DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2015-6541; Directorate Identifier 2015-NM-135-AD;
Amendment 39-18581; AD 2016-13-16]
RIN 2120-AA64

Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: We are 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 reports of a manufacturing
oversight, in which a supplier omitted the required protective finish
on certain bushings installed in the rear spar upper chord on
horizontal stabilizers, which could lead to galvanic corrosion and
consequent cracking of the rear spar upper chord. This AD requires an
inspection or records check to determine if affected horizontal
stabilizers are installed, related investigative actions, and for
affected horizontal stabilizers, repetitive inspections for any crack
of the horizontal stabilizer rear spar upper chord, and corrective
action if necessary. We are issuing this AD to detect and correct
cracking of the rear spar upper chord, which can result in the failure
of the upper chord and consequent departure of the horizontal
stabilizer from the airplane, which can lead to loss of control of the
airplane.

DATES: This AD is effective August 12, 2016.
The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of August 12,
2016.

ADDRESSES: For service information identified in this final rule,
contact Boeing Commercial Airplanes, Attention: Data & Services
Management, P.O. Box 3707, MC 2H-65, Seattle, WA 98124-2207; telephone
206-544-5000, extension 1; fax 206-766-5680; Internet https://www.
myboeingfleet.com. You may view this referenced service information
at the FAA, Transport Airplane Directorate, 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-2015-
6541.

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-2015-
6541; 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: Gaetano Settineri, Aerospace Engineer,
Airframe Branch, ANM-120S, FAA, Seattle Aircraft Certification Office
(ACO), 1601 Lind Avenue SW., Renton, WA 98057-3356; phone: 425-917-
6577; fax: 425-917-6590; email: gaetano.settineri@faa.gov.

SUPPLEMENTARY INFORMATION:

Discussion

We 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 30, 2015 (80 FR
74726) (``the NPRM''). The NPRM was prompted by reports of a
manufacturing oversight, in which a supplier omitted the required
protective finish on certain bushings installed in the rear spar upper
chord on horizontal stabilizers, which could lead to galvanic corrosion
and consequent cracking of the rear spar upper chord. The NPRM proposed
to require an inspection or records check to determine if affected
horizontal stabilizers are installed, related investigative actions,
and for affected horizontal stabilizers, repetitive inspections for any
crack of the horizontal stabilizer rear spar upper chord, and
corrective action if necessary. We are issuing this AD to detect and
correct cracking of the rear spar upper chord, which can result in the
failure of the upper chord and consequent departure of the horizontal
stabilizer from the airplane, which can lead to loss of control 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

Air Line Pilots Association International (ALPA) stated that it
supports the NPRM. Boeing stated that is concurs with the NPRM.

Effect of Winglets on Accomplishment of the Proposed Actions

Aviation Partners Boeing stated that installation of winglets per
Supplemental Type Certificate (STC) ST00830SE (http://rgl.faa.gov/
Regulatory_and_Guidance_Library/rgstc.nsf/0/
184de9a71ec3fa5586257eae00707da6/$FILE/ST00830SE.pdf) does not affect
the ability to accomplish the actions specified in the NPRM.
We concur with the commenter. We have redesignated paragraph (c) of
the proposed AD as paragraph (c)(1) and added new paragraph (c)(2) to
this AD to state that installation of STC ST00830SE (http://
rgl.faa.gov/Regulatory_and_Guidance_Library/rgstc.nsf/0/
184de9a71ec3fa5586257eae00707da6/$FILE/ST00830SE.pdf) does not affect
the ability to accomplish the actions required by this final rule.
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 the Proposed Applicability

Airlines for America (A4A) requested that we revise the
applicability of the proposed AD to state ``This AD applies to all
horizontal stabilizers with serial numbers identified in Boeing SB 737-
55A1097.'' A4A explained that the proposed AD is applicable to all
Boeing Model 737-600, -700, -700C, -800, -900, and -900ER series
airplanes; however, Boeing Alert Service Bulletin 737-55A1097, dated
July 1, 2015, provides a list of affected horizontal stabilizers by
serial number. A4A expressed that the physical plate inspections
required by paragraph (g)(1)(ii) of the proposed AD are excessive and
unneeded, as operators normally track serialized components without the
need to physically inspect the airframe. A4A further reasoned that when
paragraph (c) of the proposed AD is written against all Model 737 Next
Generation airframes, the complexity of compliance reporting becomes
more burdensome. The net result, stated A4A, is indefinite record
keeping of AD compliance for airplanes that are not equipped with
horizontal stabilizers affected by the manufacturing oversight.
We do not agree to revise the applicability of this AD as requested
by the commenter. Paragraph (g)(1) of this AD gives operators the
option of performing either a records check or an inspection. If the
operator's records are sufficient to determine the serial number of the
horizontal stabilizers on the affected airplane, then a physical
inspection is not required. Furthermore, the affected horizontal
stabilizers are rotable parts, so it is possible that an affected
horizontal stabilizer could be installed on numerous airplanes during
its service life, even on a new production airplane once it enters
service. As specified in paragraph 2.B.(2) of Chapter 6 of the AD
Manual, FAA-IR-M-8040.1C (http://rgl.faa.gov/
Regulatory_and_Guidance_Library/rgOrders.nsf/0/
66ddd8e1d2e95db3862577270062aabd/$FILE/FAA-IR-M-8040_1C.pdf), when the
unsafe condition results from the installation of the appliance or part
on an aircraft, the AD action is issued against the aircraft, not the
appliance or part. Therefore, we have determined that it is appropriate
for this AD to apply to all airplanes of the specified model types. We
have made no changes to the applicability of this AD.

Request To Allow Removal and Replacement of Affected Horizontal
Stabilizers

A4A requested that we revise paragraph (h)(2) of the proposed AD to
allow removal of an affected horizontal stabilizer, and replacement
with an unaffected or an affected horizontal stabilizer that is within
the parameters of paragraph 1.E. ``Compliance,'' of Boeing Alert
Service Bulletin 737-55A1097, dated July 1, 2015. A4A explained that
paragraph (g)(2) of the proposed AD requires that the inspection
specified in Boeing Alert Service Bulletin 737-55A1097, dated July 1,
2015, be accomplished on any horizontal stabilizer found to be within
the effectivity of Boeing Alert Service Bulletin 737-55A1097, dated
July 1, 2015, and the compliance times found in paragraph 1.E.,
``Compliance.'' A4A expressed that if cracking is found, operators must
repair in accordance with paragraph (h)(2) of the proposed AD;
paragraph (h)(2) of the proposed AD requires repair in accordance with
paragraph (j) of the proposed AD before further flight.
We agree. We have determined that removing a damaged horizontal
stabilizer and replacing it with a serviceable horizontal stabilizer,
as provided in paragraph (i) of this AD, addresses the identified
unsafe condition. We have revised paragraph (h)(2) of this AD
accordingly.

Request for Review of Other Inspection Methods

A4A requested that the FAA and Boeing review other non-destructive
test (NDT) inspection options such as an ultrasound process to satisfy
the proposed inspection requirements. A4A pointed out that paragraph
(g)(2) of the proposed AD specifies a high frequency eddy current
(HFEC) method for inspection of the rear spar upper chord. A4A
explained that the FAA should be aware that other methods, specifically
ultrasound inspection, may be better NDT diagnostic techniques, and
that an ultrasound inspection, compared to the proposed HFEC process,
may detect early crack development from the fitting holes versus
cracking that has propagated up to and near the surface of the rear
spar upper chord.
We partially agree. We agree with the commenter that other
inspection methods may be better NDT diagnostic techniques and note
that alternative methods of compliance (AMOCs) have been granted to ADs
when updated service information containing improved procedures to
address an unsafe condition becomes available.
We disagree to include other inspection options in this final rule,
because the inspection technique required in this AD adequately
addresses the unsafe condition and is accompanied by service
information, which includes detectable crack lengths and inspection
intervals. If additional service information that provides alternative
NDT inspection methods becomes available, under the provisions of
paragraph (j) of this AD, we will consider requests for approval of an
AMOC if sufficient data are submitted to substantiate that the
inspection method would provide an acceptable level of safety. We have
made no changes to this AD in this regard.

Requests for Clarification of Parts Installation Requirements

A4A requested that we reword paragraphs (g) and (i) of the proposed
AD to allow operators to maintain or install any affected horizontal
stabilizer on any airplane, provided that the horizontal stabilizer is,
or will be, inspected as specified in paragraph 1.E., ``Compliance,''
of Boeing Alert Service Bulletin 737-55A1097, dated July 1, 2015. A4A
explained that paragraphs (i)(1) and (i)(2) of the proposed AD preclude
installation of an affected horizontal stabilizer without accomplishing
the required inspection. A4A explained further that other maintenance
activity could cause a horizontal stabilizer to be removed and
reinstalled prior to reaching the compliance times specified in Boeing
Alert Service Bulletin 737-55A1097, dated July 1, 2015. With the
potential interpretation of paragraph (g)(2) of the proposed AD being
to inspect immediately, the initial inspection would be significantly
accelerated, and the inspection schedule would be altered for the
remaining life of the component.
All Nippon Airways (ANA) requested that we clarify the parts
installation restrictions specified in paragraph (i) of the proposed AD
to reduce the burden for operators. ANA explained that parts
installation is restricted based on its serial number, and that
paragraph (i)(2)(i) of the proposed AD requires initial inspection
specified in paragraph (g)(2) of the proposed AD before further flight.
ANA expressed that this requirement is applicable if the flight cycles
and/or the date of issuance the original certificate of airworthiness,
or the original export certificate of airworthiness for the horizontal
stabilizer are unknown or have already exceeded the proposed compliance
time specified in paragraph (g)(2) of the proposed AD. ANA reasoned
that, if the flight cycles and the date of issuance of the original
certificate of airworthiness or the original export certificate of
airworthiness of the horizontal stabilizer are known, and the flight
cycles and years on the horizontal stabilizer are less than the
compliance times specified in paragraph 1.E., ``Compliance,'' of Boeing
Alert Service Bulletin 737-55A1097, dated July 1, 2015, operators may
conduct the inspection specified in paragraph (g)(2) of this AD at the
time specified in paragraph (g)(2) of this AD.
We agree to clarify. An affected horizontal stabilizer that has not
reached the inspection threshold or the next repeat interval is still
in compliance with this AD at the time it is installed on the airplane.
We have revised paragraph (i)(2)(i) of this AD to read ``Initial and
repetitive HFEC inspections specified in paragraph (g)(2) of this AD
are completed within the compliance times specified in paragraph (g)(2)
of this AD.'' We also agree to clarify that the 10-year compliance time
specified in paragraph 1.E. ``Compliance,'' of Boeing Alert Service
Bulletin 737-55A1097, dated July 1, 2015, is measured using the
airplane the affected horizontal stabilizer was delivered on.

Request for Specific Repair Instructions and Terminating Action

A4A requested that repair instructions be provided either in a
revision to the service information, or via the structural repair
manual (SRM). A4A also requested that the proposed AD be revised to
include a preventive, terminating action including the option to remove
and replace the subject bushings in the upper chord fitting during a
heavy check schedule. A4A expressed that the NPRM and Boeing Alert
Service Bulletin 737-55A1097, dated July 1, 2015, provide neither
specific repair methods nor a means to terminate the inspections. A4A
reasoned that the NPRM requires corrective action for any crack that is
discovered, and that such action is to be performed in accordance with
paragraph (j) of the proposed AD, which is the AMOC section. A4A said
that, although no known inspections have revealed cracking, we (the
FAA) must believe that findings will occur, and that operators would
benefit by having guidance from Boeing without the need for an AMOC
request. Similarly, A4A expressed, without a repair plan, there should
also be a means of terminating the inspections entirely. A4A pointed to
a recent experience concerning seat track cracking that exposed the
difficulties of embarking upon a required inspection plan without a
defined recovery path. A4A referred to AD 2013-23-04, Amendment 39-
17659 (78 FR 68693, November 15, 2013) (``AD 2013-23-04''), and stated
that AD also directed operators to the AMOC process.
We do not agree. An AD is issued to address an identified unsafe
condition, as required by 14 CFR part 39. The determination of the
unsafe condition, mitigating action, and compliance times in this AD
has all been coordinated with Boeing. This AD is being issued to
address the lack of corrosion protection on a critical structural
element. As a result, dissimilar metal corrosion may cause cracking of
the horizontal stabilizer rear spar upper chord. With no service
history of cracking yet reported, it is expected that any cracking will
be limited and not result in a significant disruption to affected
operators. The inspections required by this AD provide an acceptable
level of safety for the affected airplanes. We have reviewed with
Boeing the implementation issues associated with AD 2013-23-04 and
expect that Boeing will provide us with approvable data for repair and
terminating actions in a timely manner to address any cracking found.
For these reasons, we do not consider that delaying this action
until after the possible release of revised service information is
warranted, since sufficient technology and service information
currently exist to accomplish the required actions within the
compliance time. However, under the provisions of paragraph (j) of this
AD, we will consider requests for approval of AMOCs for revised service
information, repairs, or terminating actions if sufficient data are
submitted to substantiate they would provide an acceptable level of
safety. For these reasons, we have made no changes to this AD in this
regard.

Request To Clarify Specific Parts of the Service Information

ANA stated that paragraph (g)(1)(i) of the proposed AD should refer
to Part 1, and paragraph (g)(1)(ii) of the proposed AD should refer to
Part 2 of the Accomplishment Instructions of Boeing Alert Service
Bulletin 737-55A1097, dated July 1, 2015. ANA did not provide a reason
for this request.
From these statements, we infer that ANA is requesting that we
revise paragraphs (g)(1)(i) and (g)(1)(ii) of the proposed AD. We agree
that the changes requested by ANA provide additional clarity. We have
added ``Part 1 of'' to paragraph (g)(1)(i) and ``Part 2 of'' to
paragraph (g)(1)(ii) of this AD.

Request for Assurance of Parts Availability

A4A also requested that, prior to release of the AD, we assure that
Boeing has sections of the rear spar available for the horizontal
stabilizer including a typical splice repair plan for each affected
737-NG fleet. A4A also requested that Boeing also provide or have
available, horizontal stabilizers that are service ready prior to the
release of the AD.
We do not agree. We do not consider that delaying this action until
Boeing has assured that replacement parts will be available is
warranted. This AD is issued to address an identified unsafe condition,
as required by 14 CFR part 39. The determination of the unsafe
condition, mitigating action, and compliance times in this AD has all
been coordinated with Boeing. This AD is being issued to address the
lack of corrosion protection on a critical structural element. As a
result, dissimilar metal corrosion may cause cracking of the horizontal
stabilizer rear spar upper chord. With no service history of cracking
yet reported, it is expected that any cracking will be limited and not
be a significant disruption to affected operators. We understand that
Boeing will make horizontal stabilizer parts and assemblies available
as necessary for operators to address possible on-condition actions.
However, since it is unknown how many repairs or replacements may be
necessary and what parts would be necessary for each repair, we cannot
estimate the type and number of parts needed. If parts availability
becomes an issue, under the provisions of paragraph (j) of this AD, we
may approve requests for adjustments to the compliance time for doing a
repair or replacement if data are submitted to substantiate that such
an adjustment would provide an acceptable level of safety. We have made
no changes to this AD in this regard.

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:
[Agr]re 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 737-55A1097, dated July
1, 2015. The service information describes procedures for an inspection
or records check to determine if affected horizontal stabilizers are
installed, related investigative actions, HFEC inspections for any
crack of the horizontal stabilizer rear spar upper chord, and
corrective action 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

We estimate that this AD affects 1,397 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 inspections
that would be required based on the results of the inspection or
records check. We have no way of determining the number of aircraft
that might need these inspections:

On-Condition Costs

According to the manufacturer, some of the costs of this AD may be
covered under warranty, thereby reducing the cost impact on affected
individuals. We do not control warranty coverage for affected
individuals. As a result, we have included all costs in our cost
estimate.
We have received no definitive data that would enable us to provide
cost estimates for the on-condition repairs 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.

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) 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 adding the following new airworthiness
directive (AD):