[Federal Register Volume 85, Number 186 (Thursday, September 24, 2020)]
[Rules and Regulations]
[Pages 60048-60057]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2020-19809]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2016-6139; Product Identifier 2015-NM-061-AD; Amendment 
39-21234; AD 2020-18-13]
RIN 2120-AA64


Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: The FAA is adopting a new airworthiness directive (AD) for 
certain The Boeing Company Model 737-600, -700, -700C, -800, -900, and 
-900ER series airplanes. This AD was prompted by the FAA's analysis of 
the Model 737 fuel system reviews conducted by the manufacturer. This 
AD requires modifying the fuel quantity indicating system (FQIS) to 
prevent development of an ignition source inside the center fuel tank 
due to electrical fault conditions. This AD also provides alternative 
actions for cargo airplanes.

[[Page 60049]]

The FAA is issuing this AD to address the unsafe condition on these 
products.

DATES: This AD is effective October 29, 2020.

ADDRESSES: 

Examining the AD Docket

    You may examine the AD docket on the internet at https://www.regulations.gov by searching for and locating Docket No. FAA-2016-
6139; or in person at Docket Operations between 9 a.m. and 5 p.m., 
Monday through Friday, except Federal holidays. The AD docket contains 
this final rule, any comments received, and other information. The 
address for Docket Operations is U.S. Department of Transportation, 
Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 
New Jersey Avenue SE, Washington, DC 20590.

FOR FURTHER INFORMATION CONTACT: Jon Regimbal, Aerospace Engineer, 
Propulsion Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des 
Moines, WA 98198; phone and fax: 206-231-3557; email: 
Jon.Regimbal@faa.gov.

SUPPLEMENTARY INFORMATION:

Discussion

    The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 
CFR part 39 by adding an AD that would apply to certain The Boeing 
Company Model 737-600, -700, -700C, -800, -900, and -900ER series 
airplanes. The NPRM published in the Federal Register on May 3, 2016 
(81 FR 26485). The NPRM was prompted by the FAA's analysis of the Model 
737 fuel system reviews conducted by the manufacturer. The NPRM 
proposed to require modifying the FQIS to prevent development of an 
ignition source inside the center fuel tank due to electrical fault 
conditions. The NPRM also proposed to provide alternative actions for 
cargo airplanes.
    The FAA is issuing this AD to address ignition sources inside the 
center fuel tank, which, in combination with flammable fuel vapors, 
could result in a fuel tank explosion and consequent loss of the 
airplane.

Comments

    The FAA gave the public the opportunity to participate in 
developing this final rule. The following presents the comments 
received on the NPRM and the FAA's response to each comment.

Support for the NPRM

    The Air Line Pilots Association, International (ALPA) and National 
Air Traffic Controllers Association (NATCA) supported the intent of the 
NPRM. Additional comments from NATCA are addressed below.

Request To Withdraw NPRM: Unjustified by Risk

    Airlines for America and the Cargo Airline Association, in 
consolidated comments (A4A/CAA), and KLM Royal Dutch Airlines (KLM) 
requested that the FAA withdraw the NPRM. A4A/CAA cited comments 
submitted by Boeing to Docket No. FAA-2012-0187 in which Boeing stated 
that the risk is ``less than extremely improbable'' and that Boeing 
does not believe that an unsafe condition exists. A4A/CAA noted that 
they consider the Boeing comments to be applicable to the airplane 
models in the NPRM. KLM stated that the NPRM does not clarify the 
necessity of additional actions beyond current requirements. KLM added 
that it understands that Boeing is not able to explain or substantiate 
the rationale behind the NPRM.
    The FAA disagrees with the commenters' request. The FAA notes that 
Boeing's comments were addressed in the supplemental NPRM (SNPRM) for 
Docket No. FAA 2012-0187 (80 FR 9400, February 23, 2015) in the comment 
response for ``Request To Withdraw NPRM (77 FR 12506, March 1, 2012): 
Unjustified by Risk.'' As explained in that comment response, in 
addition to examining average risk and total fleet risk, the FAA 
examines the individual flight risk on the worst reasonably anticipated 
flights. In general, the FAA issues ADs in cases where reasonably 
anticipated flights with preexisting failures (either due to latent 
failure conditions or allowable dispatch configurations) are vulnerable 
to a catastrophic event due to an additional foreseeable single failure 
condition. This is because the FAA considers operation of flights 
vulnerable to a potentially catastrophic single failure condition to be 
an excessive safety risk to the passengers on those flights. The FAA 
has determined that the current requirements, including airworthiness 
limitations and critical design configuration control limitations 
(CDCCLs) do not adequately address the unsafe condition identified in 
this AD and therefore it is necessary to issue this final rule. The FAA 
has not changed this AD regarding this issue.

Request To Withdraw NPRM: Probability Analysis Inconsistent With 
Regulatory Requirements

    A4A/CAA requested that the FAA withdraw the NPRM. A4A/CAA stated 
that the assumption of a single failure regardless of probability is 
inconsistent with 14 CFR part 25 regulatory requirements. A4A/CAA 
referred to the phrase ``regardless of probability'' associated with 
single failures. A4A/CAA acknowledged that the term is used with single 
failures in FAA Advisory Circular (AC) 25.981-1C,\1\ ``Fuel Tank 
Ignition Source Prevention Guidelines,'' but since that term does not 
appear in 14 CFR 25.981(a)(3), the commenter considered its use 
arbitrary, possibly introducing additional requirements not included in 
that section. A4A/CAA stated that the ``worst reasonably anticipated 
flight'' is a flight with a latent FQIS failure and a high-flammability 
tank, and this ``latent plus one'' failure--regardless of probability 
of a single failure--is not consistent with 14 CFR 25.981(a)(3).
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    \1\ https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_25.981-1C.pdf.
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    The FAA disagrees with the commenter's request. The FAA notes that 
the commenter's assertion about the intent of 14 CFR 25.981(a)(3) is 
incorrect based on both the language of the rule and on the published 
rulemaking documents. The absence of a probabilistic qualifier in both 
the ``from each single failure'' clause and in the ``from each single 
failure in combination with each latent failure not shown to be 
extremely remote'' clause in 14 CFR 25.981(a)(3) in fact means just 
that--there is no probabilistic qualifier intended by the regulation. 
The intent for single failures in these two scenarios to be considered 
regardless of probability of the single failure was explicitly stated 
in the NPRM for 14 CFR 25.981, as amended by amendment 25-102 (66 FR 
23085, May 7, 2001) (``amendment 25-102''). That NPRM stated, in 
pertinent part, that it would also add a new paragraph (a)(3) to 
require that a safety analysis be performed to demonstrate that the 
presence of an ignition source in the fuel tank system could not result 
from ``any single failure, from any single failure in combination with 
any latent failure condition not shown to be extremely remote, or from 
any combination of failures not shown to be extremely improbable.'' 
These new requirements would define three scenarios that must be 
addressed in order to show compliance with the proposed paragraph 
(a)(3). ``The first scenario is that any single failure, regardless of 
the probability of occurrence of the failure, must not cause an 
ignition source. The second scenario is that any single failure, 
regardless of the probability occurrence, in combination with any 
latent failure condition not shown to be at least

[[Page 60050]]

extremely remote (i.e., not shown to be extremely remote or extremely 
improbable), must not cause an ignition source. The third scenario is 
that any combination of failures not shown to be extremely improbable 
must not cause an ignition source.''
    The preamble to the final rule for amendment 25-102 made a nearly 
identical statement, including the same uses of the phrase ``regardless 
of probability.'' The FAA has determined that it is necessary to 
proceed with issuance of this final rule as proposed. Further details 
and a description of the FAA's risk assessment can be found in 
responses to similar comments in a related SNPRM that addressed the 
same unsafe condition for Model 757 airplanes, in Docket No. FAA-2012-
0187, and in the subsequently issued final rule, AD 2016-07-07, 
amendment 39-18452 (81 FR 19472, April 5, 2016) (``AD 2016-07-07''). No 
change to this AD was made in response to these comments.

Request To Withdraw NPRM: No New Data Since Fuel Tank Flammability 
Reduction (FTFR) Rulemaking

    A4A/CAA requested that the FAA withdraw the NPRM based on a lack of 
new data since the issuance of the FTFR rule (73 FR 42444, July 21, 
2008). A4A/CAA referred to the FTFR rule and decision to not require 
flammability reductions means (FRM) for all-cargo airplanes, and the 
FAA's intent to gather additional data and consideration of further 
rulemaking if flammability of these airplanes is excessive. A4A/CAA 
stated that since the FTFR rule, no additional data has been publicly 
introduced that would support or justify the applicability of this 
rulemaking to all-cargo aircraft. A4A/CAA also referred to the FAA's 
response to comments in the preamble to the SNPRM for Docket No. FAA-
2012-0187, which documented the FAA's decision on applicability of FRM 
and cost estimates. A4A/CAA stated that the FAA response was misleading 
and not factual since manufacturers did not begin detailed designs to 
address the proposed unsafe condition until after the FTFR rule was 
published. A4A/CAA added that the FAA did not discuss other changes to 
the FQIS system in the FTFR rule.
    The FAA disagrees with the commenter's request. The FAA notes that 
the FTFR rule and FQIS ADs are two different issues with separate FAA 
actions. The intent of the FTFR rule was to provide an order of 
magnitude reduction in the rate of fuel tank explosions for the 
airplanes affected by that rule through adding a new airworthiness 
standard for the flammability of fuel tanks. The FAA notes that the 
FTFR rule was never intended to be a replacement for the issuance of 
ADs to address identified unsafe conditions. An unsafe condition due to 
the identified FQIS latent-plus-single failure issue in high-
flammability fuel tanks was determined to exist during the Special 
Federal Aviation Regulation (SFAR) 88 AD Board held by the FAA in 2003 
using the guidance in FAA Policy Memorandum ANM100-2003-112-15, ``SFAR 
88--Mandatory Action Decision Criteria,'' dated February 25, 2003,\2\ 
for high-flammability fuel tanks, including the center fuel tank on 
Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes. 
That same issue was not considered to be an unsafe condition in low-
flammability wing fuel tanks based on that same policy memorandum. The 
FAA has not changed this AD regarding this issue.
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    \2\ http://rgl.faa.gov/Regulatory_and_Guidance_Library/
rgPolicy.nsf/0/dc94c3a46396950386256d5e006aed11/$FILE/Feb2503.pdf.
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Request To Withdraw NPRM: Arbitrary and Inconsistent Wire Separation 
Standards

    A4A/CAA requested that the FAA withdraw the NPRM based on a lack of 
consistent design standards for FQIS wire separation. A4A/CAA assumed 
that the approved standard for the retrofit is a 2-inch wire separation 
minimum, which the commenter considered arbitrary and inconsistently 
applied. A4A/CAA reported that the amount of wiring capable of meeting 
that separation standard varies widely among airplane models. A4A/CAA 
also acknowledged that other separation methods were used in areas not 
meeting the 2-inch wire separation requirement.
    The FAA does not agree with the commenter's request. The degree of 
physical isolation of FQIS wiring from other wiring, whether provided 
by physical distance or barrier methods, that is necessary to eliminate 
the potential for hot shorts due to wiring faults is dependent on the 
materials used, the wire securing methods, and the possible types of 
wiring faults. The FAA relied on the manufacturer to assess the details 
of the design and to propose the appropriate isolation measures. While 
2 inches of physical separation may appear to be an arbitrary number, 
it was the distance proposed by the manufacturer as appropriate for 
their design based on analysis of the design details. The FAA has not 
changed this AD regarding this issue.

Request To Withdraw NPRM: NPRM Arbitrary and Inconsistently Applied

    A4A/CAA requested that the FAA withdraw the NPRM. A4A/CAA noted 
that airplanes with FRM are not included in the applicability, and the 
NPRM would therefore not fully address the unsafe condition. A4A/CAA 
added that the distinction between high- and low-flammability exposure 
time fuel tanks as used in the NPRM is arbitrary. A4A/CAA stated that 
an arbitrary differentiation of high- versus low-flammability as 
decisional criteria for the need for corrective action does not take 
into account the actual probability of the impact of the difference in 
flammability on the potential of catastrophic failure. A4A/CAA also 
stated that allowing the proposed alternative actions for cargo 
airplanes does not fully address the unsafe condition in the NPRM. A4A/
CAA referenced the FAA's response to comments in AD 2016-07-07 
regarding this issue. The commenter summarized numerical analysis 
showing no significant difference in risk between high- and low-
flammability fuel tanks. A4A/CAA concluded that the FAA's risk analysis 
is arbitrary and an unsafe condition does not exist.
    The FAA disagrees with the assertion that the NPRM is arbitrary and 
inconsistent. The NPRM follows defined policy in FAA Policy Memorandum 
ANM100-2003-112-15, and consistently applies the policy to several 
airplane models with similar unsafe conditions, similar to AD 2016-07-
07. The FAA defined the difference between low- and high-flammability 
exposure time fuel tanks based on recommendations from the Aviation 
Rulemaking Advisory Committee Fuel Tank Harmonization Working Group 
(FTHWG). The preamble to the final rule for amendment 25-102, which 
amended 14 CFR 25.981, defined this difference as based upon comparison 
of ``the safety record of center wing fuel tanks that, in certain 
airplanes, are heated by equipment located under the tank, and unheated 
fuel tanks located in the wing.'' The FTHWG concluded that the safety 
record of fuel tanks located in the wings was adequate and that if the 
same level could be achieved in center wing fuel tanks, the overall 
safety objective would be achieved.
    In the response to comments in the preamble to the final rule for 
AD 2016-07-07 referenced by the commenter, the FAA described why FRM or 
alternative actions for cargo airplanes provide an acceptable level of 
safety, even if they do not completely eliminate the non-compliance 
with 14 CFR 25.981(a)(3).

[[Page 60051]]

    The fuel tank explosion history for turbojet/turbofan powered 
transport airplanes fueled with kerosene type fuels, outside of 
maintenance activity, has consisted of explosions of tanks that (1) are 
not conventional aluminum wing tanks and (2) spend a considerable 
amount of their operating time empty. The service history of 
conventional aluminum wing tanks has been acceptable. The intent of the 
difference in decision criteria in FAA Policy Memorandum ANM100-2003-
112-15 was intended to give credit for this satisfactory service 
experience, and to differentiate between tanks with a level of 
flammability similar to that of a conventional wing tank and those with 
a significantly higher level of flammability.
    The numerical analysis provided by the commenter is inconsistent 
with the fuel tank explosion service history. There are at least three 
identifiable physics-based reasons for that inconsistency. First, low-
flammability tanks on most types of airplanes are main tanks that are 
the last tanks used. During a large portion of their operating time, 
the systems and structural features that have the potential to be 
ignition sources in the event of a failure condition are covered with 
liquid fuel, and an ignition source, if it occurs, is likely to be 
submerged. When a potential ignition source in a main tank is 
uncovered, it is likely to be later in the flight when the tank is cool 
and no longer flammable. The commenter's analysis does not account for 
this significant effect. Second, the numerical analysis used by the 
commenter assumes that any given ignition source has a random 
occurrence in time at the estimated probability, and that, in order for 
an explosion to occur, that random occurrence of an ignition source 
needs to coincide with the tank being in a flammable state. In fact, 
many of the identified ignition threats do not simply occur briefly and 
then go away. Instead, a fault occurs that, until it is discovered and 
corrected, repeatedly creates an ignition source, and repeatedly tests 
whether flammable conditions exist.
    Third, the flammability of low-flammability fuel tanks is typically 
dependent on weather, and a low-flammability fuel tank may operate for 
months without ever becoming flammable. This is not true of most high-
flammability fuel tanks, which typically have significant on-airplane 
heat sources driving their temperature. This factor can mean that, on 
some airplanes, an in-tank latent failure can occur and, after some 
period of time, be detected and corrected without the low-flammability 
tank ever having flammable conditions. The numerical analysis provided 
by the commenter does not account for these significant factors. The 
difference in likelihood of a failure that results in repeated ignition 
source events causing a tank explosion is not simply proportional to 
difference in the fleet average flammability of the tank for the 
reasons stated above. The FAA has not changed this AD regarding this 
issue.

Request To Withdraw NPRM: Overestimate of Fleet Average Flammability 
Exposure for All-Cargo Fleet in Alaska

    A4A/CAA requested that the FAA withdraw the NPRM. The commenter 
stated that the FAA did not properly analyze the fleet average 
flammability for the center wing tank on Model 737-700 airplanes. The 
commenter stated that the known U.S. registered 737-700 all-cargo fleet 
without FRM installed will be operated almost solely in the state of 
Alaska for the foreseeable future. A4A/CAA noted that the mean average 
ambient temperature in Alaska is much lower than that used in the FAA's 
analysis. The commenter added that the air conditioning packs in an 
all-cargo configuration generate significantly less heat transfer to 
the center wing tank during normal operations than during the normal 
operations assumed by the FAA's analysis. A4A/CAA concluded that these 
factors reduce the fleet average flammability exposure for the all-
cargo Model 737-700 airplanes to the level of the main wing tanks, and 
therefore, the unsafe condition does not exist.
    The FAA does not agree to withdraw the NPRM. More than 1,100 Model 
737-700 airplanes have been produced. The FAA foresees that, as these 
airplanes are replaced in passenger service by newer airplanes, a 
significant portion of them will be converted to all-cargo service and 
will eventually fly throughout the U.S. and the world. Multiple cargo-
conversion designs for these airplanes have been approved, and other 
conversion designs are in the approval process. The FAA does not agree 
to base its decision about whether an AD is necessary for these 
airplanes on a flammability analysis based solely on the initial cargo 
conversions currently being largely operated in Alaska.
    The FAA also does not agree that a new analysis considering 
operation of only the initial cargo-converted airplanes would result in 
a determination that the center fuel tank of those airplanes has a 
level of flammability comparable to a wing tank of conventional 
aluminum construction, and that the center fuel tank on those airplanes 
could therefore legitimately be classified as a low-flammability fuel 
tank. In addition, the FAA considers the unsafe condition determination 
described in the SNPRM for Docket No. FAA-2012-0187, in the response to 
comments section under, ``Request To Withdraw NPRM (77 FR 12506, March 
1, 2012): Unjustified by Risk,'' to be applicable to these Model 737 
airplanes.

Request To Remove Certain Business Jets From the Applicability

    AMES Continuing Airworthiness Management Organization (AMES CAMO) 
requested that the proposed AD be revised to exclude Boeing Business 
Jets operated under 14 CFR part 91. AMES CAMO noted that the proposed 
AD excludes airplanes modified by the nitrogen generation system (NGS) 
system, but the NGS is mandated only on commercial airplanes operating 
under 14 CFR part 121. AMES CAMO suggested the proposed AD should only 
apply to airplanes operating under 14 CFR part 121.
    The FAA disagrees with the commenter's request. Policy Memorandum 
ANM100-2003-112-15 is applicable to large transport airplanes except 
those specifically excluded by the Special Federal Aviation Regulation 
(SFAR) No. 88 regulation (in 14 CFR part 21). The FAA did not exclude 
non-air-carrier large transport airplanes from the other ADs determined 
to be necessary as a result of SFAR 88, and included non-air-carrier 
large transport airplanes in the FRM retrofit requirements added to 14 
CFR part 125 in 2008. The unsafe condition addressed by this AD is 
applicable to Model 737 airplanes operated as business jets, except as 
specified in paragraph (c) of this AD. The FAA has not changed this AD 
regarding this issue.

Request To Require Cargo Airplane Option for All Airplanes

    Boeing and All Nippon Airways (ANA) requested that the NPRM be 
revised to make the alternative actions for cargo airplanes specified 
in paragraph (h) of the proposed AD applicable to all airplanes. Boeing 
asked that the FAA provide a technical justification why the actions in 
paragraph (h) of the proposed AD apply only to cargo airplanes. ANA 
asked that the actions in paragraph (h) of the proposed AD be allowed 
for passenger airplanes not subject to the FTFR rule, suggesting this 
would provide more choices regarding how to comply with the proposed 
AD.
    The FAA disagrees with the commenters' requests. As discussed in 
the comment response in the SNPRM

[[Page 60052]]

for Docket No. FAA-2012-0187, under the heading ``Requests To Withdraw 
NPRM (77 FR 12506, March 1, 2012) Based on Applicability'' the FAA does 
not consider the alternative action for cargo airplanes allowed by this 
AD to provide an adequate level of safety for passenger airplanes. The 
FAA is willing to accept a higher level of individual flight risk 
exposure for cargo flights that are not fail-safe due to the absence of 
passengers and the resulting significant reduction in occupant exposure 
on a cargo airplane versus a passenger airplane, and due to relatively 
low estimated individual flight risk that would exist on a cargo 
airplane after the corrective actions are taken. The FAA has not 
changed this AD regarding this issue.

Request To Exclude Certain Airplanes

    United Airlines (UAL) noted that the FRM required by 14 CFR 
121.1117 will have been installed on all affected airplanes in 
passenger configuration by December 26, 2018. The FAA infers UAL is 
requesting that the FAA revise the proposed AD to exclude airplanes 
that are affected by 14 CFR 121.1117. In addition, UAL suggested that 
the FAA either delete paragraph (g) of the proposed AD or make 
paragraph (g) of the proposed AD applicable only to airplanes in a 
cargo configuration that do not have an FRM installed and non-U.S.-
registered airplanes that do not have to comply with FRM requirements.
    The FAA disagrees with the commenter's request. There are other 
passenger-carrying airplanes operated under 14 CFR part 91 that are not 
required to install FRM. (The requirement to install FRM on all 
passenger-carrying airplanes operated by air carriers is in 14 CFR 
121.1117.) The FAA notes that foreign air carriers may not have to 
comply with that requirement or similar requirements of their own civil 
aviation authority. The European Union Aviation Safety Agency (EASA), 
for example, has chosen not to require FRM to be retrofitted to in-
service airplanes. This AD is intended to require any Model 737-600, -
700, -700C, -800, -900, and -900ER series passenger airplane that does 
not have FRM, regardless of the rules under which it is operated, to 
address the FQIS latent-plus-one unsafe condition with a corrective 
action that fully complies with the FAA's airworthiness standards. This 
requirement fulfills the FAA's International Civil Aviation 
Organization to address unsafe conditions on all of the aircraft 
manufactured by the state of design, not just those aircraft whose 
operation is under the jurisdiction of the state of design. The FAA has 
not changed this AD regarding this issue.

Request To Clarify Certification Basis for Modification Requirements

    NATCA recommended that the FAA revise paragraph (g) of the proposed 
AD to clearly state that the required FQIS design changes must comply 
with the fail-safe requirements of 14 CFR 25.901(c), as amended by 
amendment 25-46 (43 FR 50597, October 30, 1978); and 14 CFR 25.981(a) 
and (b), as amended by amendment 25-102; NATCA added that these 
provisions are required by SFAR 88.
    The FAA does not agree to change paragraph (g) of this AD. While 
the FAA agrees that modifications to comply with paragraph (g) of this 
AD should be required to comply with the referenced regulations, that 
requirement already exists in 14 CFR part 21. No change to this AD is 
necessary.

Request To Address Unsafe Condition on All Fuel Tanks

    NATCA recommended that the FAA require design changes that 
eliminate unsafe FQIS failure conditions on all fuel tanks on the 
affected models, regardless of fuel tank location or the percentage of 
time the fuel tank is flammable. NATCA referred to four fuel tank 
explosions in low-flammability exposure time fuel tanks identified by 
the FAA during FTFR rulemaking. NATCA stated that neither FRM nor 
alternative actions for cargo airplanes (e.g., BITE checks (checks of 
built-in test equipment) followed by applicable repairs before further 
flight and modification of the center fuel tank FQIS wiring within 60 
months) would bring the airplane into full regulatory compliance. NATCA 
added that the combination of failures described in the NPRM meets the 
criteria for ``known combinations'' of failures that require corrective 
action in FAA Policy Memorandum ANM100-2003-112-15.
    The FAA disagrees with the commenter's request. The FAA has 
determined that according to Policy Memorandum ANM100-2003-112-15, the 
failure condition for the airplanes affected by this AD should not be 
classified as a ``known combination.'' While the FQIS design 
architecture is similar to that of the early Boeing Model 747 
configuration that is suspected of contributing to the TWA Flight 800 
fuel tank explosion, significant differences exist in the design of 
FQIS components and wire installations between the affected Boeing 
models and the early Model 747 airplanes such that the intent of the 
``known combinations'' provision for low-flammability fuel tanks in the 
policy memorandum is not applicable. Therefore, this AD affects only 
the identified Boeing airplanes with high-flammability exposure time 
fuel tanks, as specified in paragraph (c) of this AD. The FAA provided 
a detailed response to similar comments in the preamble of the final 
rule for AD 2016-07-07. The FAA has not changed this final rule 
regarding this issue.

Request To Require Modification on All Production Airplanes

    NATCA recommended that the FAA require designs that comply with 14 
CFR 25.901(c) and 25.981(a)(3) on all newly produced transport 
airplanes. NATCA stated that continuing to grant exemptions to 14 CFR 
25.901(c), as amended by amendment 25-40 (42 FR 15042, March 17, 1977); 
and 14 CFR 25.981(a)(3), as amended by amendment 25-102; has allowed 
continued production of thousands of airplanes with this known unsafe 
condition.
    The FAA disagrees with the commenter's request. The recommendation 
to require production airplanes to fully comply with 14 CFR 25.901(c) 
and 14 CFR 25.981(a)(3) is outside the scope of this rulemaking. In 
addition, the FAA has implemented requirements for all large transport 
airplanes produced after September 2010 to include flammability 
reduction methods for tanks that would otherwise be high-flammability 
fuel tanks. Boeing incorporated this change into the Model 737 series 
airplanes that are still in production and the FAA has excluded those 
models from the applicability of this AD. The FAA has not changed this 
final rule regarding this issue.

Request To State That an Exemption is Required

    Boeing requested that paragraph (h) of the proposed AD be revised 
to state that an exemption is required to accomplish the specified 
actions. Boeing stated that the FAA has identified that the BITE 
procedure and wire separation design changes specified in the proposed 
AD are not sufficient for compliance to 14 CFR 25.981(a) at the FQIS 
level. Boeing stated that an exemption is therefore needed prior to 
approval of the related design change.
    The FAA agrees to clarify. The BITE check is not a type design 
change or alteration, so no exemption from the airworthiness standards 
is required for that action. The design data approval of any partial 
wire separation modification would require an exemption. That exemption 
would be obtained by the party seeking approval of the alteration data, 
and no further exemption would be required for the party using that 
data

[[Page 60053]]

to alter an aircraft. Obtaining such an exemption would be part of the 
certification process for such a change, so, the FAA does not find it 
necessary to include such information in paragraph (h) of this AD. In 
addition, some parties may choose to comply with the AD using a design 
change that fully complies with the airworthiness standards. The FAA 
also notes that the commenter appears to misunderstand why an exemption 
is needed for the required modification. The exemption is needed 
because, even with the modification, the FQIS does not comply with 14 
CFR 25.901(c) and 14 CFR 25.981(a). The exemption does not authorize 
evaluation of a partial system for compliance with the system level 
requirement. The FAA has not changed this AD regarding this issue.

Request To Exclude Airplanes That Have Installed an Ignition Mitigation 
Means (IMM) or Flammability Impact Mitigation Means (FIMM)

    AerSale stated that the Costs of Compliance section of the NPRM 
only cites the requirements in 14 CFR 121.1117 to install FRM, but 14 
CFR 121.1117 paragraph (d)(1) states that IMM, FRM, or FIMM may be 
installed. AerSale suggested that all IMM, FRM, or FIMM installations 
with the approval of the FAA Oversight Office would meet the 
requirements of 14 CFR 121.1117. The FAA infers AerSale is requesting 
that the proposed AD be revised to exclude airplanes on which IMM or 
FIMM has been installed.
    The FAA partially agrees with the commenter's request. The FAA 
agrees that IMM provides a level of risk reduction at least as great as 
that provided by FRM. The FAA does not agree that airplanes should be 
excluded from paragraph (c) of this AD based on the installation of 
FIMM alone. FIMM is applicable to design changes only, and is intended 
to ensure that, if a fuel tank design change would otherwise have 
increased the flammability of a fuel tank, the associated FIMM would 
ensure that the flammability of that tank is not increased by the 
design change. Therefore, FIMM itself does not address the need for FRM 
for the original tank design. The FAA has revised paragraph (c) of this 
AD to clarify that airplanes with an IMM approved by the FAA as 
compliant with certain regulations are excluded from this AD. This 
revision includes adding paragraphs (c)(1) and (2) of this AD.

Request To Record Only Certain Codes

    Boeing requested that paragraph (h)(1) of the proposed AD be 
revised to only require corrective actions if a nondispatchable fault 
code pertaining to the center wing tank is recorded (as opposed to any 
nondispatchable fault code being recorded). Boeing stated that all FQIS 
wire separation changes in the proposed AD are limited to the center 
wing tank, therefore only built-in test equipment (BITE) check messages 
pertaining to the center wing tank are applicable to the proposed AD. 
In addition, Boeing stated that a final rule should be postponed until 
the FAA develops a list of ``nondispatchable fault codes'' in 
conjunction with Boeing.
    The FAA agrees that the unsafe condition addressed by this AD is 
limited to the center wing tank. However, the FAA does not agree that 
the AD should be changed as proposed by Boeing. It is not clear to the 
FAA whether there may be FQIS BITE fault codes that are not clearly 
identified as related to the center wing tank but that may impact 
center tank circuits. Therefore, the FAA has determined that all 
nondispatchable fault codes recorded prior to the BITE check or as a 
result of the BITE check required by paragraph (h)(1) of this AD must 
be addressed. Operators or Boeing may request an alternative method of 
compliance (AMOC) under the provisions of paragraph (i) of this AD if 
they can provide sufficient data that a particular fault code does not 
pertain to the unsafe condition addressed by this AD.
    Regarding the requirement to record and address fault codes read 
immediately prior to running the BITE check procedure, the FAA notes 
that the normal Boeing procedure for performing an FQIS BITE check is 
to first erase all of the existing fault codes, then perform the BITE 
check and troubleshoot any resulting new fault codes. For this AD, the 
FAA did not want any already stored fault codes to be potentially 
ignored due to erasure at the first step because some of the failures 
of concern can be intermittent. This AD therefore requires operators to 
record the existing codes before doing the BITE check, then do the BITE 
check and record the new codes that result from that BITE check, and 
then do the appropriate troubleshooting and corrective action for both 
sets of codes per the manufacturer's guidance. The FAA has not changed 
this AD regarding this issue.
    Finally, the FAA does not agree to delay the final rule while 
Boeing proposes and obtains FAA agreement on a list of nondispatchable 
fault codes. The FAA requested service information from Boeing in 2016 
to support the option for all-cargo airplanes on all of the Boeing 
models for which similar FQIS ADs were planned. Boeing chose at that 
time to develop service information only for the Model 747-400, 757, 
and 767 airplanes because at that time only those airplanes had 
affected cargo configuration for which Boeing was the design approval 
holder. The FAA agreed at that time to not require Boeing to develop a 
BITE check service bulletin for the Model 737 airplanes because Boeing 
had not yet developed a cargo conversion service bulletin or 
supplemental type certificate (STC) for the Model 737 airplanes. The 
FAA also considered that, because the BITE check instructions already 
existed in the Model 737 AMM, a BITE check service bulletin could be 
developed quickly at a later date if needed. In addition, the process 
for obtaining FAA agreement on a list of nondispatchable fault codes 
for the models Boeing chose to support took less than 30 days. If any 
service information is developed to support compliance with paragraph 
(h) of this AD it will be evaluated for approval using the AMOC process 
specified in paragraph (i) of this AD.

Request To Clarify Required Modification

    ANA and Thomson Airways requested that the FAA provide 
clarification regarding how to accomplish the modification specified in 
paragraph (g) of the proposed AD. ANA noted that paragraph (h) of the 
proposed AD provides clear alternative actions for cargo airplanes. ANA 
stated that it could not identify how to modify the FQIS in passenger 
airplanes not subject to the FTFR rule. ANA noted that it contacted 
Boeing for clarification and Boeing stated that the FRM (which Boeing 
calls NGS) retrofit is the method of compliance for these airplanes. 
ANA asked that the FAA either clarify how to modify the FQIS system or 
accept an FRM retrofit as terminating action. Thomson Airways asked if 
the intent of the proposed AD is to install an NGS on affected 
airplanes. Thomson Airways also asked for clarification regarding the 
FQIS modification, stating that the proposed AD does not provide detail 
regarding modifying the FQIS itself, only the FQIS wiring.
    The FAA agrees to clarify. As noted in paragraph (c) of this AD, 
airplanes on which FRM or IMM that meets certain FAA airworthiness 
standards is installed are excluded from this AD. Paragraph (g) of this 
AD requires modification of the FQIS on passenger airplanes to prevent 
development of an ignition source inside the center fuel tank due to 
electrical fault conditions. The specifics of this modification may 
vary as long as the modification addresses the unsafe condition

[[Page 60054]]

identified in this AD and the procedures specified in paragraph (i) of 
this AD are used to approve the modification method. Operators may 
choose to install an FRM or IMM that meets the criteria specified in 
paragraph (c), which would then remove that airplane from the 
applicability of this AD, negating the need to do the modification 
specified in paragraph (g) of this AD. Otherwise, operators must obtain 
an AMOC as specified in paragraph (i) of this AD and modify their 
airplane accordingly. The FAA has not changed this AD regarding this 
issue.

Request To Provide a Detailed Cost-Effective Method of Compliance

    Korean Air Lines (KAL), KLM, AMES CAMO, and Duco Schiere requested 
that the FAA provide a detailed and cost-effective method of compliance 
for passenger airplanes. KAL, AMES CAMO, and Duco Schiere noted that 
the proposed AD does not provide a clear means of compliance for the 
modification, such as a Boeing service bulletin. AMES CAMO noted that 
without a clear method of compliance, it is difficult to determine the 
extent of the required work. KAL and KLM noted that the majority of 
non-FAA operators are not required to retrofit the NGS system. KLM 
stated that since 2008 the level of fuel tank safety has been improved 
by the implementation of several costly SFAR 88 service bulletins, 
implementation of airworthiness limitations into the maintenance 
program and implementation of CDCCLs into maintenance documents. KLM 
mentioned that the modification would require an airplane to be out of 
service for a lengthy time. KLM added that the modification would add 
weight to the airplane and require additional fuel usage. KAL and KLM 
requested that the FAA encourage Boeing to develop an acceptable cost-
effective method of compliance that does not require installation of an 
NGS.
    The FAA agrees that the lack of service information for FQIS 
modifications makes it difficult to assess the required work to modify 
the FQIS, and acknowledges the high cost of NGS. However, the FAA 
disagrees with the commenters' requests. For passenger-carrying 
airplanes, the cost per airplane of providing a modification of the 
FQIS that fully complies with the airworthiness standards was estimated 
by Boeing and their FQIS vendor (Goodrich) prior to the issuance of the 
NPRM to be comparable to the cost of installing NGS. Based on that cost 
estimate, Boeing proposed that they not be required to develop a fully 
compliant FQIS modification for passenger airplanes because it would 
not provide significant savings to operators and NGS would provide a 
greater safety benefit. The FAA agreed.
    The FAA's understanding is that Boeing's current position is the 
same, and that they do not plan to develop a fully compliant FQIS 
modification for passenger airplanes to address paragraph (g) of this 
AD. However, if service information is developed, approved, and 
available in the future, operators may request approval under the 
provisions of paragraph (i) of this AD to use approved service 
instructions as an AMOC for the requirements of this AD, or the FAA may 
approve the service information as a global AMOC for this AD.

Request To Clarify Intent of Different Requirements in Paragraphs (g) 
and (h) of the Proposed AD

    Boeing asked that the FAA clarify the intent of the differences 
between the requirements in paragraphs (g) and (h) of the proposed AD. 
Boeing stated that it is unclear what change is expected for compliance 
with paragraph (g) of the proposed AD versus paragraph (h) of the 
proposed AD. Boeing suggested that one possibility is that paragraph 
(g) of the proposed AD is intended to cover development of transient 
suppression, while paragraph (h) of the proposed AD is intended to 
cover compliance via FQIS wire separation and BITE checks.
    The FAA agrees to clarify. Paragraph (g) of this AD is intended to 
require, for passenger airplanes that are subject to this AD, a 
modification to the FQIS that makes it fully compliant with 14 CFR 
25.981(a), as amended by amendment 25-102. A fully compliant FQIS 
modification might include wire separation or transient suppression 
devices, but due to the system design, either option would likely 
require changes to the FQIS processor.
    Paragraph (h) of this AD is intended to allow, as an optional 
method of compliance for all-cargo airplanes only, a change that 
isolates the center fuel tank circuit wiring between the FQIS processor 
and the fuel tanks from other wiring that is connected to a sufficient 
power source to create an ignition source in the event of a hot short 
between the wiring. Such a change would not be fully compliant with the 
airworthiness regulations (hence the requirement to obtain a partial 
exemption from 14 CFR 25.901(c) and 14 CFR 25.981(a) for any such 
design change), but would provide a level of risk reduction that the 
FAA considers acceptable for all-cargo airplanes and would 
significantly reduce the costs relative to a fully compliant 
modification.

Request To Change Compliance Time

    A4A/CAA and Thomson Airways requested that the FAA extend the 
compliance time for the modifications specified in paragraphs (g) and 
(h)(2) of the proposed AD to 72 months. A4A/CAA stated that the 
compliance time should match that of AD 2016-07-07 because the unsafe 
condition and corrective actions are similar. A4A/CAA stated that 
although service information was not yet available, the compliance time 
should align with major maintenance schedules, but should be not less 
than 72 months after service information is available. Thomson Airways 
noted that 72 months would provide operators a better opportunity to 
work within existing maintenance program schedules.
    Conversely, NATCA recommended that the FAA reject requests for a 
compliance time longer than 5 years as proposed in the NPRM. Assuming 
final rule issuance in 2016, NATCA estimated that a 5-year compliance 
time would result in required compliance by 2021-25 years after the TWA 
Flight 800 fuel tank explosion that led to the requirements in SFAR 88, 
and 20 years after issuance of SFAR 88.
    The FAA agrees with Thomson Airways and A4A/CAA's request to extend 
the compliance time, and disagrees with NATCA's request. The FAA 
received similar requests to extend the compliance time from several 
commenters regarding the NPRMs for the FQIS modification on other 
airplanes. The FAA disagrees with establishing a compliance time based 
on issuance of any service information that is not yet approved or 
available. The FAA has determined that a 72-month compliance time is 
appropriate and will provide operators adequate time to prepare for and 
perform the required modifications without excessive disruption of 
operations. The FAA has determined that the requested moderate increase 
in compliance time will continue to provide an acceptable level of 
safety. The FAA has changed paragraphs (g) and (h)(2) of this AD 
accordingly.

Request To Change Compliance Time Relative to Receipt of Exemption

    Boeing requested that the FAA revise the compliance time for the 
proposed AD to ``60 months after an exemption from [14 CFR 
25.981(a)(3)] is FAA-approved.'' Boeing suggested that it would take 6 
months to develop an exemption petition and 6 months for the FAA to 
approve that exemption. Boeing

[[Page 60055]]

added that the FAA has previously identified that the BITE checks 
procedure and wire separation design were not sufficient for compliance 
with 14 CFR 25.981(a)(3).
    The FAA disagrees with the commenter's request. An AD typically 
does not include a compliance time that is based on an optional action 
that an operator or manufacturer might choose to take. In addition, the 
FAA notes that Boeing has already received exemptions for the Model 
747-400, 757, and 767 airplanes, and could quickly petition for and 
obtain approval of a similar exemption for the Model 737 airplanes 
using an almost identical petition. The FAA's flow time to disposition 
such a petition would be approximately 90 days, during which time 
Boeing could still proceed with development of the modification. In 
addition, as noted above, the compliance time for paragraph (h)(2) of 
this AD has been extended to 72 months, giving additional time for 
operators or manufacturers to obtain an exemption.

Request To Extend Repetitive BITE Check Interval

    Boeing requested that paragraph (h)(1) of the proposed AD be 
revised to extend the repetitive check interval for the BITE checks. 
Boeing requested that the repetitive interval be extended to 750 flight 
hours to match the repetitive intervals specified in the service 
information for a related AD.
    The FAA agrees for the reason provided, and because 750 flight 
hours better aligns with most operators' maintenance programs. The FAA 
intended to propose a 750 flight hour interval, but inadvertently 
specified 650 flight hour intervals in the proposed AD. The FAA has 
revised paragraph (h)(1) of this AD to specify repetitive intervals of 
750 flight hours.

Request To Revise Costs of Compliance Section To Account for Cargo 
Conversions

    A4A/CAA noted that the Costs of Compliance section in the NPRM 
stated all U.S.-registered airplanes are currently operated as 
passenger airplanes and that ``because of the requirement in 14 CFR 
121.1117 to install FRM on U.S. air-carrier passenger airplanes by the 
end of 2017, it is likely that no U.S. airplanes would actually be 
affected by this proposed AD.'' A4A/CAA noted that 14 CFR 121.1117 does 
not require FRM to be installed on all-cargo airplanes. The commenter 
stated that U.S.-registered Model 737-700 all-cargo airplanes without 
FRM installed will be operated by 2017. The FAA infers that A4A/CAA is 
requesting that the Costs of Compliance section be revised to reflect 
the number of all-cargo U.S.-registered airplanes.
    The FAA agrees that there are currently U.S.-registered Model 737-
700 all-cargo airplanes operating without FRM installed. The FAA has 
revised the Costs of Compliance section of this AD to reflect these 
airplanes.

Request To Acknowledge Impacts on Intrastate Aviation in Alaska

    A4A/CAA stated that the proposed AD will interrupt aviation 
transportation to remote Alaskan communities not serviced by other 
modes of transportation, contrary to the statement that the proposed AD 
``will not affect intrastate aviation in Alaska.'' A4A/CAA noted that, 
beginning in 2017, Model 737-700 airplanes in an all-cargo 
configuration and without FRM installed will provide transportation to 
remote Alaskan communities. A4A/CAA added that these airplanes would be 
required to be removed from service for an extended time while 
accomplishing the proposed modification, which the FAA estimates would 
take 1,200 work-hours.
    The FAA acknowledges that, since the NPRM was issued, at least one 
major operator began using converted Model 737-700 cargo airplanes for 
intrastate flights in Alaska. The few remote communities in Alaska that 
have airports suitable for a Model 737-700 are unlikely to be served 
solely by Model 737-700 airplanes. The FAA has considered the potential 
for impact to these communities due to Model 737-700 airplanes being 
temporarily out of service for the required modification actions, and 
considers the safety concern to outweigh those potential impacts. This 
AD was developed with regard to minimizing the economic impact on 
operators to the extent possible, consistent with the safety objectives 
of this AD. In any event, the Federal Aviation Regulations (14 CFR part 
39) require operators to correct an unsafe condition identified on an 
airplane to ensure operation of that airplane in an airworthy 
condition. The FAA has determined in this case that the requirements 
are necessary and the indirect costs would be outweighed by the safety 
benefits of the AD. The FAA considers the 72 month compliance adequate 
time for operators to schedule the required modifications without 
excessive disruption of service to those communities. However, if an 
operator considers that a moderate delay in the incorporation of the 
required modification would significantly reduce the impact on their 
operations or the impact on service to a remote community in Alaska 
while still providing an acceptable level of safety, that operator can 
use the procedures in paragraph (i) of this AD to explain those impacts 
and request approval of an extension of the compliance time.

Request To Require Design Changes From Manufacturers

    NATCA recommended that the FAA follow the agency's compliance and 
enforcement policy to require manufacturers to develop the necessary 
design changes soon enough to support operators' ability to comply with 
the proposed requirements. NATCA noted that SFAR 88 required 
manufacturers to develop all design changes for unsafe conditions 
identified by their SFAR 88 design reviews by December 2002, or within 
an additional 18 months if the FAA granted an extension.
    The FAA acknowledges the commenter's concerns. However, any 
enforcement action is outside the scope of this rulemaking. The FAA has 
not changed this final rule regarding this issue.

Request To Clarify the Applicability

    Duco Schiere stated the NPRM is not clear about which 
configurations (passenger/cargo, with/without NGS installed) of Model 
737 airplanes are applicable to the AD. The FAA infers the commenter is 
requesting the FAA clarify the applicability.
    The FAA agrees to clarify. This AD applies to The Boeing Company 
Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes 
(including passenger and cargo airplanes) except for airplanes having 
configurations identified in paragraphs (c)(1) and (2) of this AD. 
Airplanes with an installed NGS that meets the criteria specified in 
paragraph (c)(1) of this AD are excluded from this AD. Airplanes with 
an installed IMM such as fuel tank explosion suppression foam that 
meets the criteria specified in paragraph (c)(2) of this AD are also 
excluded from this AD.

Clarification of BITE Check Compliance Time

    The FAA has revised paragraph (h)(1) of this AD to clarify the 
compliance time for the BITE check relative to the requirement to 
record the fault codes. The FAA recognized that operators might 
interpret the proposed requirements for alternative actions for cargo 
airplanes as allowing additional flights prior to performing the BITE 
check after first recording the fault codes. The FAA intended for 
operators to perform the BITE check immediately after recording the 
fault codes to address

[[Page 60056]]

both the fault codes that exist prior to performing the BITE check and 
any new codes that are identified during the BITE check.

Clarification of Costs of Compliance

    The FAA had previously determined, as specified in the NPRM, that 
the work involved for the cargo airplane wire separation modification 
would take 230 work-hours. Boeing has since provided an updated 
estimate of 74 work-hours for the alternative modification for cargo 
airplanes. The FAA has revised the cost estimate for the modification 
accordingly in this final rule.

Conclusion

    The FAA reviewed the relevant data, considered the comments 
received, and determined that air safety and the public interest 
require adopting this final rule with the changes described previously 
and minor editorial changes. The FAA has determined that these minor 
changes:
     Are consistent with the intent that was proposed in the 
NPRM for addressing the unsafe condition; and
     Do not add any additional burden upon the public than was 
already proposed in the NPRM.
    The FAA also determined that these changes will not increase the 
economic burden on any operator or increase the scope of this final 
rule.

Costs of Compliance

    There are approximately 1,393 U.S.-registered Model 737-600, -700, 
-700C, -800, -900, and -900ER series airplanes in service. Several of 
those airplanes are currently operated as cargo airplanes. Beginning 
with line number 2620, however, Boeing has delivered airplanes with 
FRM/NGS installed. The FAA estimates that 831 affected airplanes on the 
U.S. Register were delivered without FRM installed, but the agency does 
not know the number of airplanes that have had FRM installed post-
production. Because of the requirement in 14 CFR 121.1117 to install 
FRM on U.S. air-carrier passenger airplanes by the end of 2017, it is 
likely that no U.S. passenger airplanes would actually be affected by 
this AD. However, U.S.-registered cargo airplanes may be affected by 
this AD. For any affected airplane, the FAA estimates the following 
costs to comply with this AD:

                                        Estimated Costs: Required Actions
----------------------------------------------------------------------------------------------------------------
                 Action                           Labor cost            Parts cost         Cost per product
----------------------------------------------------------------------------------------------------------------
Modification............................  1,200 work-hours x $85 per        $200,000  $302,000.
                                           hour = $102,000.
----------------------------------------------------------------------------------------------------------------


                                      Estimated Costs: Alternative Actions
----------------------------------------------------------------------------------------------------------------
                 Action                           Labor cost            Parts cost         Cost per product
----------------------------------------------------------------------------------------------------------------
BITE check..............................  1 work-hour x $85 per hour              $0  $85 per check (4 checks
                                           = $85 per check.                            per year, $340 per year).
Wire separation.........................  74 work-hours x $85 per             10,000  16,290.
                                           hour = $6,290.
----------------------------------------------------------------------------------------------------------------

Authority for This Rulemaking

    Title 49 of the United States Code specifies the FAA's authority to 
issue rules on aviation safety. 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.
    The FAA is issuing this rulemaking under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701: General requirements. 
Under that section, Congress charges the FAA with promoting safe flight 
of civil aircraft in air commerce by prescribing regulations for 
practices, methods, and procedures the Administrator finds necessary 
for safety in air commerce. This regulation is within the scope of that 
authority because it addresses an unsafe condition that is likely to 
exist or develop on products identified in this rulemaking action.

Regulatory Findings

    This AD will not have federalism implications under Executive Order 
13132. This AD will 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.
    For the reasons discussed above, I certify that this AD:
    (1) Is not a ``significant regulatory action'' under Executive 
Order 12866,
    (2) Will not affect intrastate aviation in Alaska, and
    (3) Will not have a significant economic impact, positive or 
negative, on a substantial number of small entities under the criteria 
of the Regulatory Flexibility Act.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Incorporation by 
reference, Safety.

Adoption of the Amendment

    Accordingly, under the authority delegated to me by the 
Administrator, the FAA amends 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

0
1. The authority citation for part 39 continues to read as follows:

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


Sec.  39.13  [Amended]

0
2. The FAA amends Sec.  39.13 by adding the following new airworthiness 
directive (AD):

2020-18-13 The Boeing Company: Amendment 39-21234; Docket No. FAA-
2016-6139; Product Identifier 2015-NM-061-AD.

(a) Effective Date

    This AD is effective October 29, 2020.

(b) Affected ADs

    None.

(c) Applicability

    This AD applies to The Boeing Company Model 737-600, -700, -
700C, -800, -900, and -900ER series airplanes, certificated in any 
category, excluding airplanes identified in paragraphs (c)(1) and 
(2) of this AD.
    (1) Airplanes equipped with a flammability reduction means (FRM) 
approved by the FAA as compliant with the fuel tank flammability 
reduction (FTFR) requirements of 14 CFR 25.981(b) or 26.33(c)(1).
    (2) Airplanes equipped with an ignition mitigation means (IMM) 
approved by the FAA as compliant with the FTFR requirements of 14 
CFR 25.981(c) or 26.33(c)(2).

[[Page 60057]]

(d) Subject

    Air Transport Association (ATA) of America Code 28, Fuel.

(e) Unsafe Condition

    This AD was prompted by the FAA's analysis of the Model 737 fuel 
system reviews conducted by the manufacturer. The FAA is issuing 
this AD to prevent ignition sources inside the center fuel tank, 
which, in combination with flammable fuel vapors, could result in a 
fuel tank explosion and consequent loss of the airplane.

(f) Compliance

    Comply with this AD within the compliance times specified, 
unless already done.

(g) Modification

    Within 72 months after the effective date of this AD, modify the 
fuel quantity indicating system (FQIS) to prevent development of an 
ignition source inside the center fuel tank due to electrical fault 
conditions, using a method approved in accordance with the 
procedures specified in paragraph (i) of this AD.

(h) Alternative Actions for Cargo Airplanes

    For airplanes used exclusively for cargo operations: As an 
alternative to the requirements of paragraph (g) of this AD, do the 
actions specified in paragraphs (h)(1) and (2) of this AD, using 
methods approved in accordance with the procedures specified in 
paragraph (i) of this AD. To exercise this alternative, operators 
must perform the first inspection required under paragraph (h)(1) of 
this AD within 6 months after the effective date of this AD. To 
exercise this alternative for airplanes returned to service after 
conversion of the airplane from a passenger configuration to an all-
cargo configuration more than 6 months after the effective date of 
this AD, operators must perform the first inspection required under 
paragraph (h)(1) of this AD prior to further flight after the 
conversion.
    (1) Within 6 months after the effective date of this AD, record 
the existing fault codes stored in the FQIS processor and before 
further flight thereafter do a BITE check (check of built-in test 
equipment) of the FQIS. If any nondispatchable fault code is 
recorded prior to the BITE check or as a result of the BITE check, 
before further flight, do all applicable repairs and repeat the BITE 
check until a successful test is performed with no nondispatchable 
faults found, using a method approved in accordance with the 
procedures specified in paragraph (i) of this AD. Repeat these 
actions thereafter at intervals not to exceed 750 flight hours. 
Modification as specified in paragraph (h)(2) of this AD does not 
terminate the repetitive BITE check requirement of this paragraph.
    (2) Within 72 months after the effective date of this AD, modify 
the airplane by separating FQIS wiring that runs between the FQIS 
processor and the center tank wing spar penetrations, including any 
circuits that might pass through a main fuel tank, from other 
airplane wiring that is not intrinsically safe, using methods 
approved in accordance with the procedures specified in paragraph 
(i) of this AD.

(i) Alternative Methods of Compliance (AMOCs)

    (1) The Manager, Seattle ACO Branch, FAA, has the authority to 
approve AMOCs for this AD, if requested using the procedures found 
in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request 
to your principal inspector or local Flight Standards District 
Office, as appropriate. If sending information directly to the 
manager of the certification office, send it to the attention of the 
person identified in paragraph (j) of this AD. Information may be 
emailed to: 9-ANM-Seattle-ACO-AMOC-Requests@faa.gov.
    (2) Before using any approved AMOC, notify your appropriate 
principal inspector, or lacking a principal inspector, the manager 
of the local flight standards district office/certificate holding 
district office.
    (3) An AMOC that provides an acceptable level of safety may be 
used for any repair, modification, or alteration required by this AD 
if it is approved by The Boeing Company Organization Designation 
Authorization (ODA) that has been authorized by the Manager, Seattle 
ACO Branch, FAA, to make those findings. To be approved, the repair 
method, modification deviation, or alteration deviation must meet 
the certification basis of the airplane, and the approval must 
specifically refer to this AD.

(j) Related Information

    For more information about this AD, contact Jon Regimbal, 
Aerospace Engineer, Propulsion Section, FAA, Seattle ACO Branch, 
2200 South 216th St., Des Moines, WA 98198; phone and fax: 206-231-
3557; email: Jon.Regimbal@faa.gov.

(k) Material Incorporated by Reference

    None.

    Issued on August 26, 2020.
Lance T. Gant,
Director, Compliance & Airworthiness Division, Aircraft Certification 
Service.
[FR Doc. 2020-19809 Filed 9-23-20; 8:45 am]
BILLING CODE 4910-13-P


