DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2022-1250; Project Identifier AD-2022-00763-T;
Amendment 39-22490; AD 2023-13-05]
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 all
The Boeing Company Model 737-600, -700, -700C, -800, -900, and -900ER
series airplanes. This AD was prompted by a report indicating fuselage
skin lap splice cracking was found between stations (STA) 767 and STA
787, just below S-14R fuselage skin lap splice, where a lower skin
panel buckle intersected the upper skin of the lap splice. Cracking was
also found just below S-14R between STA 747 and STA 767. This AD
requires an inspection for any repair at certain skin lap splices at S-
4, S-14, and S-24 and depending on the configuration, repetitive
inspections for buckling, wrinkling, or bulging at affected skin lap
splices and repair, repetitive inspections for cracking at affected
locations common to fuselage skin on the left and right sides and
repair, and alternative inspections and on-condition actions. The FAA
is issuing this AD to address the unsafe condition on these products.

DATES: This AD is effective October 27, 2023.
The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of October 27,
2023.

ADDRESSES:
AD Docket: You may examine the AD docket at regulations.gov under
Docket No. FAA-2022-1250; 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.
Material Incorporated by Reference:
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; website 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 at
regulations.gov under Docket No. FAA-2022-1250.

FOR FURTHER INFORMATION CONTACT: Owen Bley-Male, Aviation Safety
Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-
231-3992; email: Owen.F.Bley-Male@faa.gov.

SUPPLEMENTARY INFORMATION:

Background

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 737-600, -700, -700C, -800, -900, and -900ER series airplanes.
The NPRM published in the Federal Register on November 9, 2022 (87 FR
67581). The NPRM was prompted by a report indicating fuselage skin lap
splice cracking was found between STA 767 and STA 787, just below S-14R
fuselage skin lap splice, where a lower skin panel buckle intersected
the upper skin of the lap splice. Cracking was also found just below S-
14R between STA 747 and STA 767. In the NPRM, the FAA proposed to
require an inspection for any repair at certain skin lap splices and
depending on the configuration, repetitive inspections for buckling,
wrinkling, or bulging at affected skin lap splices and repair,
repetitive inspections for cracking at affected locations common to
fuselage skin on the left and right sides and repair, and alternative
inspections and on-condition actions. The FAA is issuing this AD to
address cracks, skin buckles, wrinkles, and bulges at fuselage
longitudinal lap splice areas at S-4, S-14 and S-24. This condition, if
not addressed, could result in cracks in fatigue-critical baseline
structure and the inability of a principal structural element to
sustain limit loads, which could adversely affect the structural
integrity of the airplane.

Discussion of Final Airworthiness Directive

Comments

The FAA received a comment from The Air Line Pilots Association,
International, who supported the NPRM without change.
The FAA received additional comments from six commenters, including
Boeing, Aviation Partners Boeing, Delta Air Lines (Delta), Southwest
Airlines (SWA), United Airlines (UAL), and Qantas. The following
presents the comments received on the NPRM and the FAA's response to
each comment.

Effect of Winglets on Accomplishment of the Proposed Actions

Aviation Partners Boeing stated that accomplishing Supplemental
Type Certificate (STC) ST00830SE does not affect the actions specified
in the proposed AD.
The FAA concurs with the commenter. The FAA has redesignated
paragraph (c) of the proposed AD as paragraph (c)(1) of this AD and
added paragraph (c)(2) to this AD to state that installation of STC
ST00830SE does not affect the ability to accomplish the actions
required by this AD. Therefore, for airplanes on which STC ST00830SE is
installed, a ``change in product'' alternative method of compliance
(AMOC) approval request is not necessary to comply with the
requirements of 14 CFR 39.17

Request To Correct Errors in Service Information or Refer to Latest
Service Information

Boeing, Delta, SWA, UAL, and Qantas requested that the NPRM be
revised to correct errors in the referenced service information.
Boeing, SWA, UAL, and Qantas requested that the NPRM be
revised to refer to a new revision of Boeing Special Attention
Requirements Bulletin 737-53-1399 RB, dated May 20, 2022. The
commenters noted that Boeing was drafting a revision to correct several
errors in the service information. Boeing stated that the revision
would modify a table and several illustrations, inspection steps, and
footnotes to clarify the inspection locations, and revise Table 1 in
the Accomplishment Instructions to correctly specify the applicable
airplanes. SWA suggested the revision would clarify the distinction
between the detailed visual and non-destructive testing (NDT)
inspections underneath the lap joint. UAL and SWA noted that certain
figures incorrectly specify to externally inspect an area that is not
visible externally. UAL added that the revision would correct errors in
the figures related to orientations of airplane sections. Delta and
Qantas noted that Table 1 of the Accomplishment Instructions of Boeing
Special Attention Requirements Bulletin 737-53-1399 RB, dated May 20,
2022, includes the compliance actions for Groups 1 through 6 airplanes,
but Table 1 specifies it applies only to Group 1 airplanes. UAL pointed
out that Table 1 in the Compliance section specifies the inspections
are applicable to all airplanes, which contradicts Table 1 in the
Accomplishment Instructions of Boeing Special Attention Requirements
Bulletin 737-53-1399 RB, dated May 20, 2022.
The FAA agrees with the commenters' request. The FAA has reviewed
Boeing Special Attention Requirements Bulletin 737-53-1399 RB, Revision
1, dated March 14, 2023, which corrects the errors noted by the
commenters. Boeing Special Attention Requirements Bulletin 737-53-1399
RB, Revision 1, dated March 14, 2023, also combines actions for several
conditions to remove duplicative instructions and revises related
figures accordingly. These revisions allow operators to do all
applicable actions for their airplane models without obtaining an AMOC,
and do not add additional work. The FAA has revised this AD to refer to
Boeing Special Attention Requirements Bulletin 737-53-1399 RB, Revision
1, dated March 14, 2023.

Request To Clarify Inspection Locations

Boeing requested that the FAA revise the Related Service
Information Under 1 CFR part 51 section of the NPRM to clarify the
inspection locations, including whether they are on the right (R) or
left (L) side of the airplane, and applicable airplane models for each
inspection location. Boeing noted that the locations specified in the
NPRM don't correspond to the service information due to errors in
Boeing Special Attention Requirements Bulletin 737-53-1399 RB, dated
May 20, 2022.
The FAA agrees to revise the Related Service Information Under 1
CFR part 51 section for the reasons provided. The FAA has revised this
section as requested.

Request To Clarify Root Cause of Cracking

Boeing asked that the FAA change a sentence in the Background
section of the NPRM to specify that the skin lap splice cracking ``may
have been the result of incorrect procedures . . .'' rather than ``was
the result of incorrect procedures . . .'' Boeing noted that it has
always stated that the root cause of cracking is unknown.
The FAA agrees that the suggested wording provides clarification on
the root cause. However, the sentence in question does not get carried
over to the final rule. This AD was not changed regarding this issue.

Request To Clarify Number of Reports

Boeing requested that the FAA revise a sentence in the Background
section of the NPRM to specify that the FAA ``received a report from
Boeing'' rather than ``received reports'' indicating fuselage skin
cracking was found. Boeing noted that it had only one report of crack
findings, with only one airplane with cracks reported.
The FAA agrees that the suggested wording is more accurate.
However, the sentence in question does not get carried over to the
final rule. This AD was not changed regarding this issue.

Request To Remove Reference to Widespread Fatigue Damage (WFD)

Boeing requested that the FAA revise the proposed AD to remove
reference to WFD. Boeing explained that, although there was one report
of multi-site damage within two bays, it does not consider this
cracking to be a WFD issue. Boeing added that, based on its reviews of
similar manufactured skins on other models of Boeing airplanes, these
skins are not susceptible to WFD. Additionally, Boeing proposed revised
language for the unsafe condition sentence in the proposed AD.
The FAA agrees with the commenter's request. After further review,
the FAA has determined that a single crack finding is not enough to
conclusively indicate that the unsafe condition is related to WFD.
However, an unsafe condition still exists because cracks in fatigue-
critical baseline structure and the inability of a principal structural
element to sustain limit loads could adversely affect the structural
integrity of the airplane. The FAA has revised the SUMMARY, Background
section, and paragraph (e) of this AD to remove reference to WFD, and
revised the Background section and paragraph (e) of this AD to clarify
the unsafe condition.

Request To Define Skin Buckles, Wrinkles, and Bulges

Qantas requested that the proposed AD be revised to provide more
precise definitions for skin buckles, wrinkles, and bulges, including
size, orientation, origin, and depth as well as whether wrinkling is
permanent or temporary. Qantas noted that skin panels have minor
imperfections due to production assembly processes, but those aren't
skin buckles, wrinkles, or bulges. Qantas states that what constitutes
a skin buckle, wrinkle, or bulge could be open to interpretation,
causing operators to perform unnecessary NDT inspection. The commenter
noted that it found a Boeing definition of a buckle and a Boeing
Service Letter that states wrinkling is temporary and will have no
detrimental long-term effect unless it becomes more severe or
permanent. The commenter added that the Boeing Service Letter suggests
that you'll find wrinkling in the vicinity of the inspection area.
Qantas suggested this would lead to unnecessary repeat inspection on
unrepaired areas and cause an undue burden.
The FAA acknowledges the commenter's concern. This terminology
reflects the suspected root cause of the discovered cracking, and
possible precursors to future cracks. This terminology also reflects
commonly used verbiage for possible structural damage, so further
explanation is unnecessary. However, the FAA notes that Boeing Special
Attention Requirements Bulletin 737-53-1399 RB, Revision 1, dated March
14, 2023, states that areas of loose paint, discoloration, loose
fasteners, lap joint separation, or disturbed sealant can be indicative
of areas where a skin buckle, wrinkle, or bulge has occurred. Skin
buckles, wrinkles, or bulges could lead to cracks that may result in
the inability of a principal structural element to sustain limit load,
which could result in reduced structural integrity of the airplane and
lead to a decompression event. Therefore, the follow-on inspections for
cracks are necessary if any skin buckles, wrinkles, or bulges are
found, regardless of size, origin, depth, or other factors. This AD was
not changed regarding this issue.

Request To Revise Certain Compliance Times

Qantas noted that, for certain conditions, the NDT inspections must
be repeated every 600 flight cycles, regardless whether any buckle,
wrinkle, or bulge is found, which is burdensome for operators. The FAA
infers the commenter is requesting an extension of the compliance time.
The FAA disagrees with the commenter's request. In developing the
compliance times for this AD, the FAA considered the recommendations of
the manufacturer as well as the urgency associated with the subject
unsafe condition. However, under the provisions of paragraph (i) of
this AD, the FAA will consider requests for approval of an extension of
the compliance time if sufficient data are submitted to substantiate
that the new compliance time would provide an acceptable level of
safety. This AD was not changed regarding this issue.

Request To Clarify Paint Thickness for Certain Inspections

SWA requested clarification regarding the acceptable level of paint
thickness for certain inspections. SWA noted that figures 17 through 19
and 28 through 35 in Boeing Special Attention Requirements Bulletin
737-53-1399 RB, dated May 20, 2022, indicate that subsequent NDT
inspections can be accomplished with paint in place. SWA added that per
the 737 NDT Manual Part 4, 53-30-06, the procedure can only be used
with paint up to 0.007 inches thick. SWA suggested adding this guidance
in a note, since the repetitive inspections occur every 600 flight
cycles.
The FAA notes that the suggested clarification has been added to a
note in the applicable figures in Boeing Special Attention Requirements
Bulletin 737-53-1399 RB, Revision 1, dated March 14, 2023. As
previously noted, this AD has been revised to refer to Boeing Special
Attention Requirements Bulletin 737-53-1399 RB, Revision 1, dated March
14, 2023. Therefore, no further change to this AD is necessary
regarding this issue.

Conclusion

The FAA reviewed the relevant data, considered any comments
received, and determined that air safety requires adopting this AD as
proposed. Accordingly, the FAA is issuing this AD to address the unsafe
condition on these products. Except for minor editorial changes, and
any other changes described previously, this AD is adopted as proposed
in the NPRM. None of the changes will increase the economic burden on
any operator.

Related Service Information Under 1 CFR Part 51

The FAA reviewed Boeing Special Attention Requirements Bulletin
737-53-1399 RB, Revision 1, dated March 14, 2023. This service
information specifies procedures for a general visual inspection for
any repair, any buckle, any wrinkle, any bulge, and any cracking at
skin lap splice at stringers S-4R (737-800 Boeing Converted Freighter
airplanes only), S-14L, S-14R, and S-24R (737-600, -700, -700C, 800, -
900, and -900ER series airplanes only). This service information also
describes procedures, depending on the configuration, for repetitive
detailed inspections for buckling, wrinkling, or bulging at unrepaired
areas of affected lap splices, and repair; repetitive detailed, high
frequency eddy current (HFEC), and ultrasonic (UT) inspections for
cracking at affected locations common to fuselage skin on the left and
right sides, and repair; and alternative inspections and on-condition
actions. 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 ADDRESSES.

Costs of Compliance

The FAA estimates that this AD affects 2,462 airplanes of U.S.
registry. The FAA estimates the following costs to comply with this AD:

Estimated Costs

The FAA has received no definitive data on which to base the cost
estimates for the on-condition repairs or for the alternative
inspections and on-condition actions specified in this AD.

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.

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
directive: