Improper Lock-Washer Installation Could Lead To Servo Malfunction

The FAA has issued an Emergency Airworthiness Directive (AD) prompted by a report that a quality escape by a supplier has occurred and certain hydraulic servo actuators (servo) 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. This condition, if not detected, could lead to a malfunction of a servo in the flight control system and subsequent loss of control of the helicopter.

The FAA has reviewed Bell Alert Service Bulletin (ASB) 407-11-96 and 427-11-35, both dated June 29, 2011, which specify the part numbers and serial numbers of the affected servos and refer to ASB 407-05-70, Revision A, dated November 10, 2005; ASB 427-05-12, Revision A, dated November 14, 2005; with HR Textron Service Bulletin (SB) 41011300-67-01, Revision 2, dated November 9, 2005; HR Textron SB 41011400-67-01, Revision 2, dated November 9, 2005; and HR Textron SB 41011700-67-01, Revision 2, dated November 9, 2005. The ASBs also specify reidentifying the servos with a “67-01” on the modification plate indicating the inspection procedures were followed.

Transport Canada, the airworthiness authority for Canada, notified the FAA that an unsafe condition may exist on these helicopter models. Transport Canada advises that a quality escape by a supplier has occurred, and a number of servos may have a loose nut, shaft, and clevis assembly. Transport Canada states in its AD that the loose connection is due to improper lock washer installation, which is not traceable or identifiable except by inspection. The authority also states a disconnect of the affected parts may lead to loss of control of the helicopter. Transport Canada classified the ASBs as mandatory and issued AD No. CF-2011-17, dated June 30, 2011, to ensure the continued airworthiness of these helicopters.

These helicopters have been approved by the aviation authority of Canada and are approved for operation in the United States. Pursuant to a bilateral agreement, Canada has notified the FAA of the unsafe condition described in the AD. The FAA is issuing this AD because it evaluated all information provided by Canada and determined the unsafe condition exists and is likely to exist or develop on other helicopters of these same type designs. Therefore, this AD requires before further flight for certain affected servos and within 25 hours time-in-service for certain other affected servos, identified by a serial number, retracting the boot and inspecting the servo as follows:

• Applying only hand pressure, determining whether the nut, shaft, or clevis assembly turns independently. If the shaft turns independently of the nut or the clevis assembly, before further flight, replacing the servo with an airworthy servo.
• If the shaft does not turn independently, inspecting to determine whether at least one tab of a lock washer is bent flush against a flat surface of the nut and at least one tab of the lock washer is bent flush against a flat surface of the clevis assembly.
• If at least one lock washer tab is not aligned and bent flush with a flat surface of the nut and at least one lock washer tab is not aligned and bent flush with a flat surface of the clevis assembly, before further flight, replacing the servo with an airworthy servo.
• If any tab of the lock washer is not bent flush against either a flat surface of the nut or clevis assembly, bending the tab flush against a flat surface.
• Reidentifying the servo by metal-impression stamping or by vibro etching “67.01” onto the modification plate.
• Before installing a servo with a P/N and S/N identified in this AD, not identified by “67-01” on the modification plate, inspecting it by following the requirements of this AD.

The actions must be done by following specified portions of the alert service bulletins described previously.

This AD differs from Transport Canada AD in that the FAA does not require that the servo be returned to the manufacturer. Also, the FAA does not limit the applicability to specific serial-numbered helicopters. The FAA has specified the inspection requirements rather than referring to the applicable service bulletins. The AD requires that the servo be replaced before further flight, and the Transport Canada AD refers to the ASB, which requires that the servo be replaced within 300 hours time-in-service.

FMI: www.faa.gov