Accident Sequence
The helicopter was damaged during a forced landing after a loss of engine power while in cruise flight. The pilot, with 14,700 hours of experience, reported that at 5,100 feet MSL, the generator warning light illuminated. After attempting to reset the generator by switching it off and on, and then recycling the circuit breaker, the pilot was unable to reset it. The pilot then pulled the generator circuit breaker and aborted the flight.
Approximately 15 minutes after the generator failure, the engine out light illuminated and an audio warning sounded. Engine and rotor RPM began to decay, and the pilot initiated an autorotation. The auto re-ignition light illuminated during the autorotation, but the engine did not spool back up to normal RPM. The pilot continued the autorotation to a small clearing.
During the final phase of the autorotation, the pilot performed a cyclic flare to increase rotor RPM and arrest forward movement. The pilot leveled the helicopter a few feet off the ground and pulled the collective to arrest descent, at which point rotor RPM decayed and the helicopter dropped from four feet AGL. The helicopter spun approximately 180 degrees from the original heading and impacted the ground nose first, on a heading of 30 degrees.
Examination Findings
Examination of the generator revealed that its drive-end bearing had failed. Bearing cage pieces fell out during disassembly. A Pc air leak test was performed, and air was detected emanating from the Pc air tube from the power turbine governor (PTG) to the engine fuel control unit (FCU). The tube was fractured approximately 90% circumferentially just above the coupling nut that attaches the tube to the FCU. Further metallurgical examination of the Pc tube revealed evidence of fatigue.
Engine Test
A new Pc air tube (same part number) was installed, and the engine was operated in a test cell in accordance with the model 250-C20B Production Test Specifications. With the engine in the standard test configuration, response times were within normal limits and the engine met all performance parameters.