Airworthiness Directives; MD Helicopters, LLC Helicopters

Issuing agencies

Abstract

The FAA is adopting a new airworthiness directive (AD) for certain MD Helicopters, LLC (MDHI) Model 369D, 369E, 369F, 369FF, 369H, and 500N helicopters. This AD was prompted by a report of mechanical damage to the main transmission drive shaft coupling (coupling). This AD requires a one-time visual inspection of the couplings for cracks of the splines and, depending on the results, replacement of the couplings. The FAA is issuing this AD to address the unsafe condition on these products.

Full Text

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<title>Federal Register, Volume 91 Issue 204 (Friday, October 23, 2026)</title>
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[Federal Register Volume 91, Number 204 (Friday, October 23, 2026)]
[Rules and Regulations]
[Pages 66783-66785]
From the Federal Register Online via the Government Publishing Office [<a href="http://www.gpo.gov">www.gpo.gov</a>]
[FR Doc No: 2026-21664]



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Rules and Regulations
                                                Federal Register
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Federal Register / Vol. 91, No. 204 / Friday, October 23, 2026 / 
Rules and Regulations

[[Page 66783]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2026-4666; Project Identifier AD-2025-01677-R; 
Amendment 39-23494; AD 2026-21-05]
RIN 2120-AA64


Airworthiness Directives; MD Helicopters, LLC Helicopters

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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

SUMMARY: The FAA is adopting a new airworthiness directive (AD) for 
certain MD Helicopters, LLC (MDHI) Model 369D, 369E, 369F, 369FF, 369H, 
and 500N helicopters. This AD was prompted by a report of mechanical 
damage to the main transmission drive shaft coupling (coupling). This 
AD requires a one-time visual inspection of the couplings for cracks of 
the splines and, depending on the results, replacement of the 
couplings. The FAA is issuing this AD to address the unsafe condition 
on these products.

DATES: This AD is effective November 27, 2026.

ADDRESSES: 
    AD Docket: You may examine the AD docket at <a href="http://regulations.gov">regulations.gov</a> under 
Docket No. FAA-2026-4666; 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, any comments received, and other 
information. The address for Docket Operations 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: Binod Singh, Aviation Safety Engineer, 
FAA, 3960 Paramount Boulevard, Lakewood, CA 90712; phone: (562) 627-
5228; email: <a href="/cdn-cgi/l/email-protection#a1c3c8cfcec58fd2c8cfc6c9e1c7c0c08fc6ced7"><span class="__cf_email__" data-cfemail="5b393235343f752832353c331b3d3a3a753c342d">[email&#160;protected]</span></a>.

SUPPLEMENTARY INFORMATION:

Background

    The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 
CFR part 39 by adding an AD that would apply to certain MDHI Model 
369D, 369E, 369F, 369FF, 369H, and 500N helicopters. The NPRM was 
published in the Federal Register on July 1, 2026 (91 FR 39903). The 
NPRM was prompted by a report that couplings having part number (P/N) 
369H5660 were found with mechanical damage to the root fillets, spline 
faces, and the major diameter. It was also reported that a new vendor 
improperly manufactured certain couplings having P/N 369H5660, with a 
total of 148 shipped to customers between June 3, 2020, and May 1, 
2022. The manufacturer reported that 86 affected couplings have been 
recovered, with the remaining 62 affected couplings at unknown 
locations.
    In the NPRM, the FAA proposed to require a one-time visual 
inspection of the couplings for cracks of the splines and, depending on 
the results, replacement of the couplings. The FAA is issuing this AD 
to address the unsafe condition on these products.

Discussion of Final Airworthiness Directive

Comments

    The FAA received one comment from an anonymous commenter. The 
following presents the comment received on the NPRM and the FAA's 
response to the comment.

Request for Clarification of the Determination for the One-Time Visual 
Inspection

    An anonymous commenter requested that the FAA clarify the 
determination for a one-time visual inspection. The commenter stated 
that the NPRM proposed to require a one-time visual inspection for 
spline cracking. The commenter specified that if spline cracking is 
caused by fatigue, alignment wear, or operational vibration, then a 
single inspection only catches existing cracks but fails to protect 
against future fatigue failures occurring hundreds of flight hours 
later. The commenter asked what data or modeling was evaluated in 
making the determination that a one-time visual inspection is safer 
than a recurring inspection interval or a time-in-service replacement 
requirement. In addition, the commenter asked whether data was gathered 
from different agencies or incident logs to further back up the 
analysis and could be used to monitor the unsafe condition for 
recurrence.
    The FAA agrees to clarify. This AD addresses a manufacturing defect 
that could lead to fatigue cracking during normal helicopter operation. 
The one-time visual inspection is to determine whether a crack is 
present in the defective part and require immediate replacement if 
cracked. If the defective part is not cracked, the compliance time for 
replacement is set significantly earlier than the calculated critical 
crack propagation threshold, which ensures that all defective parts are 
replaced before any fatigue crack reaches a critical failure point. 
Therefore, recurring inspections are unnecessary. The FAA did not 
change this AD as a result of this comment.

Additional Changes to This AD

    In the NPRM, the FAA inadvertently included multiple references 
related to material incorporated by reference (IBR). However, no IBR 
material was necessary for the NPRM or this AD. Therefore, the FAA has 
removed all IBR references from this AD. In addition, paragraph (j) 
``Material Incorporated by Reference'' of this AD has been changed to 
state ``None.''

Conclusion

    The FAA reviewed the relevant data, considered any comments 
received, and determined that air safety requires adopting this AD as 
proposed. Accordingly, the FAA is issuing this AD to address the unsafe 
condition on these products. Except for minor editorial changes, this 
AD is adopted as proposed in the NPRM. None of the changes will 
increase the economic burden on any operator.

Costs of Compliance

    The FAA estimates that this AD affects 568 helicopters of U.S. 
registry.
    The FAA estimates the following costs to comply with this AD:

[[Page 66784]]



                                                 Estimated Costs
----------------------------------------------------------------------------------------------------------------
                                                                                       Cost per    Cost on U.S.
                  Action                            Labor cost           Parts cost    product       operators
----------------------------------------------------------------------------------------------------------------
Visually inspect couplings...............  1 work-hour x $85 per hour            $0          $85         $48,280
                                            = $85.
----------------------------------------------------------------------------------------------------------------

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

                                               On-Condition Costs
----------------------------------------------------------------------------------------------------------------
              Action                      Labor cost             Parts cost              Cost per  product
----------------------------------------------------------------------------------------------------------------
Replace coupling..................  2 work-hours x $85     $1,681 (per coupling)  Up to $3,532.
                                     per hour = $170.
----------------------------------------------------------------------------------------------------------------

    The FAA has included all known costs in its cost estimate. 
According to the manufacturer, however, some of the costs of this AD 
may be covered under warranty, thereby reducing the cost impact on 
affected operators.

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

    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) Will not affect intrastate aviation in Alaska, and
    (3) 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.

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

0
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]

0
2. The FAA amends Sec.  39.13 by adding the following new airworthiness 
directive:

2026-21-05 MD Helicopters, LLC: Amendment 39-23494; Docket No. FAA-
2026-4666; Project Identifier AD-2025-01677-R.

(a) Effective Date

    This airworthiness directive (AD) is effective November 27, 
2026.

(b) Affected ADs

    None.

(c) Applicability

    This AD applies to MD Helicopters, LLC (MDHI) Model 369D, 369E, 
369F, 369FF, 369H, and 500N helicopters, certificated in any 
category, with an installed main transmission drive shaft coupling 
(coupling) part number 369H5660 with serial numbers 8564-0001 
through 8564-0308.

(d) Subject

    Joint Aircraft System Component (JASC) Code 6310, Engine/
transmission coupling.

(e) Unsafe Condition

    This AD was prompted by a report of mechanical damage to the 
coupling. The FAA is issuing this AD to detect and address couplings 
with damage to the root fillets, spline faces, and major diameter. 
The unsafe condition, if not addressed, could result in fatigue 
cracks that grow from the damaged root fillets and spline faces, 
which could lead to failure of the coupling with consequent loss of 
power to the main rotor blades and possible emergency autorotational 
landing.

(f) Compliance

    Comply with this AD within the compliance times specified, 
unless already done.

(g) Required Actions

    Within 50 hours time-in-service (TIS) or 6 months after the 
effective date of this AD, whichever occurs first, using a bright 
light and 10X magnification, visually inspect each coupling for 
cracks to the spline.
    Note 1 to paragraph (g): Examples of acceptable couplings with 
standard broaching tool marks and mechanical damage may be found in 
Figure 1 and Figure 2, as applicable, of MD Helicopters Service 
Bulletin SB369H-264R1, SB369D-230R1, SB369E-130R1, SB369F-121R1, and 
SB500N-067R1, dated May 26, 2023 (co-published as one document).
    (1) If any cracks are found on any coupling, before further 
flight, remove the affected coupling from service and replace with a 
serviceable coupling.
    (2) If there are no cracks on any coupling, within 300 hours TIS 
after the effective date of this AD, replace the affected coupling 
with a serviceable coupling.

(h) Alternative Methods of Compliance (AMOCs)

    (1) The Manager, West Certification 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 local Flight 
Standards District Office, as appropriate. If sending information 
directly to the manager of the West Certification Branch, send it to 
the attention of the person identified in paragraph (i) of this AD 
and email to: <a href="/cdn-cgi/l/email-protection#3574787a76755354541b525a43"><span class="__cf_email__" data-cfemail="20616d6f63604641410e474f56">[email&#160;protected]</span></a>.
    (2) Before using any approved AMOC, notify your appropriate 
principal inspector, or lacking a principal inspector, the manager

[[Page 66785]]

of the local flight standards district office/certificate holding 
district office.

(i) Additional Information

    For more information about this AD, contact Binod Singh, 
Aviation Safety Engineer, FAA, 3960 Paramount Boulevard, Lakewood, 
CA 90712; phone: (562) 627-5228; email: <a href="/cdn-cgi/l/email-protection#c7a5aea9a8a3e9b4aea9a0af87a1a6a6e9a0a8b1"><span class="__cf_email__" data-cfemail="67050e09080349140e09000f2701060649000811">[email&#160;protected]</span></a>.

(j) Material Incorporated by Reference

    None.

    Issued on October 20, 2026.
Steven W. Thompson,
Acting Deputy Director, Compliance & Airworthiness Division, Aircraft 
Certification Service.
[FR Doc. 2026-21664 Filed 10-22-26; 8:45 am]
BILLING CODE 4910-13-P


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