Hiller UH 12E N131HA accident near Susanville, California

Probable cause

A partial loss of engine power due to multiple overtemperature events and a hot start event, which resulted in thermally damaged internal components of the engine. Contributing to the accident was the lack of suitable terrain on which to perform the forced landing.

NTSB analysis

While operating over steep, logged forest terrain, the helicopter experienced multiple compressor stalls followed by a partial loss of power. The pilot attempted to regain engine power; however, once he determined that was not possible, he initiated a forced landing on the sloping terrain. The main rotor blades impacted terrain and the helicopter rolled over onto its left side, which resulted in substantial damage to main rotor system, fuselage, and the tail boom.

A postaccident examination of the engine revealed damage to the stage 1, 2, and 3 turbine wheels, which exhibited cracks that were consistent with multiple over-temperature operations and a hot start event. Additionally, examination of the turbine outlet temperature gauge revealed errors that registered higher than the test temperature and are most likely not causal to the over temperature and hot start events. Review of maintenance documents revealed no indication of any previous hot start events or over temperature operations.

Text from the NTSB. Open the NTSB docket for WPR20LA007 for the full record.