AD 2026-02-01

This AD is effective February 26, 2026.

The Director of the Federal Register approved the incorporation by reference of a certain publication listed in this AD as of February 26, 2026.

AD Docket: You may examine the AD docket at regulations.gov under Docket No. FAA-2025-0629; 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 mandatory continuing airworthiness information (MCAI), 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.

Material Incorporated by Reference:

• For European Union Aviation Safety Agency (EASA) material identified in this AD, contact EASA, Konrad-Adenauer-Ufer 3, 50668 Cologne, Germany; phone: +49 221 8999 000; email: ADs@easa.europa.eu; website: easa.europa.eu. You may find the EASA material on the EASA website at ad.easa.europa.eu.

• You may view this material at the FAA, Office of the Regional Counsel, Southwest Region, 10101 Hillwood Parkway, Room 6N-321, Fort Worth, TX 76177. For information on the availability of this material at the FAA, call (817) 222-5110. It is also available at regulations.gov under Docket No. FAA-2025-0629.

Aaron Nguyen, Aviation Safety Engineer, FAA, 1600 Stewart Avenue, Suite 410, Westbury, NY 11590; phone: (817) 222-5134; email: aaron.t.nguyen@faa.gov.

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to all AHD Model EC135P1, EC135P2, EC135P2+, EC135P3, EC135T1, EC135T2, EC135T2+, EC635T2+, and EC135T3 helicopters. The NPRM was published in the Federal Register on April 25, 2025 (90 FR 17352). The NPRM was prompted by AD 2023-0197, dated November 10, 2023 (EASA AD 2023-0197) (also referred to as the MCAI), issued by EASA, which is the Technical Agent for the Member States of the European Union. The MCAI advises of reports of ruptured and deformed flexible couplings, and that investigations determined that a flexible coupling installed with high axial displacement causes increased stresses and friction between its sheets. The MCAI states that this unsafe condition, if not detected and corrected, could lead to cracks and extensive deformation of flexible couplings and consequent high vibration of the tail rotor drive shaft, possibly resulting in reduced control of the helicopter.

In the NPRM, the FAA proposed to require inspecting the axial displacement of the tail rotor drive shaft and, depending on the results, taking corrective actions including inspecting the flexible couplings. The NPRM also proposed to prohibit installing a tail rotor drive shaft unless certain procedures are followed. The FAA is issuing this AD to address the unsafe condition on these products.

You may examine the MCAI in the AD docket at regulations.gov under Docket No. FAA-2025-0629.

The FAA received no comments on the NPRM or on the determination of the costs.

The FAA has revised paragraph (h) of this AD to clarify that where paragraphs (4) and (6) of EASA AD 2023-0197 specify replacing the flexible couplings in accordance with the instructions of section 4.1 of the service information referenced, this AD requires replacing the flexible couplings in accordance with the instructions of sections 4.1 and 4.2 of the service information referenced in EASA AD 2023-0197.

The FAA has also revised paragraph (h) of this AD to clarify that this AD prohibits installing a tail rotor drive shaft unless certain procedures are followed, as stated in the Preamble of the NPRM.

These products have been approved by the civil aviation authority of another country and are approved for operation in the United States. Pursuant to the FAA's bilateral agreement with this State of Design Authority, that authority has notified the FAA of the unsafe condition described in the MCAI referenced above. 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.

The FAA reviewed EASA AD 2023-0197, which specifies a one-time inspection of the flexible coupling for axial displacement and, depending on the results, corrective actions. Corrective actions include replacing both flexible couplings and correcting the axial displacement, or inspecting the flexible couplings, replacing each flexible coupling having a discrepancy, and correcting the axial displacement. For certain flexible couplings not replaced based on the results of the flexible coupling inspection, EASA AD 2023-0197 requires replacing the flexible couplings within a longer compliance time. EASA AD 2023-0197 also prohibits installing a tail rotor drive shaft on any helicopter unless certain procedures are followed. 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.

The MCAI applies to Model EC635 P2+, EC635 P3, EC635 T1, and EC635 T3 helicopters, whereas this AD does not because these models do not have an FAA type certificate.

The FAA estimates that this AD affects 69 helicopters of U.S. registry. Labor rates are estimated at $85 per hour. Based on these numbers, the FAA estimates the following costs to comply with this AD.

Inspecting both flexible couplings for axial displacement will take 8 work-hours for an estimated cost of $680 per helicopter and $46,920 for the U.S. fleet.

If required, replacing both flexible couplings will take 2 work-hours, and parts will cost $4,100 for an estimated cost of $4,270 per helicopter. Replacing one flexible coupling will take 1 work-hour and parts will cost $2,050 for an estimated cost of $2,135 per flexible coupling.

If required, correcting the axial displacement of the flexible couplings will take 5 work-hours for an estimated cost of $425 per helicopter.

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.

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.

Accordingly, under the authority delegated to me by the Administrator, the FAA amends 14 CFR part 39 as follows:

1. The authority citation for part 39 continues to read as follows:

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