Eurocopter AS 350 N5204X accident near Bisbee, Arizona

Probable cause

Failure of maintenance personnel to ensure adequate torque of a pneumatic control pipe (P2) fitting, which resulted in a loss of engine power during low altitude flight maneuvers.

NTSB analysis

The commercial pilot was maneuvering the turbine engine-equipped helicopter about 25 ft above ground level when the engine suddenly experienced a total loss of power. The pilot performed an autorotation, and the helicopter touched down hard and rolled over, coming to rest inverted. Examination of the engine revealed that the pneumatic control pipe (P2), which delivers air pressure from the centrifugal compressor on the engine to the fuel control unit (FCU), was disconnected at the engine fitting. The loss of P2 pressure to the FCU resulted in the FCU commanding the engine to spool down to ground idle speed by limiting the fuel flow. There was no evidence of malfunctions or anomalies on the pipe, threads, and union fastener. There was no evidence of torque stripe residue on the disconnected fitting, consistent with the likely scenario that, during past maintenance, the B-nut was not properly torqued and gradually vibrated off its attach fitting while in flight.

The engine manufacturer had several caution notifications within multiple maintenance task documents to prevent removal of the P2 pipe during washing and the risk of an engine failure due to insufficient torque.

Text from the NTSB. Open the NTSB docket for WPR15TA027 for the full record.