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PROPOSED AD THE BOEING COMPANY: Docket No. FAA-2021-0503; Project Identifier AD-2021-00163-T.
(a) COMMENTS DUE DATE

    The FAA  must receive comments  on  this  airworthiness directive (AD)
    action by August 16, 2021.

(b) AFFECTED ADS

    This AD replaces AD 2005-05-18, Amendment 39-14007 (70 FR 12410, March
    14, 2005) (AD 2005-05-18).

(c) APPLICABILITY

    This AD applies to The Boeing Company Model 737-600, -700, -700C, -800
    and -900 series airplanes, certificated in any category; as identified
    in Boeing Alert Service Bulletin 737-53A1251 Revision 2, dated January
    20, 2021.

(d) SUBJECT

    Air Transport Association (ATA) of America Code 53, Fuselage.

(e) UNSAFE CONDITION

    This AD was  prompted by a  report of cracks  found at several  of the
    fastener rows  in the  web lap  splices at  the dome  apex of  the aft
    pressure bulkhead, and the  determination that airplanes not  affected
    by AD  2005-05-18 are  subject to  this unsafe  condition. The  FAA is
    issuing this  AD to  address fatigue  cracks in  the webs  of the  aft
    pressure bulkhead, which  could result in  rapid decompression of  the
    airplane.

(f) COMPLIANCE

    Comply with this AD within the compliance times specified,  unless al-
    ready done.

(g) REQUIRED ACTIONS

    Except as  specified by  paragraph (h)  of this  AD: At the applicable
    times  specified  in  paragraph 1.E.,  "Compliance,"  of  Boeing Alert
    Service Bulletin 737-53A1251  Revision 2, dated  January 20, 2021,  do
    all applicable  actions identified  as "RC"  (required for compliance)
    in, and in accordance with, the Accomplishment Instructions of  Boeing
    Alert Service Bulletin 737-53A1251 Revision 2, dated January 20, 2021.
    For Group  1 airplanes,  as defined  in Boeing  Alert Service Bulletin
    737-53A1251 Revision  2, dated  January 20,  2021: Step  3.B.2. of the
    Accomplishment  Instructions  of  Boeing  Alert  Service  Bulletin 737
    -53A1251 Revision 2, dated  January 20, 2021, is  an RC step, and  the
    provisions of paragraphs (j)(5)(i) and (ii) of this AD apply.

(h) EXCEPTIONS TO SERVICE INFORMATION SPECIFICATIONS

(1) Where Boeing Alert Service Bulletin 737-53A1251 Revision 2, dated Jan-
    uary 20, 2021,  uses  the  phrase "the Revision 1 date of this service
    bulletin," this AD requires using "the effective date of this AD."

(2) Where  Boeing  Alert Service Bulletin 737-53A1251,  Revision 2,  dated
    January 20, 2021,  specifies contacting Boeing for repair instructions
    or for alternative inspections:  This AD requires doing the repair, or
    doing the alternative inspections and applicable  on-condition actions
    using a method approved in accordance with the procedures specified in
    paragraph (j) of this AD.

(i) CREDIT FOR PREVIOUS ACTIONS

(1) For airplanes having line numbers 1 through 1166 inclusive: This para-
    graph provides credit  for  the  corresponding actions of Boeing Alert
    Service Bulletin 737-53A1251 Revision 2,  dated January 20, 2021, that
    are required by paragraph (g) of this AD,  if those actions  were per-
    formed before the effective date of this AD using Boeing Service Bull-
    etin 737-53-1251, dated June 3, 2004, which was incorporated by refer-
    ence in AD 2005-05-18.

(2) This paragraph provides credit for the corresponding actions of Boeing
    Alert Service Bulletin 737-53A1251 Revision 2, dated January 20, 2021,
    that are required by paragraph (g)  of this AD,  if those actions were
    performed before  the effective  date of  this AD  using Boeing  Alert
    Service Bulletin  737-53-1251,  Revision 1,  dated September 22, 2020,
    which is not incorporated by reference in this AD.

(j) ALTERNATIVE METHODS OF COMPLIANCE (AMOCS)

(1) The  Manager,  Seattle ACO Branch,  FAA,  has the authority to approve
    AMOCs for this AD, if requested  using the procedures found in 14  CFR
    39.19. In  accordance with  14 CFR  39.19, send  your request  to your
    principal  inspector  or  responsible  Flight  Standards  Office,   as
    appropriate. If  sending information  directly to  the manager  of the
    certification  office,  send it to the attention of the person identi-
    fied in Related Information.  Information  may  be  emailed to: 9-ANM-
    Seattle-ACO-AMOC-Requests@faa.gov.

(2) Before using any approved AMOC,  notify your appropriate principal in-
    spector, or lacking a principal inspector,  the manager of the respon-
    sible Flight Standards Office.

(3) An AMOC that provides an acceptable level of safety  may  be  used for
    any repair, modification, or alteration  required by this AD if  it is
    approved by The Boeing Company Organization Designation  Authorization
    (ODA) that  has been  authorized by  the Manager,  Seattle ACO Branch,
    FAA,  to  make those  findings.  To be  approved,  the repair  method,
    modification  deviation,  or   alteration  deviation  must   meet  the
    certification  basis   of  the   airplane,  and   the  approval   must
    specifically refer to this AD.

(4) AMOCs approved for AD 2005-05-18 are approved as AMOCs  for the corre-
    sponding  provisions  of  Boeing  Alert  Service  Bulletin 737-53A1251
    Revision 2, dated January 20, 2021, that are required by paragraph (g)
    of this AD.

(5) Except as specified by paragraph (h) of this AD:  For service informa-
    tion that contains steps that  are labeled as Required for  Compliance
    (RC),  the  provisions  of paragraphs  (j)(5)(i) and  (ii) of  this AD
    apply.

(i) The steps labeled as RC,  including substeps  under an RC step and any
    figures identified in an RC step, must be done to comply with the  AD.
    If a step or substep is  labeled "RC Exempt," then the RC  requirement
    is removed  from that  step or  substep. An  AMOC is  required for any
    deviations to RC steps, including substeps and identified figures.

(ii) Steps not labeled as RC  may be deviated  from using accepted methods
     in accordance with the  operator's maintenance or inspection  program
     without  obtaining  approval  of  an  AMOC,  provided  the  RC steps,
     including  substeps  and identified  figures,  can still  be  done as
     specified,  and  the  airplane  can  be  put  back  in  an  airworthy
     condition.

(k) RELATED INFORMATION

(1) For more information about this AD,  contact  Wayne Lockett, Aerospace
    Engineer, Airframe Section,  FAA, Seattle ACO Branch, 2200 South 216th
    St., Des Moines, WA 98198;  phone and fax: 206-231-3524; email: wayne.
    lockett@faa.gov.

(2) For service information identified in this AD,  contact Boeing Commer-
    cial 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 referenced 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.

Issued on June 11, 2021.  Lance T. Gant, Director, Compliance & Airworthi-
ness Division, Aircraft Certification Service.

DATES: The FAA must receive comments on  this  proposed  AD  by August 16,
2021.
PREAMBLE 

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2021-0503; Project Identifier AD-2021-00163-T]
RIN 2120-AA64

Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

-----------------------------------------------------------------------

SUMMARY: The FAA proposes to supersede Airworthiness Directive (AD)
2005-05-18, which applies to certain The Boeing Company Model 737-600,
-700, -700C, -800, and -900 series airplanes. AD 2005-05-18 requires
repetitive inspections for cracking of the webs of the aft pressure
bulkhead at a certain body station, and corrective action if necessary.
Since the FAA issued AD 2005-05-18, cracking was found in that
inspection area on airplanes not identified in the applicability of AD
2005-05-18. This proposed AD would retain the requirements of AD 2005-
05-18, revise the applicability to include additional airplanes, and
add an inspection for existing repairs on the newly added airplanes.
The FAA is proposing this AD to address the unsafe condition on these
products.

DATES: The FAA must receive comments on this proposed AD by August 16,
2021.

ADDRESSES: You may send comments, using the procedures found in 14 CFR
11.43 and 11.45, by any of the following methods:

Federal eRulemaking Portal: Go to https://www.regulations.gov. Follow the
instructions for submitting comments.

Fax: 202-493-2251.

Mail: U.S. Department of Transportation, Docket
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New
Jersey Avenue SE, Washington, DC 20590.

Hand Delivery: Deliver to Mail address above between 9
a.m. and 5 p.m., Monday through Friday, except Federal holidays.

For service information identified in this NPRM, 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 at https://www.regulations.gov
by searching for and locating Docket No. FAA-2021-0503.

Examining the AD Docket

You may examine the AD docket at https://www.regulations.gov by
searching for and locating Docket No. FAA-2021-0503; 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 NPRM, any comments
received, and other information. The street address for Docket
Operations is listed above.

FOR FURTHER INFORMATION CONTACT: Wayne Lockett, Aerospace Engineer,
Airframe Section, FAA, Seattle ACO Branch, 2200 South 216th St., Des
Moines, WA 98198; phone and fax: 206-231-3524; email:
wayne.lockett@faa.gov.

SUPPLEMENTARY INFORMATION:

Comments Invited

The FAA invites you to send any written relevant data, views, or
arguments about this proposal. Send your comments to an address listed
under ADDRESSES. Include "Docket No. FAA-2021-0503; Project Identifier
AD-2021-00163-T" at the beginning of your comments. The most helpful
comments reference a specific portion of the proposal, explain the
reason for any recommended change, and include supporting data. The FAA
will consider all comments received by the closing date and may amend
the proposal because of those comments.

Except for Confidential Business Information (CBI) as described in
the following paragraph, and other information as described in 14 CFR
11.35, the FAA will post all comments received, without change, to
https://www.regulations.gov, including any personal information you
provide. The agency will also post a report summarizing each
substantive verbal contact received about this proposed AD.

Confidential Business Information

CBI is commercial or financial information that is both customarily
and actually treated as private by its owner. Under the Freedom of
Information Act (FOIA) (5 U.S.C. 552), CBI is exempt from public
disclosure. If your comments responsive to this NPRM contain commercial
or financial information that is customarily treated as private, that
you actually treat as private, and that is relevant or responsive to
this NPRM, it is important that you clearly designate the submitted
comments as CBI. Please mark each page of your submission containing
CBI as "PROPIN." The FAA will treat such marked submissions as
confidential under the FOIA, and they will not be placed in the public
docket of this NPRM. Submissions containing CBI should be sent to Wayne
Lockett, Aerospace Engineer, Airframe Section, FAA, Seattle ACO Branch,
2200 South 216th St., Des Moines, WA 98198; phone and fax: 206-231-
3524; email: wayne.lockett@faa.gov. Any commentary that the FAA
receives which is not specifically designated as CBI will be placed in
the public docket for this rulemaking.

Background

The FAA issued AD 2005-05-18, Amendment 39-14007 (70 FR 12410,
March 14, 2005) (AD 2005-05-18), for certain The Boeing Company Model
737-600, -700, -700C, -800, and -900 series airplanes. AD 2005-05-18
was prompted by a report of cracks found, during fatigue testing, at
several of the fastener rows in the web lap splices at the dome apex of
the aft pressure bulkhead. AD 2005-05-18 requires repetitive detailed,
low frequency eddy current (LFEC), and high frequency eddy current
(HFEC) inspections for cracking of the webs of the aft pressure
bulkhead at body station (BS) 1016, and corrective action if necessary.
The FAA issued AD 2005-05-18 to detect and correct fatigue cracks in
the webs of the aft pressure bulkhead, which could result in rapid
decompression of the airplane.

Actions Since AD 2005-05-18 Was Issued

Since the FAA issued AD 2005-05-18, cracking has been found at apex
webs on airplanes outside the applicability of AD 2005-05-18, which
includes line numbers 1 through 1166 inclusive. Line numbers 1167
through 1755 inclusive, which are included in this proposed AD, use a
revised fastener pattern in the 0.032-inch webs that was intended to
correct the cracking addressed by AD 2005-05-18. During the assembly
process on line numbers 1167 through 1755, the fasteners in the apex
dome region are subjected to fuselage pressurization fatigue cycles and
clamp-up stresses. Cracks in the inspection area of AD 2005-05-18 have now
been found on airplanes within the range of line numbers 1167 through 1755
inclusive. At one location, the crack was linked from the first to the
second fastener row. This cracking was identified during an inspection for
cracking of the web lap splice of the aft pressure bulkhead, as required
by AD 2017-10-22, Amendment 39-18896 (82 FR 23507, May 23, 2017) (AD 2017-
10-22). The inspections and intervals specified in AD 2017-10-22 are not
adequate to address the specific fatigue cracking occurring in the web
apex area that is the subject of this NPRM.

FAA's Determination

The FAA is issuing this NPRM after determining that the unsafe
condition described previously is likely to exist or develop on other
products of the same type design.

Related Service Information Under 1 CFR Part 51

The FAA reviewed Boeing Alert Service Bulletin 737-53A1251,
Revision 2, dated January 20, 2021. This service information specifies
procedures for a general visual inspection for existing repairs,
repetitive detailed and HFEC inspections for cracks around the web
fasteners, repetitive LFEC inspection for cracks around the hidden web
lap splice fastener locations, and repair of cracks. 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.

Proposed AD Requirements in This NPRM

Although this proposed AD does not explicitly restate the
requirements of AD 2005-05-18, this proposed AD would retain all of the
requirements of AD 2005-05-18. Those requirements are referenced in the
service information identified previously, which, in turn, is
referenced in paragraph (g) of this proposed AD. This proposed AD would
revise the applicability to include additional airplanes, and add an
inspection for existing repairs on those airplanes. This proposed AD
would also require accomplishment of the actions identified as "RC"
(required for compliance) in the Accomplishment Instructions of Boeing
Alert Service Bulletin 737-53A1251, Revision 2, dated January 20, 2021,
described previously, except for any differences identified as
exceptions in the regulatory text of this proposed AD.

For information on the procedures and compliance times, see this
service information at https://www.regulations.gov by searching for and
locating Docket No. FAA-2021-0503.

Differences Between This Proposed AD and the Service Information

For Group 1 airplanes, the Accomplishment Instructions of Boeing
Alert Service Bulletin 737-53A1251 Revision 2, dated January 20, 2021,
do not include any "RC" (required for compliance) steps. The RC
tagging was inadvertently removed from Boeing Alert Service Bulletin
737-53A1251 Revision 2, dated January 20, 2021. For Group 1 airplanes,
this proposed AD would therefore require treating Step 3.B.2. of the
Accomplishment Instructions of Boeing Alert Service Bulletin 737-
53A1251 Revision 2, dated January 20, 2021, as an RC step.

Costs of Compliance

The FAA estimates that this AD, if adopted as proposed, would affect
744 airplanes of U.S. registry. The FAA estimates the following costs to
comply with this proposed AD:

Estimated Costs

Action
Labor cost
Parts cost
Cost per product
Cost on U.S. operators
Detailed, HFEC, and LFEC inspections Up to 10 work-hours x $85 per hour = Up to $850 per inspection cycle $0 Up to $850 per inspection cycle Up to $632,400 per inspection cycle
General visual inspection (194 airplanes) 1 work-hour x $85 per hour = $85 0 $85 $16,490

The FAA estimates the following costs to do any necessary repairs
that would be required based on the results of the proposed
inspections. The FAA has no way of determining the number of aircraft
that might need these repairs:

On-Condition Costs

Action
Labor cost
Parts cost
Cost per product
Repair Up to 30 * work-hours x $85 per hour = Up to $2,550 Up to $30,000 * Up to $32,550 *

* Repair costs will vary depending on size of the repair required.

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

The FAA has determined that this proposed AD would not have
federalism implications under Executive Order 13132. This proposed AD
would 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 the proposed
regulation:

(1) Is not a "significant regulatory action" under Executive
Order 12866,

(2) Would not affect intrastate aviation in Alaska, and

(3) Would 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 Proposed Amendment

Accordingly, under the authority delegated to me by the
Administrator, the FAA proposes to amend 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:

a. Removing Airworthiness Directive (AD) 2005-05-18, Amendment 39-14007
(70 FR 12410, March 14, 2005), and

b. Adding the following new AD: