AD 2020-21-01 Was Prompted By Several Reported Occurrences Of Loss Of Tightening Torque Of The Shur-Lok Nut

The FAA is adopting a new airworthiness directive (AD) for certain Airbus Helicopters Model AS-365N2, AS 365N3, EC 155B, EC155B1, and SA-365N1 helicopters. This AD requires 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 T/R drive flange. The actions of this AD are intended to address an unsafe condition on these products. This AD is effective November 12, 2020.

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 Airbus Helicopters Model AS-365N2, AS 365N3, EC 155B, EC155B1, and SA-365N1 helicopters with modification 0763B64 installed, except those with modification 0763C81. The NPRM published in the Federal Register on April 23, 2020, (85 FR 22688). The NPRM proposed to require within 600 hours time-in-service, modifying the MGB T/R drive flange installation by removing the sliding flange from the flexible coupling and installing the sliding flange with aft output stop part number 365A32-7836-20 added, as per helicopter model and configuration.

The NPRM also proposed to require removing from service certain washers, degreasing the bolt threads, applying a sealant between the interlay mating surfaces, and applying torque to the nuts. The proposed requirements were intended to prevent loosening and disengagement of the Shur-Lok nut threads, possibly resulting in reduction of T/R drive control, rear transmission vibrations, and subsequent loss of control of the helicopter.

The NPRM was prompted by EASA AD No. 2019-0046, dated March 11, 2019 (EASA AD 2019-0046), issued by EASA, which is the Technical Agent for the Member States of the European Union, to correct an unsafe condition for Airbus Helicopters (formerly Eurocopter, Eurocopter France, Aerospatiale) Model SA 365 N1, AS 365 N2, AS 365 N3, EC 155 B,  and EC 155 B1 helicopters, all serial numbers, with modification 0763B64 installed, except those with 07 63C81 installed. EASA advises of reported occurrences of loss of tightening torque of the Shur-Lok 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 Shur-Lok nut locking function, which is normally ensured by two anti-rotation tabs engaged into two slots at the end of the MGB output shaft pinion. EASA states this condition could lead to the loosening and disengagement of the Shur-Lok nut threads, possibly resulting in reduction of T/R drive control, rear transmission vibrations, and subsequent loss of control of the helicopter.

To address this unsafe condition, EASA issued a series of ADs, initially with EASA AD No. 2014-0165, dated July 14, 2014 (EASA AD 2014-0165), which required a one-time inspection of the radial play inside the T/R drive flange and the condition of the Shur-Lok nut. Shortly after, EASA issued EASA AD No. 2014-0179, dated July 25, 2014 (EASA AD 2014-0179) to supersede EASA AD 2014-0165. EASA AD 2014-0179 retained the requirements of EASA AD 2014-0165 and expanded the applicability of helicopters affected by the unsafe condition. EASA later revised EASA AD 2014-0179 to Revision 1, dated July 29, 2014, to revise the applicability and specify updated related service information, and again to Revision 2, dated April 11, 2016 (EASA AD 2014-0179R2), to reduce the applicability and specify additional updated related service information. Since EASA issued EASA AD 2014-0179R2, another occurrence was reported that involved an on-ground loss of T/R synchronization, resulting from disengagement of the Shur-Lok nut. This additional occurrence prompted EASA to issue EASA AD 2019-0046 to require installation of modification 07 63C81, which consists of installing a rear output stop with 5 spigots on the T/R shaft flexible coupling.

According to Airbus Helicopters, the 5 spigots will come into contact with the row of 5 bolt heads of the front T/R shaft if the T/R drive flange moves backwards. This contact limits backward displacement of the T/R drive flange and subsequently prevents T/R drive flange disengagement.

FMI: www.regulations.gov