AD 2021-26-08 Requires Removing Certain Nuts From Service, Installing Newly Designed Nuts...

The FAA is adopting a new airworthiness directive (AD) for certain Bell Textron Canada Limited Model 206, 206A, 206A-1, 206B, 206B-1, 206L, 206L-1, 206L-3, and 206L-4 helicopters.

This AD was prompted by reports of cracked or missing nuts on the tail rotor drive shaft (TRDS) disc pack (Thomas) couplings. This AD requires removing certain nuts from service, installing newly designed nuts, and applying a specific torque and a torque stripe to each newly installed nut. This AD then requires, after the installation of each newly designed nut, inspecting the torque and, depending on the inspection results, either applying a torque stripe or performing further inspections and removing certain parts from service. Finally, this AD prohibits installing any affected nut on any TRDS Thomas coupling. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective January 27, 2022.

Supplementary Information: The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to Bell Textron Canada Limited Model 206, 206A, 206A-1, 206B, 206B-1, 206L, 206L-1, 206L-3, and 206L-4 helicopters with nut part number (P/N) MS21042L4 or P/N MS21042L5 installed on the TRDS Thomas couplings. The NPRM published in the Federal Register on September 14, 2021 (86 FR 51038). In the NPRM, the FAA proposed to require, within 600 hours time-in-service (TIS) after the effective date of this AD, removing each affected nut from service, installing a newly designed nut, and applying a specific torque and a torque stripe to each newly installed nut. The NPRM also proposed to require, within 25 hours TIS after installation of each newly designed nut, inspecting the torque of each nut, and depending on the results of the inspection, further inspections and removing certain parts from service. Finally, the NPRM proposed to prohibit installing any affected nut on any TRDS Thomas coupling.

The NPRM was prompted by a series of ADs issued by Transport Canada, which is the aviation authority for Canada. Initially, Transport Canada issued Canadian AD CF-2019-34, dated September 25, 2019 (Transport Canada AD CF-2019-34), to correct an unsafe condition for Bell Helicopter Textron Canada Limited (now Bell Textron Canada Limited) Model 206, 206A, 206A-1, 206B, 206B-1, 206L, 206L-1, 206L-3, and 206L-4 helicopters, all serial numbers. Transport Canada AD CF-2019-34 advised of reports of cracked or missing nuts at the TRDS Thomas couplings, which could have been caused by improper torque or hydrogen embrittlement. This condition, if not addressed, could result in loss of the tail rotor and subsequent loss of control of the helicopter.

After Transport Canada issued Transport Canada AD CF-2019-34, it was determined that helicopters modified in accordance with Supplemental Type Certificate (STC) SH2750NM or Transport Canada STC SH99-202, were not able to comply with Transport Canada AD CF-2019-34.

Accordingly, Transport Canada issued AD CF-2020-15, dated May 13, 2020 (Transport Canada AD CF-2020-15) which supersedes Transport Canada AD CF-2019-34, and contains a new requirement for helicopters with STC SH2750NM or Transport Canada STC SH99-202 installed or models that have been modified per Bell Service Instruction BHT-206-SI-2052, Revision 1, dated October 14, 2010 (BHT-206-SI-2052). Transport Canada advises for certain model helicopters, the newly designed nuts cannot be installed because STC SH2750NM and Transport Canada STC SH99-202 install a pulley at the Thomas coupling location causing insufficient clearance. Transport Canada further advises for certain model helicopters with STC SH2750NM or Transport Canada STC SH99-202 installed, different part-numbered nuts may be installed which were not identified in the applicable service information and are now required to be replaced with a new part-numbered nut that is not vulnerable to the unsafe condition. Accordingly, Air Comm Corporation, the STC holder for STC SH2750NM, issued new service information to address these additional issues and provide newly developed instructions which apply to certain model helicopters with STC SH2750NM or Transport Canada STC SH99-202 installed.

Additionally, Transport Canada advises that BHT-206-SI-2052, which is optional, specifies procedures for Model 206L-1 and 206L-3 helicopters to upgrade the airframe and systems and also includes installation of the Model 206L-4 TRDS Thomas coupling. According to Transport Canada, models that have incorporated BHT-206-SI-2052, with STC SH2750NM or Transport Canada STC SH99-202 installed, will have the Model 206L-4 helicopter pulley configuration and are subject to the Air Comm Corporation service information.

Accordingly, Transport Canada AD CF-2020-15 requires the replacement of the affected nuts with the newly designed nuts at each TRDS Thomas coupling.

FMI: www.regulations.gov