TriState CareFlight Agusta A119 accident

During the emergency medical service (EMS) flight, while enroute at 4,500 feet mean sea level, the pilot heard a change in the sound of the main rotor system, followed by a vibration in the cyclic. He applied aft cyclic to slow the helicopter and noticed the rotor rpm starting to decay. After he lowered the collective in an attempt to regain rotor rpm everything appeared to stabilize. The pilot then increased the collective to see if he could re-establish cruise power, but the increase in collective resulted in the low rotor aural warning activating. When the pilot lowered the collective, the warning went out, and when he tried to maintain rotor rpm by using the throttle in the manual mode there was no change, which resulted in the warning system activating again. The pilot then lowered the collective, returned the throttle to the normal position and began a descent, touching down in soft dirt and sliding forward before coming to rest upright. The helicopter was substantially damaged as a result of the hard landing, which separated the tail section. During the postaccident examination, the engine was tested in the gas generator and the power turbine modes, as well as the electronic engine control (EEC) and mechanical engine control (MEC) modes. The engine was observed to run normally to all inputs in gas generator, EEC and MEC control modes, with manual override also observed to function normally. During a subsequent engine test a reduction in gas generator speed of 8 percent occurred, with the engine fully recovering power in about 5 seconds. Additional testing could not replicate the power reduction as previously observed, and the reason for the reported loss of engine power could not be definitively determined.

Verbatim NTSB analysis and probable cause from the NTSB dataset