AD 2021-26-08 Airworthiness Directives; Bell Textron Canada Limited Helicopters

Models named: 206, 206A, 206A-1, 206B, 206B-1, 206L, 206L-1, 206L-3, 206L-4. Read the applicability below — it usually turns on serial or part numbers too.

This AD applies to Bell Textron Canada Limited Model 206, 206A, 206A-1, 206B, 206B-1, 206L, 206L-1, 206L-3, and 206L-4 helicopters, certificated in any category, with nut part number (P/N) MS21042L4 or P/N MS21042L5 installed on the tail rotor drive shaft (TRDS) disc pack (Thomas) couplings. Note 1 to paragraph (c): Helicopters with an OH-58A designation are Model 206A-1 helicopters.

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

(1) Within 600 hours time-in-service (TIS) after the effective date of this AD: (i) For helicopters that have not been modified by installing Supplemental Type Certificate (STC) SH2750NM: (A) Remove each nut P/N MS21042L4 installed on each TRDS Thomas coupling from service, and replace with nut P/N NAS9926-4L. The location of nut P/N NAS9926-4L is depicted in Detail A Figure 1 of Bell Alert Service Bulletin (ASB) 206-19-136, dated August 27, 2019 (ASB 206-19-136) or Bell ASB 206L-19-181, Revision A, dated August 29, 2019 (ASB 206L-19-181), as applicable to your model helicopter. (B) Apply a torque of 5.65-7.90 Nm (50-70 in lb) plus tare torque to each nut installed as required by paragraph (g)(1)(i)(A) of this AD, and apply a torque stripe using torque seal lacquer (C-049) or equivalent lacquer, as shown in Figure 2 of ASB 206-19-136 or ASB 206L-19-181, as applicable to your model helicopter. Note 2 to paragraph (g)(1)(i)(B): Torque stripes are referred to as witness marks in ASB 206-19-136 and ASB 206L-19-181. (ii) For Bell Textron Canada Limited Model 206, 206A, 206A-1, 206B, 206B-1, and 206L helicopters that have been modified by installing STC SH2750NM and Model 206L-1 and 206L-3 helicopters that have been modified by installing STC SH2750NM but have not been modified by accomplishing Bell Service Instruction BHT-206-SI-2052, Revision 1, dated October 14, 2010 (BHT-206-SI-2052): (A) Remove each nut P/N MS21042L4 installed on each TRDS Thomas coupling from service, except for nuts P/N MS21042L4 installed on the forward short TRDS Thomas coupling, and replace with nut P/N NAS9926-4L. The location of nut P/N NAS9926-4L is depicted in Detail A Figure 1 of ASB 206-19-136 or ASB 206L-19-181 as applicable to your model helicopter. (B) Remove each nut P/N MS21042L4 installed on the forward short TRDS Thomas coupling from service and replace with nut P/N 90-132L4. (C) For each nut installed as required by paragraphs (g)(1)(ii)(A) and (B) of this AD, apply a torque of 5.65-7.90 Nm (50-70 in lb) plus tare torque to each nut and apply a torque stripe using torque seal lacquer (C-049) or equivalent lacquer, as shown in Figure 2 of ASB 206-19-136 or ASB 206L-19-181, as applicable to your model helicopter. (iii) For Bell Textron Canada Limited Model 206L-1 and 206L-3 helicopters that have been modified by installing STC SH2750NM and have been modified by accomplishing BHT-206-SI-2052: (A) Remove each nut P/N MS21042L4 installed on each TRDS Thomas coupling from service, except for nuts P/N MS21042L4 installed on the forward short TRDS Thomas coupling, and replace with nut P/N NAS9926-4L. The location of nut P/N NAS9926-4L is depicted in Detail A Figure 1 of ASB 206L-19-181. (B) Remove each nut P/N MS21042L4 installed on the forward short TRDS Thomas coupling from service and replace with nut P/N 90-132L4. (C) For each nut installed as required by paragraphs (g)(1)(iii)(A) and (B) of this AD, apply a torque of 5.65-7.90 Nm (50-70 in lb) plus tare torque to each nut, and apply a torque stripe using torque seal lacquer (C-049) or equivalent lacquer, as shown in Figure 2 of ASB 206L-19-181. (iv) For Bell Textron Canada Limited Model 206L-4 helicopters that have been modified by installing STC SH2750NM: (A) Remove each nut P/N MS21042L4 installed on each TRDS Thomas coupling from service, except for nuts P/N MS21042L4 installed on the forward short TRDS Thomas coupling, and replace with nut P/N NAS9926-4L. The location of nut P/N NAS9926-4L is depicted in Detail A Figure 1 of ASB 206L-19-181. (B) Remove from service each nut P/N MS21042L5 installed on the forward short TRDS Thomas coupling and replace with nut P/N 90-132L5. (C) For each nut installed as required by paragraphs (g)(1)(iv)(A) and (B) of this AD, apply a torque of 5.65-7.90 Nm (50-70 in lb) plus tare torque to each nut, and apply a torque stripe using torque seal lacquer (C-049) or equivalent lacquer, as shown in Figure 2 of ASB 206L-19-181. (2) Within 25 hours TIS after installation of any nut P/N NAS9926-4L, P/N 90-132L4, or P/N 90-132L5, as required by paragraphs (g)(1)(i)(A), (ii)(A) and (B), (iii)(A) and (B), or (iv)(A) and (B) of this AD, apply a torque of 5.65 Nm (50 in lb) to each nut. (i) If the nut does not move, apply a torque stripe using torque seal lacquer (C-049) or equivalent lacquer, as shown in Figure 2 of ASB 206-19-136 or ASB 206L-19-181, as applicable to your model helicopter. (ii) If any nut moves, inspect each TRDS Thomas coupling and each bolt, nut, and washer for elongated holes and fretting on the fasteners. If any TRDS Thomas coupling has an elongated hole, remove the TRDS Thomas coupling from service. If any bolt, nut, or washer has any fretting, remove the affected part from service. (3) As of the effective date of this AD, do not install nut P/N MS21042L4 or MS21042L5 on any TRDS Thomas coupling.

This AD was prompted by reports of cracked or missing nuts installed on the TRDS Thomas couplings. The FAA is issuing this AD to prevent failure or loss of a nut on the TRDS Thomas couplings. The unsafe condition, if not addressed, could result in loss of the tail rotor and subsequent loss of control of the helicopter.

None.