DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2018-0393; Product Identifier 2018-NM-010-AD; Amendment
39-19536; AD 2018-26-06]
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 loose, worn, or
missing attachment bolts for the main landing gear (MLG) center door
assemblies. This AD requires repetitive detailed inspections of the
forward and aft MLG center door assembly attachments for loose,
missing, damaged, or bottomed-out attachment bolts, and any wear to the
retention clip assemblies as applicable; and applicable on-condition
actions. This AD also provides an
optional terminating action for the repetitive inspections. We are
issuing this AD to address the unsafe condition on these products.

DATES: This AD is effective February 1, 2019.

The Director of the Federal Register approved the incorporation by
reference of a certain publication listed in this AD as of February 1, 2019.

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, Transport
Standards 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 http://www.regulations.gov
by searching for and locating Docket No. FAA-2018-
0393.

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-2018-
0393; 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, the regulatory evaluation, any comments received, and
other information. The address for Docket Operations (phone: 800-647-
5527) 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: Alan Pohl, Aerospace Engineer,
Airframe Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des
Moines, WA 98198; phone and fax: 206-231-3527; email:
alan.pohl@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 May 11, 2018 (83 FR
21948). The NPRM was prompted by reports of loose, worn, or missing
attachment bolts for the MLG center door assemblies. The NPRM proposed
to require repetitive detailed inspections of the forward and aft MLG
center door assembly attachments for loose, missing, damaged, or
bottomed-out attachment bolts, and any wear to the retention clip
assemblies as applicable; and applicable on-condition actions. The NPRM
also provided an optional terminating action for the repetitive inspections.

We are issuing this AD to address loose, missing, damaged, or
bottomed-out attachment bolts, and any wear to the retention clip
assemblies, which could result in departure of the center and inboard
MLG door assemblies, subsequent damage to the main flap and horizontal
stabilizer, and loss of control of the airplane.

Comments

We 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

Boeing and The Air Line Pilots Association, International, each
stated that it concurred with the intent of the NPRM.

Request for Changes to Service Information

Alaska Airlines (Alaska) requested that changes be made to Boeing
Special Attention Service Bulletin 737-52-1170, Revision 1, dated
December 19, 2017 ("BSASB 737-52-1170, R1"). Alaska noted that
operators cannot comply with the requirements specified in paragraph
(g) of the proposed AD in cases where BSASB 737-52-1170, R1, directs
the operator to inspect a Group 3 airplane using Figure 3 or Figure 4
of BSASB 737-52-1170, R1, because those figures are not applicable to
Group 3 airplanes.

We agree with the commenter's observations concerning Figure 3 and
Figure 4 of BSASB 737-52-1170, R1. We contacted Boeing and have
determined that the actions for Group 2 airplanes are appropriate for
all airplanes to comply with the requirements of paragraph (g) of this
AD. We have revised paragraph (h) of this AD, "Exceptions to Service
Information Specifications," by adding paragraph (h)(2), which states
that "Where BSASB 737-52-1170, R1, limits use of Figures 3 and 4 to
Group 2 airplanes, for the purposes of this AD, those figures apply to
all airplane groups."

Request for Clarification of the Requirements of Paragraph (j) of the
Proposed AD

Alaska requested clarification of the requirements specified in
paragraph (j) of the proposed AD and clarification of which airplane
groups would be affected by these requirements. Alaska asked if "all
actions for Group 3" means that this paragraph is for Group 3
airplanes only or for all airplane groups. Alaska also noted that an
inspection of the "door assembly" implies an inspection of the door,
but BSASB 737-52-1170, R1, describes procedures for inspection of the
"door installation."

We agree with the commenter's request and have revised paragraph
(j) of this AD as follows:

As of the effective date of this AD, no person may install an
MLG assembly or MLG center door assembly on any airplane identified
in paragraphs (c)(1) through (c)(4) of this AD unless all actions
for Group 3 airplanes pertaining to that MLG center door attachment,
and identified as RC in, and in accordance with, the Accomplishment
Instructions of BSASB 737-52-1170, R1, have been accomplished on
that MLG assembly or MLG center door assembly within the compliance
times specified in Tables 4, 5, and 6, as applicable, of paragraph
1.E., "Compliance," of BSASB 737-52-1170, R1. The actions for
Group 3 airplanes apply to all airplanes for the requirement of this
paragraph.

Request To Include Identification Method for Post-Modification Door

American Airlines (American) requested that BSASB 737-52-1170, R1,
be revised to include an identifying stencil or placard that could be
placed on an affected MLG center door assembly once it has been
modified. The commenter stated that the MLG center door assembly is a
rotable part. However, neither the NPRM nor BSASB 737-52-1170, R1,
addresses the issue of a post-modification MLG center door assembly
being removed from an airplane and replaced with a pre-modification MLG
center door assembly.

We acknowledge the commenter's concern that BSASB 737-52-1170, R1,
does not address the rotability of an MLG center door assembly. We
addressed the issue of rotability in this AD in two ways. First, the
applicability in paragraph (c) of this AD includes all Model 737-600, -
700, -700C, -800, -900, and -900ER series airplanes, not just the
airplanes specified in the effectivity of BSASB 737-52-1170, R1.
Second, we added paragraph (j) of this AD, "Parts Installation Limitation".

While marking or part marking might provide some benefit for
operator awareness and recordkeeping, the issue
of rotability is approached in different ways by different operators.
When there have been similar issues regarding rotable parts, operators
expressed a preference to not have a requirement to mark and/or part
mark, although operators may do this at their own discretion. We also
note that this AD addresses not only the MLG center door assembly but
also the attachments to the MLG strut assemblies. We have not changed
this AD in regard to this issue.

Request To Extend the Compliance Time

Delta Air Lines (DAL) and SunExpress (SXS) requested that the
compliance time specified in paragraph (g) of the proposed AD be
extended. SXS requested that the compliance time for the initial
inspection be extended from 12,000 total flight cycles to 20,000 total
flight cycles, or from 800 flight cycles after the effective date of
the proposed AD to 1,500 flight cycles after the effective date of the
proposed AD, and that the interval for the repetitive inspection be
extended from 5,500 flight cycles to 6,600 flight cycles. SXS stated
that 41 airplanes in its fleet have exceeded 12,000 total flight
cycles, and it would have a short period of time to perform the
required inspection as described in BSASB 737-52-1170, R1, and it would
have to operate some airplanes a long time without the MLG shock strut
doors. SXS noted that performing operations without a MLG shock strut
door incurs a fuel burn penalty, which is approximately 0.77% more fuel
burned per flight.

DAL stated that the compliance time for the initial inspection
would require them to inspect approximately 80 airplanes in a 200-day
period, requiring them to accomplish the work for most of its airplanes
in the line environment, which increases the risk for an "airplane on
ground" situation if there is a finding on the MLG structure. DAL
noted that BSASB 737-52-1170, R1, does not provide relief for operators
when a crack or corrosion is found in the MLG lug after removal of the
bushing. For an MLG that requires re-work, DAL typically removes the
MLG, replaces it with another MLG, and sends the discrepant MLG to a
shop for repair. We infer that DAL is requesting that the compliance
time for the initial inspection be extended.

In addition, DAL pointed out that the procedures in BSASB 737-52-
1170, R1, would restrict an operator from dispatching an affected
airplane until corrective is taken to repair the MLG. DAL requested
that this situation be considered in the final rule by providing a
limited return to service. DAL stated that if several MLG lugs are
found with discrepancies, there is the potential of the operator
grounding airplanes outside of a heavy maintenance check to either
replace the MLG gear or go through its spare parts inventory.

We do not agree with the commenters' requests. We appreciate the
impact that the required actions and the associated compliance times
will have on operators. However, both the FAA and Boeing have
identified this issue as an unsafe condition, and the commenters have
not provided substantiating data for their proposals. In addition, a
limited return to service would not be appropriate for dispatching
airplanes with known cracking or corrosion. As SXS has noted, airplanes
may be operated with the MLG shock strut center and inner doors removed
until repairs can be made.

We have reviewed the related service information and note that
while repair of the MLG lug parts is required for compliance ("RC"),
certain steps are either labeled as "RC Exempt," or contain technical
instructions that are prefaced by "Refer to." Paragraph (l)(4) of
this AD and paragraph 3A., "General Information," of BSASB 737-52-
1170, R1 specify the actions labeled as "RC Exempt" are not required
in order to show compliance to this AD. When the words "refer to" are
used within an RC step and the operator has an accepted alternative
procedure, the accepted alternative procedure can be used. When the
words "in accordance with" are included in an RC step, the procedure
in the Boeing document must be used. In addition, for proposals that
provide an acceptable level of safety and have substantiating data,
operators may apply for an AMOC using the procedures specified in
paragraph (l) of this AD. We have not changed this AD in regard to this issue.

Request To Allow Installation of New, Overhauled, or Serviceable MLG

DAL requested that operators be allowed to install a new,
overhauled, or serviceable MLG instead of repairing a damaged lug and
installing a new bushing if excessive wear, galling, or cracking is
found during a detailed inspection/measurement of the MLG shock strut
bushing. DAL stated that it would have to remove the damaged MLG and
send it to the shop for repair, and it would be easier to install a
new, overhauled, or serviceable MLG than to wait for the damaged MLG to
be repaired. DAL explained that installing a new, overhauled, or
serviceable MLG provides an equivalent level of safety because the
intent of the proposed AD is to repair and install new bushings. DAL
observed that the damaged MLG would be repaired and then be ready for
use on another airplane.

We agree with the commenter's request for the reasons provided by
the commenter. We have revised paragraph (g) of this AD to clarify that
replacement of an entire MLG assembly within the required compliance
time satisfies the requirements of paragraph (g), provided that the
requirements of paragraph (j) of this AD, "Parts Installation
Limitation," are satisfied for that MLG assembly.

Since the unsafe condition is also affected by rotability, we have
revised paragraph (j) of this AD to clarify that an MLG assembly cannot
be installed on any airplane identified in paragraphs (c)(1) through
(c)(4) of this AD unless all actions for Group 3 airplanes have been
accomplished on the MLG assembly. Paragraph (j) of this AD states that:

As of the effective date of this AD, no person may install an
MLG assembly or MLG center door assembly on any airplane identified
in paragraphs (c)(1) through (c)(4) of this AD unless all actions
for Group 3 airplanes pertaining to that MLG center door attachment,
and identified as RC in, and in accordance with, the Accomplishment
Instructions of BSASB 737-52-1170, R1, have been accomplished on
that MLG assembly or MLG center door assembly within the compliance
times specified in Tables 4, 5, and 6, as applicable, of paragraph
1.E., "Compliance," of BSASB 737-52-1170, R1. The actions for
Group 3 airplanes apply to all airplanes for the requirement of this paragraph.

In the proposed AD, paragraph (j) specified only an MLG center door assembly.

Request for Clarification of Intent of Parts Installation Paragraph (j)

DAL stated that paragraph (j), "Parts Installation Paragraph," of
the proposed AD was confusing because it stated that an operator may
not install an MLG center door assembly on an airplane unless all
actions identified as RC in BSASB 737-52-1170, R1, are accomplished
within the compliance times specified in Tables 4, 5, and 6, as
applicable, of paragraph 1.E., "Compliance," of BSASB 737-52-1170,
R1. DAL observed that if an operator receives a spare door with an FAA
Form 8130, "Authorized Release Certificate--Airworthiness Approval
Tag," attached, the tag might include the AD number but the number of
flight cycles at the last inspection or total flight cycles of the door
would not be provided. DAL suggested that operators ensure that the
inspection and corrective actions are accomplished before the spare
part is installed on the airplane. Therefore, if the flight cycles on
the door are unknown, the operator would still be in
compliance with the intent of the NPRM by inspecting the door before
installation, and that it would be an equivalent level of safety that
meets the intent of the NPRM.

We appreciate the commenter's concern and the opportunity to
clarify the intent of the "Parts Installation Limitation" paragraph.
The compliance times specified in Tables 4, 5, and 6, as applicable, of
paragraph 1.E., "Compliance," of BSASB 737-52-1170, R1, are in
airplane flight cycles. There is no requirement in this AD or any
statement in BSASB 737-52-1170, R1, that it is necessary to determine
the flight cycles accumulated on the MLG door assembly. The compliance
times in this AD are based on flight cycles of the airplane instead of
the MLG door assembly. Our strategy in addressing the unsafe condition
is to first inspect all affected airplanes, and then to address future
possible unsafe conditions with the requirements in the "Parts
Installation Limitation" paragraph.

We have not made any changes to this AD in regard to this issue.

Request To Delay Issuance of Final Rule Until Service Information is
Corrected

American, subsequent to its earlier comments, requested that the
final rule not be issued until discrepancies in BSASB 737-52-1170, R1,
are rectified and the instructions made clearer. The commenter stated
that operators cannot comply with the requirements specified in the
NPRM because of discrepancies in BSASB 737-52-1170, R1. The commenter
identified the following discrepancies.

1. BSASB 737-52-1170, R1, has quantities listed in Figures 1 and
2 that are double what is actually on the aircraft. Although there
is a note that says "The QTY numbers shown below are the number or
parts necessary for each airplane," there is a Figure 1 for the
left and a Figure 2 for the right. Each side has only one of each
bolt, not two. Note that all of the other figures list the quantity
of parts that is needed for only the left side or the right side, as
applicable. Figures 1 and 2 are different than the other figures in
this regard.

2. In Figure 5 (and Figures 6, 7, and 8), step 6 says to remove
three laminated shims. The airplane only has two laminated shims.

3. In Figure 13 (and Figure 14) step #4 has you install and
torque the bolt. However, the bolt in #4 has to go through the kept
bracket and if you install the bolt first, you have to take it back
out to install the bracket. Steps #4 and #5 should be reversed.

4. In Figure 13 (and Figure 14), step #6 states to install 3
each shims, but only 2 were removed, so do we install 3 each in the
new configuration or just put 2 back?

5. In Figure 13 (and Figure 14), once we installed the forward
bolt in step 4, with the correct washers installed, the bolt
bottomed out in the barrel nut housing, since the bolt is too long.
New bolts are slightly longer than old. The bolt needs another thick
washer to fix the issue. The kits that are being delivered do not
have an adequate amount of the necessary washers.

Alaska also noted that Figure 13 and Figure 14 of BSASB 737-52-
1170, R1, depict view C with a pre-modification installation in lieu of
a post-modification installation.

We acknowledge the commenters' concerns regarding the information
in the service information that requires clarification. The amount of
clarification needed would be overly complex for inclusion in this AD.
We expect to work with Boeing to issue a global AMOC addressing any
known errors as soon as possible. In addition, we have revised
paragraph (h) of this AD, "Exceptions to Service Information
Specifications," by adding paragraph (h)(3) to provide operators with
information regarding how to address any other issues, if needed.

In light of the critical nature of the identified unsafe condition,
we do not consider it warranted to delay the issuance of this final
rule. When Boeing provides a revision to BSASB 737-52-1170, R1, we will
review it in consideration of an AMOC to this AD or may consider future
rulemaking action.

Request To Revise Compliance Time Specifications

DAL noted that paragraph (h) of the proposed AD states that "For
purposes of determining compliance with the requirements of this AD:
Where BSASB 737-52-1170, Revision 1, uses the phrase `the original
issue date of this service bulletin', this AD requires using `the
effective date of this AD'." DAL pointed out that in Table 4 of BSASB
737-52-1170, R1, the compliance times for the Group 3 airplanes, states
that the actions should be completed "Within 800 flight cycles after
the Revision 1 date of this service bulletin." DAL asked if the AD
effective date should also replace the Revision 1 date of the service
information. We infer that DAL is requesting a revision to paragraph
(h) of the proposed AD to clarify that for purposes of determining
compliance with the requirements of the final rule that the effective
date of the AD should be used instead of the original issue date or the
Revision 1 date of the service information.

We agree with the commenter's request for the reasons provided by
the commenter. We have re-designated paragraph (h) of the proposed AD
as paragraph (h)(1) in this AD, and revised the text to state that for
purposes of determining compliance with the requirement of this AD,
where BSASB 737-52-1170, Revision 1, uses the phrase "the original
issue date of this service bulletin" or "the Revision 1 date of this
service bulletin" this AD requires using "the effective date of this AD."

Observations Regarding Service Information

American stated that BSASB 737-52-1170, R1, is confusing and
unnecessarily complex. American observed that BSASB 737-52-1170, R1,
provides for 14 possible conditions, multiple options for corrective
actions, 3 multi-page logic diagrams, and 10 different parts of
instructions. American stated that the complexity could be simplified
if the service information pointed the operator straight to the
modification of the MLG center door assembly retention clip assemblies
and, if needed, repair to the lugs and replacement of the bushings.
American declared that the unnecessary complexity of the service
bulletin invites non-compliance issues.

We acknowledge the commenter's concerns regarding BSASB 737-52-
1170, R1. The reason BSASB 737-52-1170, R1, includes 14 possible
conditions, multiple options for corrective actions, 3 multi-page logic
diagrams, and 10 different parts of instructions is to provide a
comprehensive set of procedures to address the unsafe condition that
exists in the affected fleet of airplanes. We suggest that the
commenter provide its comments regarding improvements to this document
directly to Boeing. We have not changed this AD in regard to this
issue.

Effect of Winglets on Accomplishment of the Proposed Actions

Aviation Partners Boeing stated that accomplishing the Supplemental
Type Certificate (STC) ST00830SE does not affect the ability to
accomplish the actions specified in the NPRM.

We concur with the commenter. We have added paragraph (c)(5) to
this AD to state that installation of STC ST00830SE 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.

Conclusion

We 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. We have 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.

We also determined that these changes will not increase the
economic burden on any operator or increase the scope of this final rule.

Related Service Information Under 1 CFR Part 51

We reviewed Boeing Special Attention Service Bulletin 737-52-1170,
Revision 1, dated December 19, 2017. The service information describes
procedures for repetitive detailed inspections of the forward and aft
MLG center door assembly attachments for loose, missing, damaged, or
bottomed-out attachment bolts, and any wear to the retention clip
assemblies as applicable; and applicable on-condition actions. The
service information also describes procedures for modification of the
MLG center door assembly retention clip assemblies as an optional
terminating action for the repetitive inspections. 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 1,814 airplanes of U.S. registry.
We estimate the following costs to comply with this AD:

Estimated Costs for Required Actions

Estimated Costs for Optional Terminating Action

We have received no definitive data that would enable us to provide
cost estimates for the on-condition actions specified in this AD.

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 known costs in our cost
estimate.

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 and
associated appliances to the Director of the System Oversight Division.

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):