Airworthiness Directives; Siam Hiller Holdings, Inc.

[Federal Register Volume 90, Number 159 (Wednesday, August 20, 2025)]
[Rules and Regulations]
[Pages 40529-40531]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2025-15839]



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Federal Register / Vol. 90, No. 159 / Wednesday, August 20, 2025 / 
Rules and Regulations

[[Page 40529]]



DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2024-2555; Project Identifier AD-2024-00214-R; 
Amendment 39-23089; AD 2025-15-02]
RIN 2120-AA64


Airworthiness Directives; Siam Hiller Holdings, Inc.

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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

SUMMARY: The FAA is adopting a new airworthiness directive (AD) for 
certain Siam Hiller Holdings, Inc. (Siam Hiller), Model UH-12E (Army 
OH-23G and H-23F) and UH-12E-L helicopters. This AD was prompted by 
reports of cracks found in a main rotor (M/R) transmission drive shaft 
(drive shaft). This AD requires inspecting certain M/R drive shafts for 
a crack, prohibits installing certain M/R drive shafts unless the 
inspection is done, and prohibits using certain paint strippers. The 
FAA is issuing this AD to address the unsafe condition on these 
products.

DATES: This AD is effective September 24, 2025.

ADDRESSES: 
    AD Docket: You may examine the AD docket at regulations.gov under 
Docket No. FAA-2024-2555; 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: Eduardo Orozco-Duran, Aviation Safety 
Engineer, FAA, 3960 Paramount Boulevard, Lakewood, CA 90712; phone: 
(562) 627-5264; email: [email protected].

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 Siam Hiller 
Model UH-12E (Army OH-23G and H-23F) and UH-12E-L helicopters. The NPRM 
was published in the Federal Register on January 14, 2025 (90 FR 
03046). The NPRM was prompted by reports of two cracks in an M/R drive 
shaft that were found during an inspection involving a Siam Hiller 
Model UH-12E helicopter. In the NPRM, the FAA proposed to require 
inspecting certain M/R drive shafts for a crack, prohibit installing 
certain M/R drive shafts unless the inspection is done, and prohibit 
using certain paint strippers. The FAA is issuing this AD to address 
non-conforming parts and the use of improper paint stripper; and to 
detect cracking of the M/R drive shaft. The unsafe condition, if not 
addressed, could result in separation of the M/R drive shaft and M/R 
blades from the helicopter and consequent loss of control of the 
helicopter.

Discussion of Final Airworthiness Directive

Comments

    The FAA received comments from Summit Helicopters, Inc (Summit 
Helicopters). The following presents the comments received on the NPRM 
and the FAA's response to each comment.

Request To Extend Compliance Times

    Summit Helicopters stated the compliance time proposed in the NPRM 
would create significant economic impact for their company and is 
requesting an additional time margin to be added for the required 
inspections. The commenter stated that the time required to perform 
these inspections, as proposed in the NPRM, is significantly short of 
the actual time it would take to perform these inspections. Summit 
Helicopters requested to be able to delay inspection until November 
2025 or extend compliance to the next scheduled overhaul inspection. 
Additionally, the commenter provided a detailed explanation of the 
actions that would be required to comply with the actions of the AD.
    The FAA agrees to revise the initial compliance time to be within 4 
years of the effective date of this AD or at the next M/R transmission 
assembly overhaul inspection after the effective date of this AD, 
whichever comes first.

Request To Revise Cost

    Summit Helicopters stated that the cost of the actions proposed in 
the NPRM did not include the costs of baking, shipping, and other 
precondition tasks such as the removal, reinstallation, and reassembly 
of the M/R drive shaft. Summit Helicopters requested changes to the 
cost of compliance to include the above costs. Summit Helicopters 
commented that the removal and reinstallation process would take at 
least an additional 11.5 hours. In addition, Summit Helicopters has 
also commented that, based on operator experience, the labor work-hours 
required to strip, bake, inspect, prime, and paint will take 
approximately 6.5 hours instead of 5 hours.
    The FAA partially agrees. The FAA disagrees with providing cost 
information for shipping, handling, and transportation, or pre-
condition tasks such as removal, reinstallation, and reassembly. The 
cost information specified in the proposed AD describes only the direct 
costs of the specific actions required by this AD. Based on the best 
data available, the manufacturer provided the number of work-hours 
necessary to do the required actions. This number represents the time 
necessary to perform only the actions actually required by this AD. The 
FAA recognizes that, in doing the actions required by an AD, operators 
might incur incidental costs in addition to the direct costs. The cost 
analysis in AD rulemaking actions, however, typically does not include 
incidental costs such as the time necessary for planning or time 
necessitated by other administrative actions. Those incidental costs 
might vary significantly among operators. In addition, the total amount 
of work-hours will only account for the total work-hours for the 
corrective actions and not the 11.5 work-hours required for pre-
condition tasks and the 24 work-hours associated with relief baking 
span time. As stated previously, it is also important to note that 
special

[[Page 40530]]

flight permits are allowed in the AD, therefore operators are allowed 
to transport their helicopters to any repair facility to have these 
actions performed. However, the FAA agrees to update the cost of 
compliance to reflect an accurate estimate due to Summit Helicopter 
experience of work-hours required to perform corrective actions to 
paint strip, relief bake, inspect, prime and paint. The FAA also 
accepts the provided estimate as our initial analysis did not consider 
the availability of outsized ovens and will assume that it is $1,850 
for each shaft. Therefore, the FAA is updating the cost to be up to 
$1,850 based on the size of the oven and increasing the total work-
hours from 5 work-hours to 6.5 work-hours.

Request To Remove the Requirement To Use Certain Materials

    Summit Helicopters requested that the FAA provide additional 
specifications that meet the requirements of paint strippers with 
specifications TT-R-248B and TT-R-2918A Type I. The commenter stated 
that during its initial search, it was difficult to find paint stripper 
with specifications TT-R-248B and TT-R-2918A Type I. Summit Helicopters 
suggested replacing the requirement for the use of the specified paint 
stripper with one more readily available.
    The FAA disagrees with Summit Helicopters' request to provide 
additional specifications that meet the need for non-embrittling paint 
stripper, other than a paint stripper with specifications TT-R-248B or 
TT-R-2918A Type I because certain paint strippers used in aviation 
maintenance can pose risks to aircraft materials and structural 
integrity. Additionally, the FAA provides maintenance and safety 
guidelines for aircraft paint removal processes to prevent structural 
damage and ensure airworthiness, which can be found in documents such 
as Federal Specification TT-R-248B or TT-R-2918A Type I. Operators that 
prefer to accomplish this action by means other than those specified in 
this AD may submit an alternative method of compliance, as indicated in 
paragraph (i) of this final rule. However, the FAA did revise the 
reference to the paint stripper specifications in paragraphs (g)(1) and 
(h)(2) of this AD to remove the revision letter from the number, in an 
effort to improve search results for operators.

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, and 
any other changes described previously, 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 110 helicopters of U.S. 
registry. Labor costs are estimated at $85 per hour. Based on these 
numbers, the FAA estimates the following costs to comply with this AD.
    Removing the coatings, hydrogen embrittlement relief baking, and 
magnetic particle inspecting the M/R drive shaft takes 6.5 work-hours 
and parts cost up to $1,850 for an estimated cost of $2,402.50 per 
helicopter and $264,275 for the U.S. fleet. If required, replacing the 
M/R drive shaft takes 5 work-hours and parts cost $15,000 for an 
estimated cost of $15,425 per helicopter.

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:

2025-15-02 Siam Hillar Holdings, Inc.: Amendment 39-23089; Docket 
No. FAA-2024-2555; Project Identifier AD-2024-00214-R.

(a) Effective Date

    This airworthiness directive (AD) is effective September 24, 
2025.

(b) Affected ADs

    None.

(c) Applicability

    This AD Applies to Siam Hiller Holdings, Inc., Model UH-12E 
(Army OH-23G and H-23F) and UH-12E-L helicopters, certificated in 
any category, with a main rotor (M/R) transmission drive shaft 
(drive shaft) having part number (P/N) 23600 installed.
    Note 1 to paragraph (c): Hiller Aircraft Corporation Main Rotor 
Transmission Assembly Overhaul Manual, Manual 63-20, for UH-12E 
Series Helicopters, accepted May 6, 2015, contains additional 
information pertaining to inspecting the M/R drive shaft and refers 
to an M/R drive shaft as a transmission M/R mast and M/R drive 
shaft, interchangeably.

(d) Subject

    Joint Aircraft System Component (JASC) Code 6230, Main Rotor 
Mast/Swashplate.

(e) Unsafe Condition

    This AD was prompted by reports of cracks in the M/R drive 
shaft. The FAA is issuing this AD to address non-conforming parts 
and the use of improper paint stripper; and to detect cracking of 
the M/R drive shaft. The unsafe condition, if not addressed, could 
result in separation of the M/R drive shaft and M/R blades from the 
helicopter and consequent loss of control of the helicopter.

[[Page 40531]]

(f) Compliance

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

(g) Required Actions

    At the next overhaul inspection of the M/R transmission assembly 
or within 4 years after the effective date of this AD, whichever 
comes first, with the M/R drive shaft removed, inspect the M/R drive 
shaft by accomplishing the actions required by paragraphs (g)(1) 
through (3) of this AD.
    (1) Remove all coatings from all surfaces of the M/R drive shaft 
by using paint stripper TT-R-248 or TT-R-2918 Type I.
    (2) Hydrogen embrittlement relief bake the M/R drive shaft for 
24 hours minimum at 375 [deg]F <plus-minus> 25 [deg]F.
    (3) Magnetic particle inspect all surfaces of the M/R drive 
shaft for a crack. If there is a crack, before further flight, 
remove the M/R drive shaft from service and install an airworthy M/R 
drive shaft.
    Note 2 to paragraph (g): Hiller Aircraft Corporation Main Rotor 
Transmission Assembly Overhaul Manual, Manual 63-20, for UH-12E 
Series Helicopters, accepted May 6, 2015, contains additional 
information pertaining to inspecting the M/R drive shaft, applying 
appropriate coatings to the M/R drive shaft, and returning the 
helicopter to service.

(h) Parts Installation Limitations

    (1) As of the effective date of this AD, do not install an M/R 
drive shaft having P/N 23600 on any helicopter unless the actions 
required by paragraphs (g)(1) through (3) of this AD have been 
accomplished.
    (2) As of the effective date of this AD, do not use any paint 
stripper other than TT-R-248 or TT-R-2918 Type I to remove coatings 
from all areas of the M/R drive shaft.

(i) 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 (j)(1) of this 
AD and email to: [email protected].
    (2) Before using any approved AMOC, notify your appropriate 
principal inspector, or lacking a principal inspector, the manager 
of the local flight standards district office/certificate holding 
district office.

(j) Additional Information

    (1) For more information about this AD, contact Eduardo Orozco-
Duran, Aviation Safety Engineer, FAA, 3960 Paramount Boulevard, 
Lakewood, CA 90712; phone: (562) 627-5264; email: [email protected].
    (2) For Hiller Aircraft Corporation material identified in this 
AD that is not incorporated by reference, contact Hiller Aircraft 
Corporation, 925 M Street, Firebaugh, CA 93622; phone: (559) 659-
5959; or website: hilleraircraftcorporation.com/.

(k) Material Incorporated by Reference

    None.

    Issued on August 15, 2025.
Steven W. Thompson,
Acting Deputy Director, Compliance & Airworthiness Division, Aircraft 
Certification Service.
[FR Doc. 2025-15839 Filed 8-19-25; 8:45 am]
BILLING CODE 4910-13-P

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