedures found in 49 CFR part 7.

Availability of Rulemaking Documents

    You can get an electronic copy of rulemaking documents using the 
Internet by--
    1. Searching the Federal eRulemaking Portal (http://
www.regulations.gov);
    2. Visiting the FAA's Regulations and Policies web page at http://
www.faa.gov/regulations_policies/; or
    3. Accessing the Government Printing Office's web page at http://
www.gpoaccess.gov/fr/index.html.
    You can also get a copy by sending a request to the Federal 
Aviation Administration, Office of Rulemaking, ARM-1, 800 Independence 
Avenue, SW., Washington, DC 20591, or by calling (202) 267-9680. Make 
sure to identify the docket number or notice number of this rulemaking.
    You may access all documents the FAA considered in developing this 
proposed rule, including economic analyses and technical reports, from 
the internet through the Federal eRulemaking Portal referenced in 
paragraph (1).
    The following appendix will not appear in the Code of Federal 
Regulations.

Appendix 1 to the Preamble--Definition of Terms Used in This Preamble

    For the purposes of this preamble, the following definitions are 
applicable. These definitions of terms are intended for use only with 
this preamble:
    a. Appendix C Icing Conditions: The environmental conditions 
defined in appendix C of 14 CFR part 25.
    b. Appendix O Icing Conditions: The environmental conditions 
defined in appendix O of 14 CFR part 25.
    c. Drizzle Drop: A drop of water measuring 100 [micro]m to 500 
[micro]m (0.1-0.5 mm) in diameter.
    d. Freezing Drizzle (FZDZ): Supercooled drizzle drops that remain 
in liquid form and freeze upon contact with objects colder than 
0[deg]C.
    e. Freezing Rain (FZRA): Supercooled rain drops that remain in 
liquid form and freeze upon contact with objects colder than 0[deg]C.
    f. Icing Conditions: The presence of atmospheric moisture and 
temperature conducive to airplane icing.
    g. Icing Conditions Detector: A device that detects the presence of 
atmospheric moisture and temperature conducive to airplane icing.
    h. Irreversible Flight Controls: Flight controls in the normal 
operating configuration that have loads generated at the control 
surfaces of an airplane which are reacted against the actuator and its 
mounting and cannot be transmitted directly back to the flight deck 
controls. This term refers to flight controls in which all of the force 
necessary to move the pitch, roll, or yaw control surfaces is provided 
by hydraulic or electric actuators, the motion of which is controlled 
by signals from the flight deck controls.
    i. Liquid Water Content (LWC): The total mass of water contained in 
liquid drops within a unit volume or mass of air, usually given in 
units of grams of water per cubic meter (g/m\3\).
    j. Mean Effective Diameter (MED): The calculated drop diameter that 
divides the total liquid water content present in the drop size 
distribution in half. Half the water volume will be in larger drops and 
half the volume in smaller drops. This value is calculated, as opposed 
to being arrived at by measuring actual drop size. The MED is based on 
an assumed Langmuir drop size distribution. The fact that it is a 
calculated measurement is how it differs from median volume diameter, 
which is based on actual drop size.
    k. Median Volume Diameter (MVD): The drop diameter that divides the 
total liquid water content present in the drop distribution in half. 
Half the water volume will be in larger drops and half the volume in 
smaller drops. The value is obtained by actual drop size measurements.
    l. Mixed Phase Icing Environment: A combination of supercooled 
liquid and ice crystals.
    m. Rain Drop: A drop of water greater than 500 [micro]m (0.5 mm) in 
diameter.
    n. Reversible Flight Controls: Flight controls in the normal 
operating configuration that have force or motion originating at the 
airplane's control surface (for example, through aerodynamic loads, 
static imbalance, or trim tab inputs) that is transmitted back to 
flight deck controls. This term refers to flight deck controls 
connected to the pitch, roll, or yaw control surfaces by direct 
mechanical linkages, cables, or push-pull rods in such a way that pilot 
effort produces motion or force about the hinge line.
    (1) Aerodynamically boosted flight controls: Reversible flight 
control systems that employ a movable tab on the trailing edge of the 
main control surface linked to the pilot's controls or to the structure 
in such a way as to produce aerodynamic forces that move, or help to 
move, the surface. Among the various forms are flying tabs, geared or 
servo tabs, and spring tabs.
    (2) Power-assisted flight controls: Reversible flight control 
systems in which some means is provided, usually a hydraulic actuator, 
to apply force to a control surface in addition to that supplied by the 
pilot to enable large surface deflections to be obtained at high 
speeds.
    o. Supercooled Large Drops (SLD): Supercooled liquid water that 
includes freezing rain or freezing drizzle.
    p. Supercooled Water: Liquid water at a temperature below the 
freezing point of 0[deg]C.

List of Subjects

14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements, Safety, Transportation.

14 CFR Part 33

    Aircraft, Aviation safety.

The Proposed Amendment

    In consideration of the foregoing, the Federal Aviation 
Administration proposes to amend Chapter I of Title 14, Code of Federal 
Regulations parts 25 and 33 as follows:

PART 25--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES

    1. The authority citation for part 25 continues to read as follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702 and 44704.

    2. Amend Sec.  25.21 by revising paragraphs (g)(1) and (g)(2) and 
adding paragraphs (g)(3) and (g)(4) to read as follows:


Sec.  25.21  Proof of compliance.

* * * * *
    (g) * * *
    (1) Paragraphs (g)(3) and (g)(4) of this section apply only to 
airplanes with one or both of the following attributes:
    (i) Takeoff maximum gross weight is less than 60,000 lbs; or
    (ii) The airplane is equipped with reversible flight controls.

[[Page 37324]]

    (2) Each requirement of this subpart, except Sec. Sec.  25.121(a), 
25.123(c), 25.143(b)(1) and (2), 25.149, 25.201(c)(2), 25.207(c) and 
(d), 25.239, and 25.251(b) through (e), must be met in the icing 
conditions specified in appendix C of this part. Compliance must be 
shown using the ice accretions defined in part II of appendix C of this 
part, assuming normal operation of the airplane and its ice protection 
system in accordance with the operating limitations and operating 
procedures established by the applicant and provided in the Airplane 
Flight Manual.
    (3) If the applicant does not seek certification for flight in all 
icing conditions defined in appendix O of this part, each requirement 
of this subpart, except Sec. Sec.  25.105, 25.107, 25.109, 25.111, 
25.113, 25.115, 25.121, 25.123, 25.143(b)(1), (b)(2), and (c)(1), 
25.149, 25.201(c)(2), 25.207(c) and (d), 25.239, and 25.251(b) through 
(e), must be met in the appendix O icing conditions for which 
certification is not sought in order to allow a safe exit from those 
conditions. Compliance must be shown using the ice accretions defined 
in part II, paragraphs (b) and (d) of appendix O of this part, assuming 
normal operation of the airplane and its ice protection system in 
accordance with the operating limitations and operating procedures 
established by the applicant and provided in the Airplane Flight 
Manual.
    (4) If the applicant seeks certification for flight in any portion 
of the icing conditions of appendix O of this part, each requirement of 
this subpart, except Sec. Sec.  25.123(c), 25.143(b)(1) and (2), 
25.149, 25.201(c)(2), 25.207(c) and (d), 25.239, and 25.251(b) through 
(e), must be met in the appendix O icing conditions for which 
certification is sought. Compliance must be shown using the ice 
accretions defined in part II, paragraphs (c) and (d) of appendix O of 
this part, assuming normal operation of the airplane and its ice 
protection system in accordance with the operating limitations and 
operating procedures established by the applicant and provided in the 
Airplane Flight Manual.
    3. Amend Sec.  25.105 by revising paragraph (a)(2) introductory 
text to read as follows:


Sec.  25.105  Takeoff.

    (a) * * *
    (2) In icing conditions, if in the configuration used to show 
compliance with Sec.  25.121(b), and with the most critical of the 
takeoff ice accretion(s) defined in appendices C and O of this part, as 
applicable, in accordance with Sec.  25.21(g):
* * * * *
    4. Amend Sec.  25.111 by revising paragraphs (c)(5)(i) and 
(c)(5)(ii) to read as follows:


Sec.  25.111  Takeoff path.

* * * * *
    (c) * * *
    (5) * * *
    (i) With the most critical of the takeoff ice accretion(s) defined 
in appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g), from a height of 35 feet above the takeoff surface up 
to the point where the airplane is 400 feet above the takeoff surface; 
and
    (ii) With the most critical of the final takeoff ice accretion(s) 
defined in appendices C and O of this part, as applicable, in 
accordance with Sec.  25.21(g), from the point where the airplane is 
400 feet above the takeoff surface to the end of the takeoff path.
* * * * *
    5. Amend Sec.  25.119 by revising paragraph (b) to read as follows:


Sec.  25.119  Landing climb: All-engines-operating.

* * * * *
    (b) In icing conditions with the most critical of the landing ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), and with a climb speed of 
VREF determined in accordance with Sec.  25.125(b)(2)(ii).
    6. Amend Sec.  25.121 by revising paragraphs (b)(2)(ii) 
introductory text, (c)(2)(ii) introductory text, and (d)(2)(ii) to read 
as follows:


Sec.  25.121  Climb: One-engine-inoperative.

* * * * *
    (b) * * *
    (2) * * *
    (ii) In icing conditions with the most critical of the takeoff ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), if in the configuration used to show 
compliance with Sec.  25.121(b) with this takeoff ice accretion:
* * * * *
    (c) * * *
    (2) * * *
    (ii) In icing conditions with the most critical of the final 
takeoff ice accretion(s) defined in appendices C and O of this part, as 
applicable, in accordance with Sec.  25.21(g), if in the configuration 
used to show compliance with Sec.  25.121(b) with the takeoff ice 
accretion used to show compliance with Sec.  25.111(c)(5)(i):
* * * * *
    (d) * * *
    (2) * * *
    (ii) In icing conditions with the most critical of the approach ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g). The climb speed selected for non-
icing conditions may be used if the climb speed for icing conditions, 
computed in accordance with paragraph (d)(1)(iii) of this section, does 
not exceed that for non-icing conditions by more than the greater of 3 
knots CAS or 3 percent.
    7. Amend Sec.  25.123 by revising paragraph (b)(2) introductory 
text to read as follows:


Sec.  25.123  En-route flight paths.

* * * * *
    (b) * * *
    (2) In icing conditions with the most critical of the en route ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), if:
* * * * *
    8. Amend Sec.  25.125 by revising paragraphs (a)(2), (b)(2)(ii)(B), 
and (b)(2)(ii)(C) to read as follows:


Sec.  25.125  Landing.

    (a) * * *
    (2) In icing conditions with the most critical of the landing ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), if VREF for icing 
conditions exceeds VREF for non-icing conditions by more 
than 5 knots CAS at the maximum landing weight.
    (b) * * *
    (2) * * *
    (ii) * * *
    (B) 1.23 VSR0 with the most critical of the landing ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), if that speed exceeds 
VREF selected for non-icing conditions by more than 5 knots 
CAS; and
    (C) A speed that provides the maneuvering capability specified in 
Sec.  25.143(h) with the most critical of the landing ice accretion(s) 
defined in appendices C and O of this part, as applicable, in 
accordance with Sec.  25.21(g).
* * * * *
    9. Amend Sec.  25.143 by revising paragraphs (c) introductory text, 
(i)(1), and (j) introductory text to read as follows:


Sec.  25.143  Controllability and maneuverability--General.

* * * * *
    (c) The airplane must be shown to be safely controllable and 
maneuverable with the most critical of the ice accretion(s) appropriate 
to the phase of flight as defined in appendices C and O of this part, 
as applicable, in accordance

[[Page 37325]]

with Sec.  25.21(g), and with the critical engine inoperative and its 
propeller (if applicable) in the minimum drag position:
* * * * *
    (i) * * *
    (1) Controllability must be demonstrated with the most critical of 
the ice accretion(s) for the particular flight phase as defined in 
appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g);
* * * * *
    (j) For flight in icing conditions before the ice protection system 
has been activated and is performing its intended function, it must be 
demonstrated in flight with the most critical of the ice accretion(s) 
defined in appendix C, part II, paragraph (e) of this part and appendix 
O, part II, paragraph (d) of this part, as applicable, in accordance 
with Sec.  25.21(g), that:
* * * * *
    10. Amend Sec.  25.207 by revising paragraphs (b), (e)(1) through 
(5), and (h) introductory text to read as follows:


Sec.  25.207  Stall warning.

* * * * *
    (b) The warning must be furnished either through the inherent 
aerodynamic qualities of the airplane or by a device that will give 
clearly distinguishable indications under expected conditions of 
flight. However, a visual stall warning device that requires the 
attention of the crew within the cockpit is not acceptable by itself. 
If a warning device is used, it must provide a warning in each of the 
airplane configurations prescribed in paragraph (a) of this section at 
the speed prescribed in paragraphs (c) and (d) of this section. Except 
for the stall warning prescribed in paragraph (h)(3)(ii) of this 
section, the stall warning for flight in icing conditions must be 
provided by the same means as the stall warning for flight in non-icing 
conditions.
* * * * *
    (e) * * *
    (1) The most critical of the takeoff ice and final takeoff ice 
accretions defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g), for each configuration used in the 
takeoff phase of flight;
    (2) The most critical of the en route ice accretion(s) defined in 
appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g), for the en route configuration;
    (3) The most critical of the holding ice accretion(s) defined in 
appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g), for the holding configuration(s);
    (4) The most critical of the approach ice accretion(s) defined in 
appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g), for the approach configuration(s); and
    (5) The most critical of the landing ice accretion(s) defined in 
appendices C and O of this part, as applicable, in accordance with 
Sec.  25.21(g), for the landing and go-around configuration(s).
* * * * *
    (h) The following stall warning margin is required for flight in 
icing conditions before the ice protection system has been activated 
and is performing its intended function. Compliance must be shown using 
the most critical of the ice accretion(s) defined in appendix C, part 
II, paragraph (e) of this part and appendix O, part II, paragraph (d) 
of this part, as applicable, in accordance with Sec.  25.21(g). The 
stall warning margin in straight and turning flight must be sufficient 
to allow the pilot to prevent stalling without encountering any adverse 
flight characteristics when:
* * * * *
    11. Amend Sec.  25.237 by revising paragraph (a)(3)(ii) to read as 
follows:


Sec.  25.237  Wind velocities.

    (a) * * *
    (3) * * *
    (ii) Icing conditions with the most critical of the landing ice 
accretion(s) defined in appendices C and O of this part, as applicable, 
in accordance with Sec.  25.21(g).
* * * * *
    12. Amend Sec.  25.253 by revising paragraph (c) introductory text 
to read as follows:


Sec.  25.253  High-speed characteristics.

* * * * *
    (c) Maximum speed for stability characteristics in icing 
conditions. The maximum speed for stability characteristics with the 
most critical of the ice accretions defined in appendices C and O of 
this part, as applicable, in accordance with Sec.  25.21(g), at which 
the requirements of Sec. Sec.  25.143(g), 25.147(e), 25.175(b)(1), 
25.177 and 25.181 must be met, is the lower of:
* * * * *
    13. Amend Sec.  25.773 by revising paragraph (b)(1)(ii) to read as 
follows:


Sec.  25.773  Pilot compartment view.

* * * * *
    (b) * * *
    (1) * * *
    (ii) The icing conditions specified in appendix C and the following 
icing conditions specified in appendix O of this part, if certification 
for flight in icing conditions is sought:
    (A) For airplanes certificated in accordance with Sec.  
25.1420(a)(1), the icing conditions that the airplane is certified to 
safely exit following detection.
    (B) For airplanes certificated in accordance with Sec.  
25.1420(a)(2), the icing conditions that the airplane is certified to 
safely operate in and the icing conditions that the airplane is 
certified to safely exit following detection.
    (C) For airplanes certificated in accordance with Sec.  
25.1420(a)(3) and for airplanes not subject to Sec.  25.1420, all icing 
conditions.
* * * * *
    14. Amend Sec.  25.903 by adding paragraph (a)(3) to read as 
follows:


Sec.  25.903  Engines.

    (a) * * *
    (3) Each turbine engine must comply with one of the following 
paragraphs:
    (i) Section 33.68 of this chapter in effect on [effective date of 
final rule], or as subsequently amended; or
    (ii) Section 33.68 of this chapter in effect on February 23, 1984, 
or as subsequently amended before [effective date of final rule], 
unless that engine's ice accumulation service history has resulted in 
an unsafe condition; or
    (iii) Section 33.68 of this chapter in effect on October 1, 1974, 
or as subsequently amended prior to February 23, 1984, unless that 
engine's ice accumulation service history has resulted in an unsafe 
condition; or
    (iv) Be shown to have an ice accumulation service history in 
similar installation locations which has not resulted in any unsafe 
conditions.
* * * * *
    15. Amend Sec.  25.929 by revising paragraph (a) to read as 
follows:


Sec.  25.929  Propeller deicing.

    (a) If certification for flight in icing is sought there must be a 
means to prevent or remove hazardous ice accumulations that could form 
in the icing conditions defined in appendices C and O of this part on 
propellers or on accessories where ice accumulation would jeopardize 
engine performance.
* * * * *
    16. Amend Sec.  25.1093 by revising paragraph (b) to read as 
follows:


Sec.  25.1093  Induction system icing protection.

* * * * *
    (b) Turbine engines. Each engine, with all icing protection systems 
operating, must:
    (1) Operate throughout its flight power range, including the 
minimum descent idling speeds, in the icing

[[Page 37326]]

conditions defined in appendices C and O of this part, and appendix D 
of part 33 of this chapter, and in falling and blowing snow within the 
limitations established for the airplane for such operation, without 
the accumulation of ice on the engine, inlet system components or 
airframe components that would do any of the following:
    (i) Adversely affect installed engine operation or cause a 
sustained loss of power or thrust; or an unacceptable increase in gas 
path operating temperature; or an airframe/engine incompatibility; or
    (ii) Result in unacceptable temporary power loss or engine damage; 
or
    (iii) Cause a stall, surge, or flameout or loss of engine 
controllability (for example, rollback).
    (2) Idle for a minimum of 30 minutes on the ground in the following 
icing conditions shown in Table 1, unless replaced by similar test 
conditions that are more critical. These conditions must be 
demonstrated with the available air bleed for icing protection at its 
critical condition, without adverse effect, followed by an acceleration 
to takeoff power or thrust. During the idle operation the engine may be 
run up periodically to a moderate power or thrust setting in a manner 
acceptable to the Administrator. The applicant must document the engine 
run-up procedure (including the maximum time interval between run-ups 
from idle, run-up power setting, and duration at power) and associated 
minimum ambient temperature demonstrated for the maximum time interval, 
and these conditions must be used in establishing the airplane 
operating limitations in accordance with Sec.  25.1521.

                                                       Table 1--Icing Conditions for Ground Tests
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                            Water concentration      Mean effective
             Condition              Total air temperature        (minimum)          particle diameter                     Demonstration
--------------------------------------------------------------------------------------------------------------------------------------------------------
(i) Rime ice condition............  0 to 15 [deg]F (-18    Liquid--0.3 g/m\3\...  15-25 microns.......  By test, analysis or combination of the two.
                                     to -9 [deg]C).
(ii) Glaze ice condition..........  20 to 30 [deg]F (-7    Liquid--0.3 g/m\3\...  15-25 microns.......  By test, analysis or combination of the two.
                                     to -1 [deg]C).
(iii) Large drop condition........  15 to 30 [deg]F (-9    Liquid--0.3 g/m\3\...  100 microns           By test, analysis or combination of the two.
                                     to -1 [deg]C).                                (minimum).
--------------------------------------------------------------------------------------------------------------------------------------------------------

* * * * *
    17. Amend Sec.  25.1323 by revising paragraph (i) to read as 
follows:


Sec.  25.1323  Airspeed indicating system.

* * * * *
    (i) Each system must have a heated pitot tube or an equivalent 
means of preventing malfunction in mixed phase and ice crystal 
conditions as defined in Table 1 of this section, the icing conditions 
defined in appendix C of this part, and the following icing conditions 
specified in appendix O of this part:
    (1) For airplanes certificated in accordance with Sec.  
25.1420(a)(1), the icing conditions that the airplane is certified to 
safely exit following detection.
    (2) For airplanes certificated in accordance with Sec.  
25.1420(a)(2), the icing conditions that the airplane is certified to 
safely operate in and the icing conditions that the airplane is 
certified to safely exit following detection.
    (3) For airplanes certificated in accordance with Sec.  
25.1420(a)(3) and for airplanes not subject to Sec.  25.1420, all icing 
conditions.

                                             Table 1--Icing Conditions for Airspeed Indicating System Tests


--------------------------------------------------------------------------------------------------------------------------------------------------------
Air temperature                               Altitude range               Ice water      Liquid     Horizontal extent    Ice median mass...      Liquid
                                                                             content       water                          dimension.........   water MVD
                                                                                         content
--------------------------------------------------------------------------------------------------------------------------------------------------------
([deg]C)                          (ft)..............  (m)...............      g/m\3\      g/m\3\        (km)   (n miles)  ([mu]m)...........     ([mu]m)
--------------------------------------------------------------------------------------------------------------------------------------------------------
0 to -20........................  10,000 to 30,000..  3,000 to 9,000....           4           1           5           3  100 to 1,000......          20
                                                                                   1           1         100          50
                                                                                 0.5         0.5         500         300
-20 to -40......................  15,000 to 40,000..  4,500 to 12,000...           5           0           5           3
                                                                                   2           0          20          10
                                                                                   1           0         100          50
                                                                                 0.5           0         500         300
--------------------------------------------------------------------------------------------------------------------------------------------------------

* * * * *
    18. Add Sec.  25.1324 to read as follows:


Sec.  25.1324  Angle of attack system.

    Each angle of attack system sensor must be heated or have an 
equivalent means of preventing malfunction in the mixed phase and ice 
crystal conditions as defined in Sec.  25.1323, the icing conditions 
defined in appendix C of this part, and the following icing conditions 
specified in appendix O of this part:
    (a) For airplanes certificated in accordance with Sec.  
25.1420(a)(1), the icing conditions that the airplane is certified to 
safely exit following detection.
    (b) For airplanes certificated in accordance with Sec.  
25.1420(a)(2), the icing conditions that the airplane is certified to 
safely operate in and the icing conditions that the airplane is 
certified to safely exit following detection.
    (c) For airplanes certificated in accordance with Sec.  
25.1420(a)(3) and for airplanes not subject to Sec.  25.1420, all icing 
conditions.
    19. Amend Sec.  25.1325 by revising paragraph (b) to read as 
follows:


Sec.  25.1325  Static pressure systems.

* * * * *
    (b) Each static port must be designed and located so that:
    (1) The static pressure system performance is least affected by 
airflow variation, or by moisture or other foreign matter, and
    (2) The correlation between air pressure in the static pressure 
system and true ambient atmospheric static pressure is not changed when 
the

[[Page 37327]]

airplane is exposed to the icing conditions defined in appendix C of 
this part, and the following icing conditions specified in appendix O 
of this part:
    (i) For airplanes certificated in accordance with Sec.  
25.1420(a)(1), the icing conditions that the airplane is certified to 
safely exit following detection.
    (ii) For airplanes certificated in accordance with Sec.  
25.1420(a)(2), the icing conditions that the airplane is certified to 
safely operate in and the icing conditions that the airplane is 
certified to safely exit following detection.
    (iii) For airplanes certificated in accordance with Sec.  
25.1420(a)(3) and for airplanes not subject to Sec.  25.1420, all icing 
conditions.
* * * * *
    20. Add Sec.  25.1420 to read as follows:


Sec.  25.1420  Supercooled large drop icing conditions.

    (a) If certification for flight in icing conditions is sought, in 
addition to the requirements of Sec.  25.1419, an airplane with a 
maximum takeoff weight less than 60,000 pounds or with reversible 
flight controls must be capable of operating in accordance with 
paragraphs (a)(1), (2), or (3), of this section.
    (1) Operating safely after encountering the icing conditions 
defined in appendix O of this part:
    (i) There must be a means provided to detect that the airplane is 
operating in appendix O icing conditions; and
    (ii) Following detection of appendix O icing conditions, the 
airplane must be capable of operating safely while exiting all icing 
conditions.
    (2) Operating safely in a portion of the icing conditions defined 
in appendix O of this part as selected by the applicant.
    (i) There must be a means provided to detect that the airplane is 
operating in conditions that exceed the selected portion of appendix O 
icing conditions; and
    (ii) Following detection, the airplane must be capable of operating 
safely while exiting all icing conditions.
    (3) Operating safely in the icing conditions defined in appendix O 
of this part.
    (b) To establish that the airplane can operate safely as required 
in paragraph (a) of this section, an analysis must be performed to 
establish that the ice protection for the various components of the 
airplane is adequate, taking into account the various airplane 
operational configurations. To verify the analysis, one, or more as 
found necessary, of the following methods must be used:
    (1) Laboratory dry air or simulated icing tests, or a combination 
of both, of the components or models of the components.
    (2) Laboratory dry air or simulated icing tests, or a combination 
of both, of models of the airplane.
    (3) Flight tests of the airplane or its components in simulated 
icing conditions, measured as necessary to support the analysis.
    (4) Flight tests of the airplane with simulated ice shapes.
    (5) Flight tests of the airplane in natural icing conditions, 
measured as necessary to support the analysis.
    (c) For an airplane certified in accordance with paragraph (a)(2) 
or (a)(3) of this section, the requirements of Sec.  25.1419 (e), (f), 
(g), and (h) must be met for the icing conditions defined in appendix O 
of this part in which the airplane is certified to operate.
    21. Amend Sec.  25.1521 by redesignating paragraph (c)(3) as (c)(4) 
and revising it, and by adding new paragraph (c)(3) to read as follows:


Sec.  25.1521  Powerplant limitations.

* * * * *
    (c) * * *
    (3) Maximum time interval between engine run-ups from idle, run-up 
power setting, duration at power, and the associated minimum ambient 
temperature demonstrated for the maximum time interval, for ground 
operation in icing conditions, as defined in Sec.  25.1093(b)(2).
    (4) Any other parameter for which a limitation has been established 
as part of the engine type certificate except that a limitation need 
not be established for a parameter that cannot be exceeded during 
normal operation due to the design of the installation or to another 
established limitation.
* * * * *
    22. Amend Sec.  25.1533 by adding paragraph (c) to read as follows:


Sec.  25.1533  Additional operating limitations.

* * * * *
    (c) For airplanes certified in accordance with Sec.  25.1420(a)(1) 
or (a)(2), an operating limitation must be established to require 
exiting all icing conditions if icing conditions defined in appendix O 
of this part are encountered for which the airplane has not been 
certified to safely operate.
    23. Amend part 25 by adding Appendix O to part 25 to read as 
follows:

Appendix O to Part 25--Supercooled Large Drop Icing Conditions

    Appendix O consists of two parts. Part I defines appendix O as a 
description of supercooled large drop (SLD) icing conditions in 
which the drop median volume diameter (MVD) is less than or greater 
than 40 [micro]m, the maximum mean effective drop diameter (MED) of 
appendix C continuous maximum (stratiform clouds) icing conditions. 
For appendix O, SLD icing conditions consist of freezing drizzle and 
freezing rain occurring in and/or below stratiform clouds. Part II 
defines ice accretions used to show compliance with part 25, subpart 
B, airplane performance and handling qualities requirements.

Part I--Meteorology

    Appendix O icing conditions are defined by the parameters of 
altitude, vertical and horizontal extent, temperature, liquid water 
content, and water mass distribution as a function of drop diameter 
distribution.
    (a) Freezing Drizzle (Conditions with spectra maximum drop 
diameters from 100 [micro]m to 500 [micro]m):
    (1) Pressure altitude range: 0 to 22,000 feet MSL.
    (2) Maximum vertical extent: 12,000 feet.
    (3) Horizontal extent: standard distance of 17.4 nautical miles.
    (4) Total liquid water content.

    Note: Liquid water content (LWC) in grams per cubic meter (g/
m\3\) based on horizontal extent standard distance of 17.4 nautical 
miles.

BILLING CODE 4910-13-P

[[Page 37328]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.051

    (5) Drop diameter distribution:

[[Page 37329]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.052

    (6) Altitude and temperature envelope:

[[Page 37330]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.053

    (b) Freezing Rain (Conditions with spectra maximum drop 
diameters greater than 500 [micro]m):
    (1) Pressure altitude range: 0 to 12,000 ft MSL.
    (2) Maximum vertical extent: 7,000 ft.
    (3) Horizontal extent: standard distance of 17.4 nautical miles.
    (4) Total liquid water content.

    Note:  LWC in grams per cubic meter (g/m\3\) based on horizontal 
extent standard distance of 17.4 nautical miles.


[[Page 37331]]


[GRAPHIC] [TIFF OMITTED] TP29JN10.054

    (5) Drop Diameter Distribution

[[Page 37332]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.055

    (6) Altitude and temperature envelope:

[[Page 37333]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.056

    (c) Horizontal extent.
    The liquid water content for freezing drizzle and freezing rain 
conditions for horizontal extents other than the standard 17.4 
nautical miles can be determined by the value of the liquid water 
content determined from Figure 1 or Figure 4, multiplied by the 
factor provided in Figure 7.

[[Page 37334]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.057

Part II--Airframe Ice Accretions for Showing Compliance With Subpart B

    (a) General.
    The most critical ice accretion in terms of airplane performance 
and handling qualities for each flight phase must be used to show 
compliance with the applicable airplane performance and handling 
qualities requirements for icing conditions contained in subpart B 
of this part. Applicants must demonstrate that the full range of 
atmospheric icing conditions specified in part I of this appendix 
have been considered, including drop diameter distributions, liquid 
water content, and temperature appropriate to the flight conditions 
(for example, configuration, speed, angle-of-attack, and altitude).
    (1) For an airplane certified in accordance with Sec.  
25.1420(a)(1), the ice accretions for each flight phase are defined 
in part II, paragraph (b) of this appendix.
    (2) For an airplane certified in accordance with Sec.  
25.1420(a)(2), the most critical ice accretion for each flight phase 
defined in part II, paragraphs (b) and (c) of this appendix, must be 
used. For the ice accretions defined in part II, paragraph (c) of 
this appendix, only the portion of part I of this appendix in which 
the airplane is capable of operating safely must be considered.
    (3) For an airplane certified in accordance with Sec.  
25.1420(a)(3), the ice accretions for each flight phase are defined 
in part II, paragraph (c) of this appendix.
    (b) Ice accretions for airplanes certified in accordance with 
Sec.  25.1420(a)(1) or (a)(2).
    (1) En route ice is the en route ice as defined by part II, 
paragraph (c)(3), of this appendix, for an airplane certified in 
accordance with Sec.  25.1420(a)(2), or defined by part II, 
paragraph (a)(3), of appendix C of this part, for an airplane 
certified in accordance with Sec.  25.1420(a)(1), plus:
    (i) Pre-detection ice as defined by part II paragraph (b)(5) of 
this appendix; and
    (ii) The ice accumulated during the transit of one cloud with a 
horizontal extent of 17.4 nautical miles in the most critical of the 
icing conditions defined in part I of this appendix and one cloud 
with a horizontal extent of 17.4 nautical miles in the continuous 
maximum icing conditions defined in appendix C of this part.
    (2) Holding ice is the holding ice defined by part II, paragraph 
(c)(4), of this appendix, for an airplane certified in accordance 
with Sec.  25.1420(a)(2), or defined by part II, paragraph (a)(4) of 
appendix C of this part, for an airplane certified in accordance 
with Sec.  25.1420(a)(1), plus:
    (i) Pre-detection ice as defined by part II, paragraph (b)(5) of 
this appendix; and
    (ii) The ice accumulated during the transit of one cloud with a 
17.4 nautical miles horizontal extent in the most critical of the 
icing conditions defined in part I of this appendix and one cloud 
with a horizontal extent of 17.4 nautical miles in the continuous 
maximum icing conditions defined in appendix C of this part. The 
total exposure to the icing conditions need not exceed 45 minutes.
    (3) Approach ice is the more critical of the holding ice defined 
by part II, paragraph (b)(2) of this appendix, or the ice calculated 
in the applicable paragraph (b)(3)(i) or (ii) of part II of this 
appendix:
    (i) For an airplane certified in accordance with Sec.  
25.1420(a)(2), the ice accumulated during descent from the maximum 
vertical extent of the icing conditions defined in part I of this 
appendix to 2,000 feet above the landing surface in the cruise 
configuration, plus transition to the approach configuration, plus:
    (A) Pre-detection ice, as defined by part II, paragraph (b)(5) 
of this appendix; and
    (B) The ice accumulated during the transit at 2,000 feet above 
the landing surface of one cloud with a horizontal extent of 17.4 
nautical miles in the most critical of the icing conditions defined 
in part I of this appendix and one cloud with a horizontal extent of 
17.4 nautical miles in the continuous

[[Page 37335]]

maximum icing conditions defined in appendix C of this part.
    (ii) For an airplane certified in accordance with Sec.  
25.1420(a)(1), the ice accumulated during descent from the maximum 
vertical extent of the maximum continuous icing conditions defined 
in part I of appendix C to 2,000 feet above the landing surface in 
the cruise configuration, plus transition to the approach 
configuration, plus:
    (A) Pre-detection ice, as defined by part II, paragraph (b)(5) 
of this appendix; and
    (B) The ice accumulated during the transit at 2,000 feet above 
the landing surface of one cloud with a horizontal extent of 17.4 
nautical miles in the most critical of the icing conditions defined 
in part I of this appendix and one cloud with a horizontal extent of 
17.4 nautical miles in the continuous maximum icing conditions 
defined in appendix C of this part.
    (4) Landing ice is the more critical of the holding ice as 
defined by part II, paragraph (b)(2) of this appendix, or the ice 
calculated in the applicable paragraph (b)(4)(i) or (ii) of part II 
of this appendix:
    (i) For an airplane certified in accordance with Sec.  
25.1420(a)(2), the ice accretion defined by part II, paragraph 
(c)(5)(i) of this appendix, plus a descent from 2,000 feet above the 
landing surface to a height of 200 feet above the landing surface 
with a transition to the landing configuration in the icing 
conditions defined in part I of this appendix, plus:
    (A) Pre-detection ice, as defined in part II, paragraph (b)(5) 
of this appendix; and
    (B) The ice accumulated during an exit maneuver, beginning with 
the minimum climb gradient required by Sec.  25.119, from a height 
of 200 feet above the landing surface through one cloud with a 
horizontal extent of 17.4 nautical miles in the most critical of the 
icing conditions defined in part I of this appendix and one cloud 
with a horizontal extent of 17.4 nautical miles in the continuous 
maximum icing conditions defined in appendix C of this part.
    (ii) For an airplane certified in accordance with Sec.  
25.1420(a)(1), the ice accumulated in the maximum continuous icing 
conditions defined in appendix C of this part, during a descent from 
the maximum vertical extent of the icing conditions defined in 
appendix C of this part, to 2,000 feet above the landing surface in 
the cruise configuration, plus transition to the approach 
configuration and flying for 15 minutes at 2,000 feet above the 
landing surface, plus a descent from 2,000 feet above the landing 
surface to a height of 200 feet above the landing surface with a 
transition to the landing configuration, plus:
    (A) Pre-detection ice, as described by part II, paragraph (b)(5) 
of this appendix; and
    (B) The ice accumulated during an exit maneuver, beginning with 
the minimum climb gradient required by Sec.  25.119, from a height 
of 200 feet above the landing surface through one cloud with a 
horizontal extent of 17.4 nautical miles in the most critical of the 
icing conditions defined in part I of this appendix and one cloud 
with a horizontal extent of 17.4 nautical miles in the continuous 
maximum icing conditions defined in appendix C of this part.
    (5) Pre-detection ice is the ice accretion before detection of 
appendix O conditions that require exiting per Sec.  25.1420(a)(1) 
and (a)(2). It is the pre-existing ice accretion that may exist from 
operating in icing conditions in which the airplane is approved to 
operate prior to encountering the icing conditions requiring an 
exit, plus the ice accumulated during the time needed to detect the 
icing conditions, followed by two minutes of further ice 
accumulation to take into account the time for the flight crew to 
take action to exit the icing conditions, including coordination 
with air traffic control.
    (i) For an airplane certified in accordance with Sec.  
25.1420(a)(1), the pre-existing ice accretion must be based on the 
icing conditions defined in appendix C of this part.
    (ii) For an airplane certified in accordance with Sec.  
25.1420(a)(2), the pre-existing ice accretion must be based on the 
more critical of the icing conditions defined in appendix C of this 
part, or the icing conditions defined in part I of this appendix in 
which the airplane is capable of safely operating. The pre-detection 
ice accretion applies in showing compliance with Sec. Sec.  
25.143(k) and 25.207(k), and as part of the ice accretion 
definitions of part II, paragraph (b)(1) through (b)(4) of this 
appendix.
    (c) Ice accretions for airplanes certified in accordance with 
Sec. Sec.  25.1420(a)(2) or 25.1420(a)(3). For an airplane certified 
in accordance with Sec.  25.1420(a)(2), only the portion of the 
icing conditions of part I of this appendix in which the airplane is 
capable of operating safely must be considered.
    (1) Takeoff ice is the most critical ice accretion on 
unprotected surfaces, and any ice accretion on the protected 
surfaces appropriate to normal ice protection system operation, 
occurring between liftoff and 400 feet above the takeoff surface, 
assuming accretion starts at liftoff in the icing conditions defined 
in part I of this appendix.
    (2) Final takeoff ice is the most critical ice accretion on 
unprotected surfaces, and any ice accretion on the protected 
surfaces appropriate to normal ice protection system operation, 
between 400 feet and either 1,500 feet above the takeoff surface, or 
the height at which the transition from the takeoff to the en route 
configuration is completed and VFTO is reached, whichever 
is higher. Ice accretion is assumed to start at liftoff in the icing 
conditions defined in part I of this appendix.
    (3) En route ice is the most critical ice accretion on the 
unprotected surfaces, and any ice accretion on the protected 
surfaces appropriate to normal ice protection system operation, 
during the en route flight phase in the icing conditions defined in 
part I of this appendix.
    (4) Holding ice is the most critical ice accretion on the 
unprotected surfaces, and any ice accretion on the protected 
surfaces appropriate to normal ice protection system operation, 
resulting from 45 minutes of flight within a cloud with a 17.4 
nautical miles horizontal extent in the icing conditions defined in 
part I of this appendix, during the holding phase of flight.
    (5) Approach ice is the ice accretion on the unprotected 
surfaces, and any ice accretion on the protected surfaces 
appropriate to normal ice protection system operation, resulting 
from the more critical of the:
    (i) Ice accumulated in the icing conditions defined in part I of 
this appendix during a descent from the maximum vertical extent of 
the icing conditions defined in part I of this appendix, to 2,000 
feet above the landing surface in the cruise configuration, plus 
transition to the approach configuration and flying for 15 minutes 
at 2,000 feet above the landing surface; or
    (ii) Holding ice as defined by part II, paragraph (c)(4) of this 
appendix.
    (6) Landing ice is the ice accretion on the unprotected 
surfaces, and any ice accretion on the protected surfaces 
appropriate to normal ice protection system operation, resulting 
from the more critical of the:
    (i) Ice accretion defined by part II, paragraph (c)(5)(i), of 
this appendix, plus ice accumulated in the icing conditions defined 
in part I of this appendix during a descent from 2,000 feet above 
the landing surface to a height of 200 feet above the landing 
surface with a transition to the landing configuration, followed by 
a go-around at the minimum climb gradient required by Sec.  25.119, 
from a height of 200 feet above the landing surface to 2,000 feet 
above the landing surface, flying for 15 minutes at 2,000 feet above 
the landing surface in the approach configuration, and a descent to 
the landing surface (touchdown) in the landing configuration; or
    (ii) Holding ice as defined by part II paragraph (c)(4) of this 
appendix.
    (7) For both unprotected and protected parts, the ice accretion 
for the takeoff phase must be determined for the icing conditions 
defined in part I of this appendix, using the following assumptions:
    (i) The airfoils, control surfaces, and, if applicable, 
propellers are free from frost, snow, or ice at the start of 
takeoff;
    (ii) The ice accretion begins at liftoff;
    (iii) The critical ratio of thrust/power-to-weight;
    (iv) Failure of the critical engine occurs at VEF; 
and
    (v) Crew activation of the ice protection system is in 
accordance with a normal operating procedure provided in the 
Airplane Flight Manual, except that after beginning the takeoff 
roll, it must be assumed that the crew takes no action to activate 
the ice protection system until the airplane is at least 400 feet 
above the takeoff surface.
    (d) The ice accretion before the ice protection system has been 
activated and is performing its intended function is the critical 
ice accretion formed on the unprotected and normally protected 
surfaces before activation and effective operation of the ice 
protection system in the icing conditions defined in part I of this 
appendix. This ice accretion only applies in showing compliance to 
Sec. Sec.  25.143(j) and 25.207(h).
    (e) In order to reduce the number of ice accretions to be 
considered when demonstrating compliance with the requirements of 
Sec.  25.21(g), any of the ice accretions defined in this appendix 
may be used for any other flight phase if it is shown to be more 
critical than the specific ice accretion defined for that flight 
phase. Configuration differences and their effects on ice accretions 
must be taken into account.
    (f) The ice accretion that has the most adverse effect on 
handling qualities may be

[[Page 37336]]

used for airplane performance tests provided any difference in 
performance is conservatively taken into account.

PART 33--AIRWORTHINESS STANDARDS: AIRCRAFT ENGINES

    24. The authority citation for part 33 continues to read as 
follows:

    Authority: 49 U.S.C. 106(g), 40113, 44701, 44702, 44704.

    25. Revise Sec.  33.68 to read as follows:


Sec.  33.68  Induction system icing.

    Each engine, with all icing protection systems operating, must:
    (a) Operate throughout its flight power range, including the 
minimum descent idle rotor speeds achievable in flight, in the icing 
conditions defined in appendices C and O of part 25 of this chapter, 
and appendix D of this part 33, without the accumulation of ice on the 
engine components that:
    (1) Adversely affects engine operation or that causes an 
unacceptable permanent loss of power or thrust or unacceptable increase 
in engine operating temperature; or
    (2) Results in unacceptable temporary power loss or engine damage; 
or
    (3) Causes a stall, surge, or flameout or loss of engine 
controllability (for example, rollback). The applicant must account for 
in-flight ram effects (for example; scoop factor amplification, water 
temperature, air density) in any critical point analysis or test 
demonstration of these flight conditions.
    (b) Operate throughout its flight power range, including minimum 
descent idle rotor speeds achievable in flight, in the icing conditions 
defined in appendices C and O of part 25 of this chapter. In addition,
    (1) It must be shown through Critical Point Analysis (CPA) that the 
complete ice envelope has been analyzed, and that the most critical 
points must be demonstrated by engine test, analysis or a combination 
of the two to operate acceptably. Extended flight in critical flight 
conditions such as hold, descent, approach, climb, and cruise, must be 
addressed, for the ice conditions defined in these appendices.
    (2) It must be shown by engine test, analysis or a combination of 
the two that the engine can operate acceptably for the following 
durations:
    (i) At engine powers that can sustain level flight: A duration that 
achieves repetitive, stabilized operation in the icing conditions 
defined in appendices C and O of part 25 of this chapter.
    (ii) At engine power below that which can sustain level flight:
    (A) Demonstration in altitude flight simulation test facility: A 
duration of 10 minutes consistent with a simulated flight descent of 
10,000 ft (3 km) in altitude while operating in Continuous Maximum 
icing conditions defined in appendix C of part 25 of this chapter, plus 
40 percent liquid water content margin, at the critical level of 
airspeed and air temperature, or
    (B) Demonstration in ground test facility: A duration of 3 cycles 
of alternating icing exposure corresponding to the liquid water content 
levels and standard cloud lengths in Intermittent Maximum and 
Continuous Maximum icing conditions defined in appendix C of part 25 of 
this chapter, at the critical level of air temperature.
    (c) In addition to complying with Sec.  33.68(b), the following 
conditions shown in Table 1 of this section unless replaced by similar 
CPA test conditions that are more critical or produce an equivalent 
level of severity, must be demonstrated by an engine test:

                         Table 1--Conditions That Must Be Demonstrated by an Engine Test
----------------------------------------------------------------------------------------------------------------
                                                  Supercooled     Median volume
                                  Total air          water        drop diameter
          Condition              temperature     concentrations   (3             Duration
                                                   (minimum)         microns)
----------------------------------------------------------------------------------------------------------------
1. Glaze ice conditions......  21 to 25 [deg]F  2 g/m\3\.......  25 microns.....  (a) 10 minutes for power below
                                (-6 to -4                                          sustainable level flight
                                [deg]C).                                           (idle descent).
                                                                                  (b) Must show repetitive,
                                                                                   stabilized operation for
                                                                                   higher powers (50%, 75%, 100%
                                                                                   MC).
2. Rime ice conditions.......  -10 to 0 [deg]F  1 g/m\3\.......  15 microns.....  (a) 10 minutes for power below
                                (-23 to -18                                        sustainable level flight
                                [deg]C).                                           (idle descent).
                                                                                  (b) Must show repetitive,
                                                                                   stabilized operation for
                                                                                   higher powers (50%, 75%, 100%
                                                                                   MC).
3. Glaze ice holding           Turbofan, only:  Alternating      20 microns.....  Must show repetitive,
 conditions (Turboprop and      10 to 18         cycle: 0.3 g/                     stabilized operation (or 45
 turbofan, only).               [deg]F (-12 to   m\3\ (6                           minutes max).
                                -8 [deg]C).      minute) 1.7 g/
                               Turboprop,        m\3\ (1
                                only: 2 to 10    minute).
                                [deg]F (-17 to
                                -12 [deg]C).
4. Rime ice holding            Turbofan, only:  0.25 g/m\3\....  20 microns.....  Must show repetitive,
 conditions (Turboprop and      -10 to 0                                           stabilized operation (or 45
 turbofan, only).               [deg]F (-23 to                                     minutes max).
                                -18 [deg]C)
                               Turboprop,
                                only: 2 to 10
                                [deg]F (-17 to
                                -12 [deg]C).
----------------------------------------------------------------------------------------------------------------

     (d) The engine should be run at ground idle speed for a minimum of 
30 minutes at each of the following icing conditions shown in Table 2 
of this section with the available air bleed for icing protection at 
its critical condition, without adverse effect, followed by 
acceleration to takeoff power or thrust. During the idle operation the 
engine may be run up periodically to a moderate power or thrust setting 
in a manner acceptable to the Administrator. The applicant must 
document any demonstrated run ups and minimum ambient temperature 
capability during the conduct of icing testing in the engine operating 
manual as mandatory in icing conditions. The applicant must 
demonstrate, with consideration of expected airport elevations, the 
following:

[[Page 37337]]



                                              Table 2--Demonstration Methods for Specific Icing Conditions
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                             Supercooled water
             Condition              Total air temperature      concentrations        Mean effective                       Demonstration
                                                                 (minimum)          particle diameter
--------------------------------------------------------------------------------------------------------------------------------------------------------
1. Rime ice condition.............  0 to 15 [deg]F (-8 to  Liquid--0.3 g/m\3\...  15-25 microns.......  By engine test.
                                     -9 [deg]C).
2. Glaze ice condition............  20 to 30 [deg]F (-7    Liquid--0.3 g/m\3\...  15-25 microns.......  By engine test.
                                     to -1 [deg]C).
3. Snow ice condition.............  26 to 32 [deg]F (-3    Ice--0.9 g/m\3\......  100 microns           By test, analysis or combination of the two.
                                     to 0 [deg]C).                                 (minimum).
4. Large drop glaze ice condition.  15 to 30 [deg]F (-9    Liquid--0.3 g/m\3\...  100 microns           By test, analysis or combination of the two.
                                     to -1 [deg]C).                                (minimum); 3000
                                                                                   microns (maximum).
--------------------------------------------------------------------------------------------------------------------------------------------------------

     (e) The applicant must demonstrate by test, analysis, or 
combination of the two, acceptable operation in ice crystals and mixed 
phase icing conditions throughout part 33, appendix D, icing envelope 
throughout its flight power range, including minimum descent idling 
speeds.
    26. Amend Sec.  33.77 by adding paragraph (a) and by revising 
paragraphs (c) introductory text, (c)(1), (d), and (e)(1) through (4) 
to read as follows:


Sec.  33.77  Foreign object ingestion--ice.

    (a) Compliance with the requirements of this paragraph shall be 
demonstrated by engine ice ingestion test or by validated analysis 
showing equivalence of other means for demonstrating soft body damage 
tolerance.
* * * * *
    (c) Ingestion of ice under the conditions of this section may not 
--
    (1) Cause an immediate or ultimate unacceptable sustained power or 
thrust loss; or
* * * * *
    (d) For an engine that incorporates a protection device, compliance 
with this section need not be demonstrated with respect to ice formed 
forward of the protection device if it is shown that--
    (1) Such ice is of a size that will not pass through the protective 
device;
    (2) The protective device will withstand the impact of the ice; and
    (3) The ice stopped by the protective device will not obstruct the 
flow of induction air into the engine with a resultant sustained 
reduction in power or thrust greater than those values defined by 
paragraph (c) of this section.
    (e) * * *
    (1) The minimum ice quantity and dimensions will be established by 
the engine size as defined in Table 1 of this section.
    (2) The ingested ice dimensions are determined by linear 
interpolation between table values, and are based on the actual 
engine's inlet hilite area.
    (3) The ingestion velocity will simulate ice from the inlet being 
sucked into the engine.
    (4) Engine operation will be at the maximum cruise power or thrust 
unless lower power is more critical.

     Table 1--Minimum Ice Slab Dimensions Based on Engine Inlet Size
------------------------------------------------------------------------
                                            Thickness    Width    Length
   Engine inlet hilite area (sq inch)        (inch)      (inch)   (inch)
------------------------------------------------------------------------
0.......................................          0.25        0      3.6
80......................................          0.25        6      3.6
300.....................................          0.25       12      3.6
700.....................................          0.25       12      4.8
2800....................................          0.35       12      8.5
5000....................................          0.43       12     11.0
7000....................................          0.50       12     12.7
7900....................................          0.50       12     13.4
9500....................................          0.50       12     14.6
11300...................................          0.50       12     15.9
13300...................................          0.50       12     17.1
16500...................................          0.5        12     18.9
20000...................................          0.5        12     20.0
------------------------------------------------------------------------

    27. Amend part 33 by adding appendix D to read as follows:

Appendix D to Part 33--Mixed Phase And Ice Crystal Icing Envelope (Deep 
Convective Clouds)

    Ice crystal conditions associated with convective storm cloud 
formations exist within the part 25, appendix C, Intermittent 
Maximum Icing envelope (including the extension to -40 deg C) and 
the Mil Standard 210 Hot Day envelope. This ice crystal icing 
envelope is depicted in Figure D1, below.

[[Page 37338]]

[GRAPHIC] [TIFF OMITTED] TP29JN10.058

    Within the envelope, total water content (TWC) in g/m\3\ has 
been determined based upon the adiabatic lapse defined by the 
convective rise of 90% relative humidity air from sea level to 
higher altitudes and scaled by a factor of 0.65 to a standard cloud 
length of 17.4 nautical miles. Figure D2 displays TWC for this 
distance over a range of ambient temperature within the boundaries 
of the ice crystal envelope specified in Figure D1.
[GRAPHIC] [TIFF OMITTED] TP29JN10.059


[[Page 37339]]


    Ice crystal size median mass dimension (MMD) range is 50-200 
microns (equivalent spherical size) based upon measurements near 
convective storm cores.
    The TWC can be treated as completely glaciated (ice crystal) 
except as noted in the Table 1.

               TABLE 1--Supercooled Liquid Portion of TWC
------------------------------------------------------------------------
                                   Horizontal cloud
   Temperature range--deg C             length            LWC--g/m\3\
------------------------------------------------------------------------
0 to -20......................  
