Airworthiness Directives; Turbomeca S.A. Arrius 2F Turboshaft Engines

The FAA is revising an existing airworthiness directive (AD) for Turbomeca S.A. Arrius 2F turboshaft engines. That AD currently requires replacing certain O-rings on the check valve piston in the lubrication unit, at repetitive intervals. This AD requires the same actions except it reduces the applicability from all Turbomeca S.A. Arrius 2F turboshaft engines, to Turbomeca S.A. Arrius 2F turboshaft engines that have not incorporated modification Tf75. This AD results from Turbomeca S.A. introducing a check valve piston design requiring no O-ring. We are issuing this AD to prevent an uncommanded in-flight shutdown of the engine, which could result in a forced autorotation landing and damage to the helicopter.