On January 4, 1999, a Hughes 369D (registration N5013P) was involved in an aviation accident near Woodruff, UT. Investigators recorded the probable cause as: A partial loss of engine power due to erosion in the compressor assembly module. Factors were a missing rotating labyrinth seal, the installation of which had not been performed by unknown maintenance personnel. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced multiple compressor stalls during a low-altitude maneuver, leading to rotor rpm decay and a subsequent ground impact and rollover. Post-accident testing revealed the engine was 20 horsepower below overhaul specifications, with case liner erosion and a missing labyrinth seal.
Accident Sequence
While maneuvering at low altitude, the pilot heard a series of compressor stalls, accompanied by a rapid decay of rotor rpm. The helicopter subsequently struck the ground, spun around, and rolled over onto its right side.
Engine Examination
Functional testing of the engine revealed it was 20 horsepower below overhaul specifications. The compressor exhibited repeated stalls. Disassembly of the compressor disclosed erosion of the case liner, most noticeably at the 5th and 6th stages. Additionally, the rotating labyrinth seal was missing.
Resolution
After replacing the missing labyrinth seal and installing a slave compressor module, the engine tested to normal operational parameters.