Mcdonnell Douglas Helicopter 369E N504MP accident near Mesa, Arizona

Probable cause

The loss of engine power during cruise flight due to the decoupling of the engine’s turbine and compressor sections. Contributing to the decoupling was the excessive wear of the turbine-to-compressor coupling components due maintenance personnel’s placement of an incorrect shim during a compressor section overhaul and a latent misalignment within the exhaust collector.

NTSB analysis

The commercial pilot was conducting a nighttime law enforcement patrol flight in the helicopter. The pilot reported that, about 1.4 hours into the flight, the engine lost all power, and he subsequently performed an autorotation to an unlit empty field. The helicopter landed hard, and the main rotor blades subsequently struck and severed the tailboom.

Postaccident examination of the engine revealed excessive wear in the turbine-to-compressor coupling components. The turbine-to-compressor coupling had moved, which caused its forward splines to contact the spur adapter gear shoulder and its rear splines to disengage from the turbine-splined adapter. The disengagement caused the decoupling of the turbine section from the compressor section and the subsequent loss of engine power.

The excessive wear was determined to be the result of a misalignment of the engine's centerline shafting components. Two areas of misalignment were found. The first misalignment was caused by the incorrect placement of two shims at the interface between the rear diffuser and gearbox. A review of the engine maintenance logbooks revealed that maintenance personnel had placed these shims during the last compressor section overhaul, about 2 years or 1,432.1 flight hours before the accident.

The second misalignment was found between the forward and rear pilot diameter bores of the exhaust collector assembly, which exhibited excessive runout when compared to engineering drawings. The maintenance records showed that, about 9 months before the accident, the exhaust collector assembly was reworked in accordance with the engine manufacturer's maintenance manual, which did not call for a runout check of the forward-to-rear pilot diameters but rather a runout check of both pilot diameters independently with reference to the bearing bore. At the time of rework, the runout-to-bearing bore values were within tolerance. Therefore, it is likely that the discrepancy was latent, predating the accident by a long period of time, and was not significant enough to cause previous shafting misalignment. Although neither of the misalignments individually were likely significant, cumulatively, they produced sufficient misalignment to cause the observed wear.

Text from the NTSB. Open the NTSB docket for WPR14TA236 for the full record.