Hughes 369D N5027P accident near Waldorf, Maryland

Probable cause

Fatigue failure of the overrunning clutch outer race and a subsequent inflight loss of power due to abnormal bending loads as a result of improper reassembly of the clutch subassembly following maintenance and the presence of microcracks on the clutch’s outer race.

NTSB analysis

The flight crew of the helicopter detected an abnormal noise while in cruise flight and the pilot felt a vibration in the collective. The crew chose to discontinue the mission and return to the departure airport; however, about 4 nautical miles from the airport, there was a loud bang/pop noise and the helicopter yawed left. The pilot entered an autorotation and selected a forced landing site adjacent to a pond in a residential area.

Home security video captured the final seconds of the flight. The sounds of the helicopter’s engine and the turning main rotor could be heard before the helicopter appeared in the frame, descending steeply in a level attitude. The helicopter dropped from sight behind a house and reappeared just as the helicopter’s nose flared upward and the tail rotor impacted the surface of the pond about the same time. The tailboom was separated by the turning main rotor at or about water contact. The helicopter’s skids impacted the fence that bordered the pond, and the helicopter subsequently came to rest upright. The engine, which continued to run briefly, was shut down by the pilot and the damaged main rotor blades stopped turning. The crewmembers egressed the helicopter without assistance.

Examination of the helicopter revealed the overrunning clutch outer race’s splined shaft had fractured, which disconnected engine power from the main transmission.

Examination of the fracture surface showed signatures consistent with fatigue. The examination of the disassembled clutch subassembly revealed off-axis and asymmetric wear, indicating that the overrunning clutch was likely reassembled improperly after its last inspection, which occurred about three months and 22 flight hours before the accident. Additionally, secondary microcracks were observed on the shaft portion of the clutch outer race's splined shaft that likely occurred during its manufacture. The improper reassembly of the clutch subassembly following the inspection likely resulted in abnormal loading of the clutch outer race shaft during operation. This abnormal loading, in addition to the preexisting microcracks, initiated fatigue cracking of the clutch outer race, resulting in its failure during the accident flight.

Text from the NTSB. Open the NTSB docket for ERA23LA293 for the full record.