Summary
We propose to supersede an existing airworthiness directive (AD) that applies to all Turbomeca S.A. Arriel 2D turboshaft engines. The existing AD currently requires replacing the hydromechanical metering unit (HMU) at a reduced life. Since we issued that AD, further cases of deterioration of HMU rotating components have been reported before the HMU reached the replacement interval in that AD. The proposed AD would maintain the existing AD requirements and would also require inspections of the HMU. We are proposing this AD to prevent an uncommanded in-flight shutdown of the engine, and possible loss of the helicopter.