Incident Overview
During external load operations, the pilot radioed that the engine had failed. Witnesses reported visually observing the main rotor slowing and the individual blades becoming visible before the helicopter descended steeply into a reservoir from about 200 feet above ground level.
Post-Impact Examination
Examination of the wreckage revealed that the main rotor driveshaft, main rotor blades, and tail rotor blades exhibited signatures consistent with low rotor rpm at impact.
Engine Failure Analysis
Engine examination determined that the gas producer and power turbine assembly failed as a result of the cumulative effects of thermal stress due to over-temperature operation. Further examination revealed that the exhaust gas temperature (EGT) harness on the engine had a low signal output, resulting in a low EGT indication in the cockpit. This caused the company pilots to unknowingly operate the engine in excess of maximum EGT.
Operational Practices
According to other company pilots who flew this helicopter on typical external load operations, the engine would reach its EGT limit before the maximum torque was achieved. The operator did not routinely perform health indicator tests (HIT checks), which were developed by the U.S. Army to detect changes in engine power output. Review of the helicopter's FAA Type Certificate Data Sheet (H15NM) revealed the requirement for HIT checks to be accomplished prior to each takeoff and recorded in a log record.