AD 2011-15-51 76 FR 66609 Active

This document publishes in the Federal Register an amendment adopting Airworthiness Directive (AD) 2011-15-51, which was sent previously to all known U.S. owners and operators of the specified Bell Model 407 and 427 helicopters by individual letters. This AD requires inspecting certain hydraulic servo actuators to determine whether the shaft turns independently of the nut or the clevis assembly. If the shaft turns independently, this AD requires replacing the servo with an airworthy servo. If the shaft does not turn independently, the AD requires inspecting to determine the condition of the lock washers. Based on the condition of the lock washers, the AD requires either replacing the servo with an airworthy servo, or if any tab of the lock washer is not flush against a flat surface of the nut or clevis assembly, bending it flush against a flat surface. The AD also requires reidentifying the servo by metal-impression stamping or by vibro- etching "67.01" onto the modification plate. Also, the AD requires before installing a servo with a part number or serial number identified in this AD, not identified by "67-01" on the modification plate, inspecting it by following the requirements of this AD. This AD is prompted by a report that a quality escape by a supplier has occurred and certain servos may have a loose nut, shaft, and clevis assembly due to improper lock-washer installation. An investigation after an accident revealed the clevis nut on the servo was loose. The actions specified by this AD are intended to prevent a malfunction of a servo in the flight control system and subsequent loss of control of the helicopter.