Incident Summary
A helicopter sustained substantial damage when its tail boom was severed during a hard landing following an in-flight engine failure. The pilot initiated an autorotation after the engine lost power, but the aircraft spun abruptly and the main rotor blades struck the tail boom and the ground.
Sequence of Events
The pilot reported observing an engine chip light and hearing a whining sound from the engine while at an altitude of approximately 700 feet above ground level (AGL). About one minute later, an engine out light illuminated, and the engine failed completely. The pilot had already begun descending to land in an open field.
To execute an autorotation, the pilot lowered the collective and applied aft cyclic to reduce forward airspeed. As the helicopter neared the ground, he pulled collective pitch to cushion the landing. However, the helicopter abruptly spun 90 degrees to the left. The main rotor blades struck the tail boom and the ground, causing the tail boom to separate from the helicopter.
Findings
Following the accident, an inspection and teardown of the engine revealed that the 2½ bearing had failed in fatigue. Further investigation of maintenance records showed that a chip light had been reported on the same helicopter four days prior to the accident flight. Before the helicopter was returned to service after that event, a magnetic plug inspection and cleaning, as well as an engine ground test run, were performed.
The Allison 250 Series Operation and Maintenance Manual specifies that when a chip light is encountered after a gearbox flush and a second ground test run has been performed, or when a total of four chip lights have been encountered within 50 hours of engine operation, the engine must be removed from service and sent to the manufacturer's authorized maintenance center.