DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
[Docket No. FAA-2019-1071; Project Identifier AD-2019-NM-165-AD;
Amendment 39-21494; AD 2021-07-17]
Airworthiness Directives; The Boeing Company Airplanes
AGENCY: Federal Aviation Administration (FAA), DOT.
ACTION: Final rule.
SUMMARY: The FAA is adopting a new airworthiness directive (AD)
The Boeing Company Model 737-900ER series airplanes. This AD was
prompted by reports of significant corrosion of electrical connectors
located in the main landing gear (MLG) wheel well. This AD requires
repetitive records checks to determine exposure to certain deicing
fluids or repetitive inspections for corrosion of the electrical
connectors, and corrective actions if necessary. The FAA is issuing
this AD to address the unsafe condition on these products.
DATES: This AD is effective May 25, 2021.
The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of May 25,
ADDRESSES: For service information identified in this final rule,
contact Boeing Commercial Airplanes, Attention: Contractual & Data
Services (C&DS), 2600 Westminster Blvd., MC 110-SK57, Seal Beach, CA
90740-5600; telephone 562-797-1717; internet https://www.myboeingfleet.
com. You may view this service information at the
FAA, Airworthiness Products Section, Operational Safety Branch, 2200
South 216th St., Des Moines, WA. For information on the availability of
this material at the FAA, call 206-231-3195. It is also available on
the internet at https://www.regulations.gov by searching for and
locating Docket No. FAA-2019-1071.
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-2019-
1071; 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: Julio C. Alvarez, Aerospace Engineer,
Systems and Equipment Section, FAA, Seattle ACO Branch, 2200 South
216th St., Des Moines, WA 98198; phone and fax: 206-231-3657; email:
The FAA issued a notice of proposed rulemaking (NPRM) to amend 14
CFR part 39 by adding an AD that would apply to all The Boeing Company
Model 737-900ER series airplanes. The NPRM published in the Federal
Register on January 10, 2020 (85 FR 1290). The NPRM was prompted by
reports of significant corrosion of electrical connectors located in
the MLG wheel well. The NPRM proposed to require repetitive records
checks to determine exposure to certain deicing fluids or repetitive
inspections for corrosion of the electrical connectors, and corrective
actions if necessary.
The FAA is issuing this AD to address corrosion and subsequent
moisture ingress that may lead to electrical shorting of the connectors
and incorrect functioning of critical systems necessary for safe flight
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
Air Line Pilots Association, International (ALPA) and two other
commenters supported the NPRM.
Request To Revise Resistance Values
Boeing requested that Boeing Alert Service Bulletin 737-24A1148,
Revision 1, dated July 10, 2003, which incorrectly specified a maximum
electrical bonding resistance of 5 milliohms for aluminum and 10
milliohms for stainless steel, be replaced with Boeing Alert Service
Bulletin 737-24A1148, Revision 2, dated September 14, 2020, which
updates the maximum allowable resistance values to 3 milliohms for both
aluminum and stainless steel, per AWL No. 28-AWL-04, as identified in
Subsection G of Boeing Temporary Revision (TR) 09-020, dated March
2008, to the Boeing 737-600/700/800/900 Maintenance Planning Document
(MPD), D626A001-Certification Maintenance Requirements (CMR), Revision
March 2008. Boeing also advised that operators who have incorporated
Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10,
2003, should restore the fuel quantity indicating system (FQIS)
aluminum and stainless steel connectors to a maximum resistance of 3
milliohms at the next inspection interval per Boeing Alert Service
Bulletin 737-24A1148, Revision 2, dated September 14, 2020. Boeing
observed that the maximum allowable resistance values of 5 milliohms
(aluminum) and 10 milliohms (stainless steel) specified in Boeing Alert
Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003, are
greater than the values specified in AWL No. 28-AWL-04, and that the
proposed AD is in conflict with AD 2008-10-10 R1, Amendment 39-16164
(75 FR 1529, January 12, 2010) (AD 2008-10-10 R1), and AD 2018-20-24,
Amendment 39-19458 (83 FR 51815, October 15, 2018) (AD 2018-20-24),
both of which reference AWL No. 28-AWL-04. ADs 2008-10-10 and 2018-20-24
currently require revising the Airworthiness Limitations (AWLs) section
Instructions for Continued Airworthiness by incorporating new limitations
tank systems to satisfy Special Federal Aviation Regulation No. 88 (in
CFR part 21) for FQIS connectors in unpressurized area. The critical
design configuration control limitations (CDCCLs) require that if the
FQIS connector D4850 is disturbed or repaired, the electrical bonding
resistance value from the backshell to the structure must be 3
milliohms or less.
The FAA agrees with the request to require the revised service
information, which updates the maximum allowable resistance values to
milliohms. These values are consistent with AD 2008-10-10 R1 and AD
2018-20-24 and SFAR No. 88 requirements. The FAA has revised paragraph
(g)(2) of this AD to specify Boeing Alert Service Bulletin 737-24A1148,
Revision 2, dated September 14, 2020, and has also updated the
``Related Service Information under CFR part 51'' paragraph in this
Request To Specify Initial Compliance Time
Delta Air Lines (DAL) suggested that paragraph (g) specify the
compliance time as within 24 months of issuance of the original
certificate of airworthiness or 12 months after the effective date of
this AD, whichever comes later. DAL noted that newer airplanes having
accumulated less than 24 months since the issuance of the original
certificate of airworthiness should not require inspection prior to the
accumulation of 24 months since the issuance of the original
certificate of airworthiness, because these aircraft were in a known
corrosion-free condition upon completion of production. This condition,
DAL asserted, is equivalent to an aircraft that has just been inspected
as required by this AD. DAL maintained that because the repeat interval
after initial inspection given in the proposed rule is 24 months, an
equivalent level of safety would be provided if the initial inspection
were accomplished within 24 months of delivery or 12 months after the
effective date of this AD, whichever occurs later.
The FAA agrees with the request. An affected airplane that has just
received an original airworthiness certificate or original export
certificate of airworthiness is in a condition equivalent to that of
completing the repetitive inspection; therefore, conducting an initial
inspection within 24 months is appropriate to address the concern of
the AD for that airplane. The initial compliance time specified in
paragraph (g) of this AD has been changed to include this information.
Request To Modify Compliance Times
DAL suggested that paragraph (g)(2) of the proposed AD be changed
to add a new paragraph (g)(2)(iv) specifying ``If connectors or
contacts show signs of corrosion or connector resistance measurements
are greater than the specified milliohm limit given in Boeing Alert
Service Bulletin 737-24A1148, Revision 1, accomplish appropriate
corrective action(s) prior to further flight in accordance with Boeing
Alert Service Bulletin 737-24A1148, Revision 1.'' DAL observed that the
applicable times specified in paragraphs (g)(2)(i) through (iii) of the
proposed AD are specific to corrosion of the backshells, and that the
proposed AD does not clearly define corrective actions for the
connectors and contacts, which would also be inspected in paragraph
(g)(2) of the proposed AD. DAL contended that the intended action of
the proposed AD is for discrepant connectors and contacts to have
corrective action accomplished prior to further flight.
The FAA disagrees with the request to modify paragraph (g)(2) of
this AD. The service information adequately specifies when to perform
corrective actions for connectors and contacts. The FAA has not changed
this AD with regard to this request.
Request To Address Discrepancies in NPRM
DAL noted the following discrepancies with Boeing Alert Service
Bulletin 737-24A1148, Revision 1, dated July 10, 2003: The Work
Instructions, paragraph 3.B of the Accomplishment Instructions,
indicate that the accomplishment instructions are divided into two
parts; however, they are actually divided into three parts. Also, part
3 of the Work Instructions does not give the instruction to remove
electrical power from the airplane, yet Figure 5, Step 10, of the ASB
instructs to restore electrical power. DAL noted that if electrical
power was not removed, it cannot be restored. The FAA infers a request
to address the discrepancies in the proposed AD.
The FAA agrees with addressing the stated discrepancies in this AD.
The 57 discrepancies have been rectified in Boeing Alert Service
Bulletin 737-24A1148, Revision 2, dated September 14, 2020, and this AD
has been changed to reference that revision.
Request To Include AMOC Notices
DAL requested that the proposed AD be revised to include references
to two FAA-approved AMOCs to the mandated service information: AMOC
Notice 737-24A1148-AMOC-01, dated May 06, 2013, and FAA AMOC Letter
130S- 09-9, dated April 28, 2009. DAL observed that the AMOCs are
applicable service documentation that should be specifically included
in the final rule to facilitate operators in correctly incorporating
the requirements of the final rule.
The FAA partially agrees with the request. To facilitate operators
in correctly incorporating the requirements of the final rule,
paragraph (i)(4) of this AD has been changed to include reference to
FAA AMOC Letter 130S-09-9, dated April 28, 2009. However, Boeing AMOC
Notice 737-24A1148-AMOC-01, dated May 6, 2013, has been incorporated
into the revised service bulletin that is mandated by this AD and
therefore is not included in paragraph (i)(4) of this AD.
Request To Give Credit for Previous Actions
United Airlines (UAL) generally supported the NPRM and requested
that credit be given for actions previously accomplished in their
maintenance inspection program per their publication EA 2400-01516,
addressing Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated
July 10, 2003. UAL stated it has incorporated inspections of its 737-
900ER fleet starting May 2010, using their publication EA 2400-01516.
The FAA agrees that operators should get credit for performing
actions of Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated
July 10, 2003, referenced in the NPRM. As stated previously, this final
rule has been revised to refer to the latest service bulletin, Boeing
Alert Service Bulletin 737-24A1148, Revision 2, dated September 14,
2020, which contains updated service information to address the unsafe
condition. Operators who previously performed the work before the
effective date of this AD using Boeing Alert Service Bulletin 737-
24A1148, dated December 6, 2001, or Revision 1, dated July 10, 2003,
may receive credit for accomplishment of the initial detailed
inspection specified in paragraph (g)(2) of this AD; however, the
updated FQIS connector D4850 resistance values must be used at the
next repetitive inspection interval, in accordance with Boeing Alert
Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020. The
FAA has added paragraph (h) of this AD to provide credit for Boeing
Alert Service Bulletin 737-24A1148, dated December 6, 2001, and Boeing
Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003.
Effect of Winglets on Accomplishment of the Proposed Actions
Aviation Partners Boeing stated that the installation of winglets
per Supplemental Type Certificate STC ST00830SE does not affect the
accomplishment of the manufacturer's service instructions. Blended
winglets are part of the production type certificate for the 737-900ER
and are not an STC installation.
The FAA agrees with the commenter that STC ST00830SE does not
affect the accomplishment of the manufacturer's service instructions.
Therefore, the installation of STC ST00830SE does not affect the
ability to accomplish the actions required by this AD. The FAA has not
changed this AD as a result of this comment.
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
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
Related Service Information Under 1 CFR Part 51
The FAA reviewed Boeing Alert Service Bulletin 737-24A1148,
Revision 2, dated September 14, 2020. This service information
describes procedures for repetitive records checks to determine
exposure to certain deicing fluids or repetitive inspections for
corrosion of the electrical connectors, and corrective actions if
necessary. This service information is reasonably available because the
interested parties have access to it through their normal course of
business or by the means identified in the ADDRESSES section.
Costs of Compliance
The FAA estimates that this AD affects 346 airplanes of U.S.
registry. The FAA estimates the following costs to comply with this AD:
||Cost per product
||Cost on U.S. operators
|Repetitive records check
||1 work-hour x $85 per hour =
$85 per inspection
||$85 per inspection cycle
||Up to $29,410 per inspection
|Repetitive detailed inspection
||3 work-hours x $85 per hour =
$255 per inspection cycle
||$255 per inspection
||Up to $88,230 per
The FAA estimates the following costs
to do any necessary repairs
or replacements that would be required based on the results of the
inspection. The FAA has no way of determining the number of aircraft
that might need these repairs or replacements:
|Cleaning or replacement
||Up to 5 work-hours x $85 per
hour = Up to $425
||Up to $831
||Up to $1,256
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.
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
(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
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
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]
2. The FAA amends Sec. 39.13 by adding the following new airworthiness