AD/S-76/74 Main Rotor Assembly Lower Bifilar Support (Cth)

AIRWORTHINESS DIRECTIVE

For the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.001(1) of CASR 1998.  The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section.

Sikorsky S-76 Series Helicopters

AD/S-76/74 Main Rotor Assembly Lower Bifilar Support 1/2006 DM

Applicability:

Model S-76A, B, and C helicopters, equipped with a main rotor hub pilot fitting, part number 76103-08003-101 that has 1500 or more hours time in service.

Requirement:

Inspect the lower bifilar arm assembly for a crack in the lug attachment area in accordance with paragraphs 3.A.(1) through 3.A.(6) of the Accomplishment Instructions of Sikorsky Alert Service Bulletin (ASB) No. 76-65-62, dated
14 December 2004, or later FAA approved revision.

If a crack is evident on any bifilar arm assembly lug, before further flight, replace the bifilar arm assembly with a serviceable bifilar arm assembly.

If no cracking is evident at the initial inspection, perform a one-time torque test.  Perform the torque test and the additional torque procedures as stated in the Accomplishment Instructions of paragraphs 3.B.(1) through 3.B.(3) of ASB No.
76-65-62.  The torque test is not required at the recurring inspection intervals of the lower bifilar arm assembly.

Note:  FAA AD 2005-22-01 Amdt 39-14345 refers.

Compliance:

Inspect within 50 hours time in service after 1 December 2005, and thereafter at intervals not to exceed 50 hours time in service.

This Airworthiness Directive becomes effective on 1 December 2005.

Background: The FAA received two reports before the ASB was issued, and two subsequent reports of cracked bifilar arm assemblies.  The additional reports indicate that cracks have become more severe.  This Directive is effective for helicopters with certain pilot fittings, and this is different to the applicability of the ASB.  Investigation has shown the root cause of the failure is not the bifilar itself, but the mating part that attaches the bifilar to the main rotor hub, called the pilot fitting.  This fitting is transferring the load too abruptly into the bifilar.  The frequency of reports and severity of failures prompted this Directive.  In the most recent report, all of the six lugs had separated from the bifilar.

David Villiers
Delegate of the Civil Aviation Safety Authority

29 November 2005