DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2025-0469; Project Identifier AD-2024-00640-T]
RIN 2120-AA64

Airworthiness Directives; The Boeing Company Airplanes

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: The FAA proposes to adopt a new airworthiness directive (AD)
for all The Boeing Company Model 737-600, -700, -700C, -800, -900, and
-900ER series airplanes. This proposed AD was prompted by a report of
under-torqued and missing nuts on the horizontal stabilizer trim
actuator (HSTA) lower gimbal assembly. This proposed AD would require a
review of maintenance records to determine if an overhauled HSTA was
inspected and corrective actions were accomplished using certain
service information, and applicable on-condition actions. 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 May 30, 2025.

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 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.

AD Docket: You may examine the AD docket at regulations.gov under
Docket No. FAA-2025-0469; 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.

Material Incorporated by Reference:

For the Boeing material identified in this proposed AD,
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 material 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-2025-0469.

FOR FURTHER INFORMATION CONTACT: Luis Cortez-Muniz, Aviation Safety
Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-
231-3958; email: luis.a.cortez-muniz@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 the ADDRESSES section. Include "Docket No. FAA-2025-0469;
Project Identifier AD-2024-00640-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 this 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
regulations.gov, including any personal information you provide. The
agency will also post a report summarizing each substantive verbal
contact received about this NPRM.

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 Luis
Cortez-Muniz, Aviation Safety Engineer, FAA, 2200 South 216th St., Des
Moines, WA 98198; phone: 206-231-3958; email: luis.a.cortez-muniz@faa.gov. Any
commentary that the FAA receives that is not specifically designated as CBI will
be placed in the public docket for this rulemaking.

Background

The FAA has received a report of under-torqued and missing nuts on
the HSTA lower gimbal assembly during a routine lubrication of the
HSTA. The yoke assembly was found separated from the fork assembly at
one corner but was still engaged on the related trunnion on the HSTA.
Loss of the HSTA primary load path can result in limit cycle
oscillation (LCO). LCO may be detectable by flight crew through
airframe vibration or pitch oscillations but could go undetected for
many fight cycles. Incorrectly installed nuts on the HSTA lower gimbal
assembly, if not addressed, could result in a HSTA lower gimbal
assembly disconnect resulting in loss of load path redundancy
preventing continued safe flight and landing.

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.

Material Incorporated by Reference Under 1 CFR Part 51

The FAA reviewed Boeing Alert Requirements Bulletin 737-27A1330 RB,
dated October 7, 2024. This material specifies procedures for a review
of maintenance records to determine if an overhauled HSTA was inspected
and corrective actions were accomplished using certain service
information. On-condition actions include general visual inspections,
torque value measurements, replacement of under-torqued nuts, HSTA
replacement, and repair of cracking and corrosion.

This material 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

This proposed AD would require accomplishing the actions specified
in the material already described, except as discussed under
"Differences Between this Proposed AD and the Referenced Material,"
and 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 material at regulations.gov under Docket
No. FAA-2025-0469.

Differences Between This Proposed AD and the Referenced Material

The effectivity of Boeing Alert Requirements Bulletin 737-27A1330
RB, dated October 7, 2024, is limited to Model 737-600, -700, -700C, -
800, -900, and -900ER series airplanes, having certain line numbers.
However, the applicability of this proposed AD includes all Model 737-
600, -700, -700C, -800, -900, and -900ER series airplanes. Because the
affected parts are rotable parts, the FAA has determined that these
parts could later be installed on airplanes that were initially
delivered with acceptable parts, thereby subjecting those airplanes to
the unsafe condition.

Costs of Compliance

The FAA estimates that this AD, if adopted as proposed, would
affect 2,060 airplanes of U.S. registry. The FAA estimates the
following costs to comply with this proposed AD:

Estimated Costs

The FAA estimates the following costs to do any necessary
replacements that would be required based on the results of the
proposed inspection. The agency has no way of determining the number of
aircraft that might need this replacement:

On-Condition Costs

The FAA has received no definitive data on which to base the cost
estimates for the on-condition repairs specified in this proposed 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

The FAA 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 this 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 adding the following new airworthiness
directive: