AD 2021-26-18 Prompted By Several Reported Occurrences Of Loss Of Tightening Torque Of The Shur-Lok Nut

The FAA is superseding Airworthiness Directive (AD) 2020-21-01 for certain Airbus Helicopters Model AS-365N2, AS 365N3, EC 155B, EC155B1, and SA-365N1 helicopters.

AD 2020-21-01 required modifying the main gearbox (MGB) tail rotor (T/R) drive flange installation. This AD was prompted by several reported occurrences of loss of tightening torque of the Shur-Lok nut, which serves as a retainer of the MGB T/R drive flange. This AD continues to require modifying the MGB T/R drive flange installation, and includes additional helicopters in the applicability  for the required actions. The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective February 9, 2022.

Supplementary Information: The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to supersede AD 2020-21-01, Amendment 39-21274 (85 FR 63440, October 8, 2020) (AD 2020-21-01). AD 2020-21-01 applied to certain Airbus Helicopters Model AS-365N2, AS 365N3, EC 155B, EC155B1, and SA-365N1 helicopters. The NPRM published in the Federal Register on September 30, 2021 (86 FR 54139). In the NPRM, the FAA proposed to continue to require modifying the MGB T/R drive flange installation, and also proposed to include additional helicopters in the  applicability for the required actions. The NPRM was prompted by several reported occurrences of loss of tightening torque of the Shur-Lok nut, which serves as a retainer of the MGB T/R drive flange.

EASA AD 2020-0287, dated December 21, 2020 (EASA AD 2020-0287), was issued by EASA, which is the Technical Agent for the Member States of the European Union, to correct an unsafe condition for certain AS 365 N2, AS 365 N3, SA 365 C1, SA 365 C2, SA 365 C3, SA 365 N and SA 365 N1 helicopters; and all EC 155 B and EC 155 B1 helicopters. Model SA 365 C3 helicopters are not certificated by the FAA and are not included on the U.S. type certificate data sheet; this AD therefore does not include those helicopters in the applicability. EASA AD 2020-0287 supersedes EASA AD 2020-0212, dated October 5, 2020, which required modification of the MGB T/R drive flange installation. EASA advises of reported occurrences of loss of tightening torque of the ShurLok nut, which serves as a retainer of the T/R drive flange of the MGB. EASA also advises of subsequent investigation that determined that these occurrences were the result of failure of the ShurLok nut locking function, which is normally ensured by two antirotation tabs engaged into two slots at the end of the MGB output shaft pinion. EASA states this condition could lead to the loosening of the Shur-Lok nut and disengagement of the Shur-Lok nut threads, possibly resulting in reduction of T/R drive control, rear transmission vibrations, and subsequent reduced control of the helicopter.

Accordingly, EASA AD 2020-0287 retains the modification of the MGB T/R drive flange installation. EASA AD 2020-0287 also includes additional helicopters in the applicability for the required actions (Model SA-365C1, SA-365C2, and SA-365N helicopters on which Airbus Helicopters modification 0763B64 has been embodied; and Model EC 155B and EC155B1 helicopters without modification 0763B64 embodied).

FMI: www.regulations.gov