[Federal Register Volume 82, Number 210 (Wednesday, November 1, 2017)]
[Rules and Regulations]
[Pages 50496-50500]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2017-23699]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 25

[Docket No. FAA-2017-0717; Special Conditions No. 25-704-SC]


Special Conditions: The Boeing Company Model 777-8 and 777-9 
Airplanes; Interaction of Systems and Structures

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final special conditions; request for comments.

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SUMMARY: These special conditions are issued for The Boeing Company 
(Boeing) Model 777-8 and 777-9 airplanes. These airplanes will have 
novel or unusual design features when compared to the state of 
technology envisioned in the airworthiness standards for transport-
category airplanes. These design features include systems that, 
directly or as a result of failure or malfunction, affect airplane 
structural performance. The applicable airworthiness regulations do not 
contain adequate or appropriate safety standards for these design 
features. These special conditions contain the additional safety 
standards that the Administrator considers necessary to establish a 
level of safety equivalent to that established by the existing 
airworthiness standards.

DATES: This action is effective on Boeing on November 1, 2017. We must 
receive your comments by December 18, 2017.

ADDRESSES: Send comments identified by docket number FAA-2017-0717 
using any of the following methods:
     Federal eRegulations 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 (DOT), 1200 New Jersey Avenue SE., Room 
W12-140, West Building Ground Floor, Washington, DC 20590-0001.
     Hand Delivery or Courier: Take 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.
    Privacy: The FAA will post all comments it receives, without 
change, to http://www.regulations.gov/, including any personal 
information the commenter provides. Using the search function of the 
docket Web site, anyone can find and read the electronic form of all 
comments received into any FAA docket, including the name of the 
individual sending the comment (or signing the comment for an 
association, business, labor union, etc.). DOT's complete Privacy Act 
Statement can be found in the Federal Register published on April 11, 
2000 (65 FR 19477-19478).
    Docket: Background documents or comments received may be read at 
http://www.regulations.gov/ at any time. 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: Mark Freisthler, FAA, Airframe and

[[Page 50497]]

Cabin Safety Branch, ANM-115, Transport Airplane Directorate, Aircraft 
Certification Service, 1601 Lind Avenue SW., Renton, Washington 98057-
3356; telephone 425-227-1119; facsimile 425-227-1320.

SUPPLEMENTARY INFORMATION: The substance of these special conditions 
has been subject to the public-comment process in several prior 
instances with no substantive comments received. The FAA therefore 
finds it unnecessary to delay the effective date and that good cause 
exists for making these special conditions effective upon publication 
in the Federal Register.

Comments Invited

    We invite interested people to take part in this rulemaking by 
sending written comments, data, or views. The most helpful comments 
reference a specific portion of the special conditions, explain the 
reason for any recommended change, and include supporting data.
    We will consider all comments we receive by the closing date for 
comments. We may change these special conditions based on the comments 
we receive.

Background

    On April 19, 2017 (for the Model 777-8 airplane), and May 12, 2015 
(for the 777-9 airplane), Boeing applied for an amendment to Type 
Certificate (TC) No. T00001SE to include the new Model 777-8 and 777-9 
airplanes. These airplanes are derivatives of the Model 777-300ER 
airplane currently approved under TC No. T00001SE. The Model 777-9 
airplane is a stretched-fuselage, large, twin-engine airplane with 
seating for 408 passengers and a maximum takeoff weight of 775,000 
pounds.
    The Model 777-8 airplane, a shortened-body derivative of the Model 
777-9 airplane, is a large, twin-engine airplane with seating for 359 
passengers and a maximum takeoff weight of 775,000 pounds.

Type Certification Basis

    Under the provisions of title 14, Code of Federal Regulations (14 
CFR) 21.101, Boeing must show that the Model 777-8 and 777-9 airplanes 
meet the applicable provisions of the regulations listed in TC No. 
T00001SE, or the applicable regulations in effect on the date of 
application for the change, except for earlier amendments as agreed 
upon by the FAA.
    If the Administrator finds that the applicable airworthiness 
regulations (i.e., 14 CFR part 25) do not contain adequate or 
appropriate safety standards for the Model 777-8 and 777-9 airplanes 
because of a novel or unusual design feature, special conditions are 
prescribed under the provisions of Sec.  21.16.
    Special conditions are initially applicable to the model for which 
they are issued. Should the type certificate for that model be amended 
later to include any other model that incorporates the same novel or 
unusual design features, or should any other model already included on 
the same type certificate be modified to incorporate the same novel or 
unusual design feature, these special conditions would also apply to 
the other model under Sec.  21.101.
    In addition to the applicable airworthiness regulations and special 
conditions, the Model 777-8 and 777-9 airplanes must comply with the 
fuel-vent and exhaust-emission requirements of 14 CFR part 34, and the 
noise-certification requirements of 14 CFR part 36.
    The FAA issues special conditions, as defined in 14 CFR 11.19, in 
accordance with Sec.  11.38, and they become part of the type 
certification basis under Sec.  21.101.

Novel or Unusual Design Features

    The Model 777-8 and 777-9 airplanes will incorporate the following 
novel or unusual design features:
    These Boeing airplanes have full-time, digital, electronic flight-
control systems (EFCS) affecting the pitch, yaw, and roll axes of the 
airplanes. In addition, the airplanes are equipped with on-ground load-
alleviation systems to reduce braking loads. The current regulations 
are inadequate for considering the effects of these systems and their 
effects upon structural performance. These special conditions define 
the criteria to be used in the assessment of the effects of these 
systems on structures.
    The general approach of accounting for the effect of system 
failures on structural performance would be extended to include any 
partial or complete system failure, alone or in combination with other 
partial or complete system failures, as would affect structural 
performance.

Discussion

    Active flight-control systems are capable of providing automatic 
responses to external inputs from sources other than pilots. These 
systems have been expanded in function, effectiveness, and reliability 
such that fly-by-wire flight controls, without a manual backup system 
in the event of system failures, are becoming standard equipment on 
larger transport-category airplanes. As a result of these advancements 
in flight-control technology, the current safety standards contained in 
part 25 do not provide an adequate basis to address an acceptable level 
of safety for airplanes equipped with these advanced systems. Instead, 
certification of these systems has been achieved by issuance of special 
conditions under the provisions of Sec.  21.16.
    For example, stability-augmentation systems (SAS), and to a lesser 
extent load-alleviation systems (LAS), have been used on transport-
category airplanes for many years. Past approvals of these systems were 
based on both special conditions and individual findings of equivalent 
level of safety with existing rules.
    Although autopilots are also considered active control systems, 
typically their control authority has been limited such that the 
consequences of system failures could be readily counteracted by the 
pilot. Now, autopilot functions are integrated into the primary flight 
controls and are given sufficient control authority to maneuver the 
airplane to its structural design limits. This advanced technology, 
with its expanded authority, requires a new approach to account for the 
interaction of control systems and structures.
    The usual deterministic approach to defining the loads envelope 
contained in part 25 does not fully account for system effectiveness 
and system reliability. These automatic systems may be inoperative or 
may operate in a degraded mode with less than full system authority. 
Therefore, it is necessary to determine the structural factors of 
safety and operating margins such that the joint probability of 
structural failures, due to application of loads during system 
malfunctions, is not greater than that found in airplanes equipped with 
earlier-technology control systems. To achieve this objective, it is 
necessary to define the failure conditions, with their associated 
frequency of occurrence, to determine the structural factors of safety 
and operating margins that will ensure an acceptable level of safety.
    Earlier automatic control systems usually provided two states: 
Either fully functioning, or a total loss of function. Flightcrew 
readily detected these conditions. The new, active, flight-control 
systems have failure modes that allow the system to function in the 
degraded mode without full authority. This degraded mode is not readily 
detectable by the flightcrew. Therefore, monitoring systems are 
required on these new systems to provide an annunciation of a condition 
of degraded system capability.

[[Page 50498]]

    In these special conditions, and in the current standards and 
regulations, the term ``any'' requires the applicant to address all 
items covered by the term, rather than addressing only a portion of the 
items.

Applicability

    As discussed above, these special conditions are applicable to 
Boeing Model 777-8 and 777-9 airplanes. Should Boeing apply at a later 
date for a change to the type certificate to include another model 
incorporating the same novel or unusual design feature, these special 
conditions would apply to that model as well.

Conclusion

    This action affects only a certain novel or unusual design feature 
on one model of airplane. It is not a rule of general applicability.

List of Subjects in 14 CFR Part 25

    Aircraft, Aviation safety, Reporting and recordkeeping 
requirements.
    The authority citation for these special conditions is as follows:

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

The Special Conditions

    Accordingly, pursuant to the authority delegated to me by the 
Administrator, the following special conditions are issued as part of 
the type certification basis for Boeing Model 777-8 and 777-9 
airplanes.

Interaction of Systems and Structures

    For airplanes equipped with systems that affect structural 
performance, either directly or as a result of a failure or 
malfunction, the influence of these systems and their failure 
conditions must be taken into account when showing compliance with the 
requirements of part 25, subparts C and D.
    For airplanes equipped with flight-control systems, autopilots, 
stability-augmentation systems, load-alleviation systems, fuel-
management systems, and other systems that either directly, or as a 
result of failure or malfunction, affect structural performance, the 
following criteria must be used for showing compliance. If these 
special conditions are used for other systems, it may be necessary to 
adapt the criteria to the specific system.
    1. The criteria defined herein only address the direct structural 
consequences of the system responses and performance. They cannot be 
considered in isolation, but should be included in the overall safety 
evaluation of the airplane. These criteria may, in some instances, 
duplicate standards already established for this evaluation. These 
criteria are only applicable to structure the failure of which could 
prevent continued safe flight and landing. Specific criteria that 
define acceptable limits on handling characteristics or stability 
requirements, when operating in the system-degraded or inoperative 
mode, are not provided in these special conditions.
    2. Depending upon the specific characteristics of the airplane, 
additional studies that go beyond the criteria provided in these 
special conditions may be required to demonstrate the airplane's 
capability to meet other realistic conditions, such as alternative gust 
or maneuver descriptions for an airplane equipped with a load-
alleviation system.
    3. The following definitions are applicable to these special 
conditions.
    a. Structural performance: Capability of the airplane to meet the 
structural requirements of part 25.
    b. Flight limitations: Limitations that can be applied to the 
airplane flight conditions following an in-flight occurrence, and that 
are included in the airplane flight manual (e.g., speed limitations, 
avoidance of severe weather conditions, etc.).
    c. Operational limitations: Limitations, including flight 
limitations, that can be applied to the airplane operating conditions 
before dispatch (e.g., fuel, payload and master minimum-equipment list 
limitations).
    d. Probabilistic terms: Terms such as probable, improbable, and 
extremely improbable, as used in these special conditions, are the same 
as those used in Sec.  25.1309.
    e. Failure condition: This term is the same as that used in Sec.  
25.1309. However, these special conditions apply only to system-failure 
conditions that affect the structural performance of the airplane 
(e.g., system-failure conditions that induce loads, change the response 
of the airplane to inputs such as gusts or pilot actions, or lower 
flutter margins).

Effects of Systems on Structures

    1. General. The following criteria will be used in determining the 
influence of a system and its failure conditions on the airplane 
structure.
    2. System fully operative. With the system fully operative, the 
following apply:
    a. Limit loads must be derived in all normal operating 
configurations of the system from all the limit conditions specified in 
part 25, subpart C (or defined by special conditions or findings of 
equivalent level of safety in lieu of those specified in subpart C), 
taking into account any special behavior of such a system or associated 
functions, or any effect on the structural performance of the airplane 
that may occur up to the limit loads. In particular, any significant 
nonlinearity (rate of displacement of control surface, thresholds, or 
any other system nonlinearities) must be accounted for in a realistic 
or conservative way when deriving limit loads from limit conditions.
    b. The airplane must meet the strength requirements of part 25 
(static strength, residual strength), using the specified factors to 
derive ultimate loads from the limit loads defined above. The effect of 
nonlinearities must be investigated beyond limit conditions to ensure 
that the behavior of the system presents no anomaly compared to the 
behavior below limit conditions. However, conditions beyond limit 
conditions need not be considered when it can be shown that the 
airplane has design features that will not allow it to exceed those 
limit conditions.
    c. The airplane must meet the aeroelastic stability requirements of 
Sec.  25.629.
    3. System in the failure condition. For any system-failure 
condition not shown to be extremely improbable, the following apply:
    a. At the time of occurrence. Starting from 1g level flight 
conditions, a realistic scenario, including pilot corrective actions, 
must be established to determine the loads occurring at the time of 
failure and immediately after the failure.
    i. For static-strength substantiation, these loads, multiplied by 
an appropriate factor of safety that is related to the probability of 
occurrence of the failure, are ultimate loads to be considered for 
design. The factor of safety is defined in Figure 1, below.

[[Page 50499]]

[GRAPHIC] [TIFF OMITTED] TR01NO17.000

    ii. For residual-strength substantiation, the airplane must be able 
to withstand two thirds of the ultimate loads defined in special 
condition 3.a.i. For pressurized cabins, these loads must be combined 
with the normal operating differential pressure.
    iii. Freedom from aeroelastic instability must be shown up to the 
speeds defined in Sec.  25.629(b)(2). For failure conditions that 
result in speeds beyond VC/MC, freedom from 
aeroelastic instability must be shown to increased speeds, so that the 
margins intended by Sec.  25.629(b)(2) are maintained.
    iv. Failures of the system that result in forced structural 
vibrations (oscillatory failures) must not produce loads that could 
result in detrimental deformation of primary structure.
    b. For the continuation of the flight. For the airplane in the 
system-failed state, and considering any appropriate reconfiguration 
and flight limitations, the following apply:
    i. The loads derived from the following conditions (or defined by 
special conditions or findings of equivalent level of safety in lieu of 
the following conditions) at speeds up to VC/MC 
(or the speed limitation prescribed for the remainder of the flight) 
must be determined:
    1. the limit symmetrical maneuvering conditions specified in 
Sec. Sec.  25.331 and 25.345.
    2. the limit gust and turbulence conditions specified in Sec. Sec.  
25.341 and 25.345.
    3. the limit rolling conditions specified in Sec.  25.349, and the 
limit unsymmetrical conditions specified in Sec. Sec.  25.367, and 
25.427(b) and (c).
    4. the limit yaw-maneuvering conditions specified in Sec.  25.351.
    5. the limit ground-loading conditions specified in Sec. Sec.  
25.473, 25.491, 25.493(d), and 25.503.
    ii. For static-strength substantiation, each part of the structure 
must be able to withstand the loads in special condition 3.b.i., 
multiplied by a factor of safety depending on the probability of being 
in this failure state.
    The factor of safety is defined in Figure 2, below.
    [GRAPHIC] [TIFF OMITTED] TR01NO17.001
    
Qj = (Tj)(Pj)

Where:

Tj = Average time spent in failure mode j (in hours)
Pj = Probability of occurrence of failure mode j (per 
hour)

    Note: If Pj is greater than 10-3 per 
flight hour, then a 1.5 factor of safety must be applied to all 
limit load conditions specified in part 25, subpart C.

    iii. For residual-strength substantiation, the airplane must be 
able to withstand two-thirds of the ultimate loads defined in paragraph 
3.b.ii. of these special conditions. For pressurized cabins, these 
loads must be combined with the normal operating differential pressure.
    iv. If the loads induced by the failure condition have a 
significant effect on fatigue or damage tolerance, then their effects 
must be taken into account.
    v. Freedom from aeroelastic instability must be shown up to a speed 
determined from Figure 3, below. Flutter clearance speeds V' and V'' 
may be based on the speed limitation specified for the remainder of the 
flight using the margins defined by Sec.  25.629(b).

[[Page 50500]]

[GRAPHIC] [TIFF OMITTED] TR01NO17.002

V' = Clearance speed as defined by Sec.  25.629(b)(2).
V'' = Clearance speed as defined by Sec.  25.629(b)(1).
Qj = (Tj)(Pj)

Where:

Tj = Average time spent in failure mode j (in hours)
Pj = Probability of occurrence of failure mode j (per 
hour)

    Note: If Pj is greater than 10-3 per 
flight hour, then the flutter clearance speed must not be less than 
V''.

    vi. Freedom from aeroelastic instability must also be shown up to 
V' in Figure 3, above, for any probable system-failure condition, 
combined with any damage required or selected for investigation by 
Sec.  25.571(b).
    c. Consideration of certain failure conditions may be required by 
other sections of part 25 regardless of calculated system reliability. 
Where analysis shows the probability of these failure conditions to be 
less than 10-9 per flight hour, criteria other than those 
specified in this paragraph may be used for structural substantiation 
to show continued safe flight and landing.
    4. Failure indications. For system-failure detection and 
indication, the following apply:
    a. The system must be checked for failure conditions, not extremely 
improbable, that degrade the structural capability below the level 
required by part 25, or that significantly reduce the reliability of 
the remaining system. As far as reasonably practicable, the flightcrew 
must be made aware of these failures before flight. Certain elements of 
the control system, such as mechanical and hydraulic components, may 
use special periodic inspections, and electronic components may use 
daily checks, in lieu of detection and indication systems, to achieve 
the objective of this requirement. These certification-maintenance 
requirements must be limited to components that are not readily 
detectable by normal detection-and-indication systems, and where 
service history shows that inspections will provide an adequate level 
of safety.
    b. The existence of any failure condition, not extremely 
improbable, during flight, that could significantly affect the 
structural capability of the airplane, and for which the associated 
reduction in airworthiness can be minimized by suitable flight 
limitations, must be signaled to the flightcrew. For example, failure 
conditions that result in a factor of safety between the airplane 
strength and the loads of part 25, subpart C below 1.25, or flutter 
margins below V'', must be signaled to the crew during flight.
    5. Dispatch with known failure conditions. If the airplane is to be 
dispatched in a known system-failure condition that affects structural 
performance, or that affects the reliability of the remaining system to 
maintain structural performance, then the provisions of these special 
conditions must be met, including the provisions of special condition 2 
for the dispatched condition, and special condition 3 for subsequent 
failures.
    a. Expected operational limitations may be taken into account in 
establishing Pj as the probability of failure occurrence for 
determining the safety margin in Figure 1.
    b. Flight limitations and expected operational limitations may be 
taken into account in establishing Qj as the combined probability of 
being in the dispatched failure condition, and the subsequent failure 
condition, for the safety margins in Figures 2 and 3.
    c. These limitations must be such that the probability of being in 
this combined failure state, and then subsequently encountering limit 
load conditions, is extremely improbable. No reduction in these safety 
margins is allowed if the subsequent system-failure rate is greater 
than 10-3 per flight hour.

    Issued in Renton, Washington, on October 23, 2017.
Victor Wicklund,
Manager, Transport Standards Branch, Policy and Innovation Division, 
Aircraft Certification Service.
[FR Doc. 2017-23699 Filed 10-31-17; 8:45 am]
 BILLING CODE 4910-13-P


