DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2016-9301; Product Identifier 2015-NM-193-AD; Amendment
39-19056; AD 2017-19-26]
RIN 2120-AA64

Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), Department of
Transportation (DOT).

ACTION: Final rule.

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SUMMARY: We are superseding Airworthiness Directive (AD) 2008-12-04,
which applied to certain The Boeing Company Model 737-600, -700, -700C,
-800, and -900 series airplanes. AD 2008-12-04 required various
repetitive inspections to detect cracks along the chem-milled steps of
the fuselage skin, and to detect missing or loose fasteners in the area
of a certain preventive modification or repairs; replacement of the
time-limited repair with a permanent repair, if applicable; and
applicable corrective actions which would end certain repetitive
inspections. This AD reduces the post-modification inspection
compliance times, limits installation of the preventive modification to
airplanes with fewer than 30,000 total flight cycles, and adds
repetitive inspections for modified airplanes. This AD was prompted by
an evaluation by the design approval holder (DAH) that indicated that
the upper skin panel at the chem-milled step above the lap joint is
subject to widespread fatigue damage (WFD) if the modification was
installed after 30,000 total flight cycles. We are issuing this AD to
address the unsafe condition on these products.

DATES: This AD is effective October 30, 2017.

The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of October 30,
2017.

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; telephone 562-797-1717; Internet https://www.myboeingfleet.com.
You may view this referenced service information at the FAA, Transport
Standards Branch, 1601 Lind Avenue SW., Renton, WA. For information on
the availability of this material at the FAA, call 425-227-1221. It is
also available on the internet at http://www.regulations.gov by
searching for and locating Docket No. FAA-2016-9301.

Examining the AD Docket

You may examine the AD docket on the Internet at http://www.regulations.gov
by searching for and locating Docket No. FAA-2016-
9301; or in person at the Docket Management Facility between 9 a.m. and
5 p.m., Monday through Friday, except Federal holidays. The AD docket
contains this AD, the regulatory evaluation, any comments received, and
other information. The address for the Docket Office (phone: 800-647-
5527) is Docket Management Facility, 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: Alan Pohl, Aerospace Engineer,
Airframe Section, FAA, Seattle ACO Branch, 1601 Lind Avenue SW.,
Renton, WA 98057-3356; phone: 425-917-6450; fax: 425-917-6590; email:
alan.pohl@faa.gov.

SUPPLEMENTARY INFORMATION:

Discussion

We issued a notice of proposed rulemaking (NPRM) to amend 14 CFR
part 39 to supersede AD 2008-12-04, Amendment 39-15547 (73 FR 32991,
June 11, 2008) (``AD 2008-12-04''). AD 2008-12-04 applied to certain
The Boeing Company Model 737-600, -700, -700C, -800, and -900 series
airplanes. The NPRM published in the Federal Register on November 22,
2016 (81 FR 83745) (``the NPRM''). The NPRM was prompted by an
evaluation by the DAH that indicated that the upper skin panel at the
chem-milled step above the lap joint is subject to WFD if the
modification was installed after 30,000 total flight cycles. The NPRM
proposed to continue to require various repetitive inspections to
detect cracks along the chem-milled steps of the fuselage skin, and to
detect missing or loose fasteners in the area of the preventive
modification or repairs; replacement of the time-limited repair with a
permanent repair, if applicable; and applicable corrective actions
which would end certain repetitive inspections. The NPRM also proposed
to reduce the post-modification inspection compliance times, limit
installation of the preventive modification to airplanes with fewer
than 30,000 total flight cycles, and add repetitive inspections for
modified airplanes. We are issuing this AD to detect and correct
cracking of the upper skin panel at the chem-milled step above the lap
joint, which could result in reduced structural integrity of the
airplane.

Comments

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

Support for the NPRM

Boeing and United Airlines supported the NPRM.

Effect of Winglets on Accomplishment of the Proposed Actions

Aviation Partners Boeing stated that installation of winglets, as
provided in Supplemental Type Certificate (STC) ST00830SE, does not
affect the ability to accomplish the actions proposed in the NPRM.
We agree with the commenter. We have redesignated paragraph (c) of
the proposed AD as (c)(1) 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 Revise Certain Compliance Time Provisions

Southwest Airlines (SWA) asked that we revise certain compliance
language in paragraph (p)(4) of the proposed AD, which stipulated that
post-repair or post-mod inspections be done at the time specified in
the service information or at the time specified in the previously
approved AMOC, ``whichever occurs first.'' SWA stated that previously
approved AMOCs for post-repair or post-modification supplemental
inspections that comply with certain regulations may contain unique
damage tolerance inspection programs that demonstrate a level of safety
equivalent to that of AD 2008-12-04. SWA added that altering those
supplemental inspections to post-repair or post-modification
inspections as specified in Boeing Alert Service Bulletin 737-53A1232,
Revision 3, dated July 27, 2015, when those are done first, could
result in incorrect inspection methods to geometrical structure that
does not conform to the repair or modification definitions specified in
Revisions 1 and 3 of that service information.
We partially agree with the commenter's request. We have determined
that repairs and preventive modifications should be handled separately.
Fleet experience and subsequent analysis of Model 737-200, -200C, -300,
-400, and -500 airplanes, which have similar chem-milled step details,
have shown that certain post-preventative modification inspection
programs may not adequately address the unsafe condition. Therefore,
paragraph (p)(4) of this AD has been changed to remove the language
``preventative modifications'' and remove the reference to the service
information and ``whichever occurs first'' from the compliance time
specified. In addition, we have added paragraph (p)(5) to this AD to
address only the preventive modifications without change to the service
information and ``whichever occurs first'' language.

Request To Retain Certain Exceptions

Additionally, SWA asked that paragraphs (j) and (k) of AD 2008-12-
04 be included in the proposed AD. Paragraph (j) of AD 2008-12-04
provides an allowance for repairs that are FAA-approved and that have a
minimum of three rows of fasteners above and below the chem-milled
step. SWA stated that paragraph (k) of AD 2008-12-04 provides a means
of inspections without an AMOC when an external repair is covering the
chem-milled step, but that the doubler does not span the step by a
minimum of three rows of fasteners above and below the chem-milled
step. SWA added that both paragraphs (j) and (k) of AD 2008-12-04 are
missing from the proposed AD and should be added, with certain
clarifications, to paragraph (j) of the proposed AD. First, the repair
is an external doubler repair. Second, in lieu of the doing the post-
repair supplemental inspections in accordance with table 2 of paragraph
1.E., ``Compliance,'' of Boeing Alert Service Bulletin 737-53A1232, Revision
3, dated July 27, 2015, the inspections should be done in accordance
with 14 CFR 121.1109(c)(2) or 14 CFR 129.109(b)(2) supplemental
inspection requirements, or in accordance with FAA-approved damage
tolerance inspection requirements.
SWA also stated that if paragraphs (j) and (k) of AD 2008-12-04 are
not restated for compliance with existing FAA-approved repairs,
operators will be required to seek AMOC approvals for such existing
repairs prior to the inspection threshold or repeat interval of table 1
of paragraph 1.E., ``Compliance,'' of Boeing Alert Service Bulletin
737-53A1232, Revision 3, dated July 27, 2015. SWA stated that not
including the exceptions in paragraphs (j) and (k) of AD 2008-12-04
could potentially lead to disruption of operations if it is necessary
to request AMOC approvals during repair discovery, or could burden
operators with records research to identify these repairs for AMOC
approvals prior to the required compliance times.
We agree that an allowance can be made for repairs that meet the
criteria specified in paragraph (j) of AD 2008-12-04. These repairs
address the unsafe condition identified in this AD. Therefore, we have
added paragraph (l)(3) to this AD to include the provision of paragraph
(j) of AD 2008-12-04 for repairs that were accomplished before the
effective date of this AD.
We disagree that post-repair inspections for these repairs should
be done in accordance with 14 CFR 121.1109(c)(2) or 14 CFR
129.109(b)(2) supplemental inspection requirements. Post-repair
inspections for repairs that meet the criteria of paragraph (j) of this
AD are to be accomplished in accordance with table 2 of paragraph 1.E.,
``Compliance,'' of Boeing Alert Service Bulletin 737-53A1232, Revision
3, dated July 27, 2015. This is consistent with the DAH's current
recommendation as well as the requirements of paragraph (j) of AD 2008-
12-04. Paragraph (l)(3) of this AD reflects these provisions.
We also disagree with the commenter's request to change the word
``repair'' to ``external doubler repair'' in paragraph (l)(3) of this
AD because we are retaining the provisions of paragraph (j) of AD 2008-
12-04.
We also agree to add certain provisions of paragraph (k) of AD
2008-12-04 to this AD. We have added paragraph (l)(4) to this AD to
address certain repairs as defined in paragraph (k) of AD 2008-12-04.
However, paragraph (l)(4) of this AD does not include a reference to
Boeing Model 737 Non-destructive Test (NDT) Manual, Part 6, Subject 53-
30-20, and instead requires that the inspection be done using FAA-
approved procedures. We have also added Note 1 to paragraph (l)(4) of
this AD to specify that guidance on the inspection specified in
paragraph (l)(4) of this AD can be found in Boeing Model 737 NDT
Manual, Part 6, Subject 53-30-20.

Clarification of Paragraph (i)(1) of This AD

We have revised the language in paragraph (i)(1) of this AD to
clarify which modifications are exempt from the actions required by
paragraph (i)(1) of this AD.

Conclusion

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

Related Service Information Under 1 CFR Part 51

We reviewed Boeing Alert Service Bulletin 737-53A1232, Revision 3,
dated July 27, 2015. This service information describes procedures for
an external detailed inspection and an external nondestructive
inspection (NDI) for cracks in the fuselage skin at chem-milled steps.
Corrective actions include a permanent or time-limited repair, a
preventive modification, and replacement of loose and missing
fasteners. Related investigative actions include internal and external
detailed inspections of the repair area. 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

We estimate that this AD affects 376 airplanes of U.S. registry.
We estimate the following costs to comply with this AD:

Estimated Costs

We estimate the following costs to do any necessary repairs and
replacements that would be required based on the results of the
inspections. We have no way of determining the number of aircraft that
might need these replacements:

On-Condition Costs

We have received no definitive data that would enable us to provide
cost estimates for the related investigative actions, certain repairs,
and other applicable 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.
We are 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 is issued in accordance with authority delegated by the
Executive Director, Aircraft Certification Service, as authorized by
FAA Order 8000.51C. In accordance with that order, issuance of ADs is
normally a function of the Compliance and Airworthiness Division, but
during this transition period, the Executive Director has delegated the
authority to issue ADs applicable to transport category airplanes to
the Director of the System Oversight Division.

Regulatory Findings

We have determined that 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) Is not a ``significant rule'' under DOT Regulatory Policies and
Procedures (44 FR 11034, February 26, 1979),
(3) Will not affect intrastate aviation in Alaska, and
(4) 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

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 removing Airworthiness Directive (AD)
2008-12-04, Amendment 39-15547 (73 FR 32991, June 11, 2008), and adding
the following new AD: