Helicopter Hard Landing After Engine Power Loss Due to Planetary Gear Failure (N114FD)

Event Details

A helicopter sustained a hard landing following an engine power loss while in the traffic pattern. The pilot reported that he had just picked up the aircraft after maintenance on the audio and visual low rpm warning systems and was positioning to return to the airport. As he entered the pattern, he heard a bang and a loud grinding sound from the rear of the aircraft, followed by a loss of engine power. The pilot entered an autorotation to a clear area on the airport. Near touchdown, he realized the selected area was sloped and increased collective to extend the glide to a flat area, resulting in a hard landing.

Examination Findings

A teardown and examination of the engine revealed that the type and degree of damage were indicative of the separation of a portion of a planetary reduction gear. This separation caused a piece of inlet housing to enter the compressor, damaging compressor airfoils and leading to power loss. The separation was determined to be the result of a fatigue fracture originating from the aft root corner of one gear tooth. The root cause of the fatigue fracture could not be determined due to smearing of the initiation site. The planetary gears installed in the engine were military direct purchase breakout parts. The engine manufacturer does not approve the use of military direct purchase hardware in FAA certified or military surplus engines used in public-use operations.