On August 7, 1982, a Hughes 369D (registration N10966) operated by Olympic Air, Inc. was involved in an aviation accident near Shelton, WA. Investigators recorded the probable cause as: Over-heating and coking of oil caused thermal and mechanical distress to the turbine-to-compressor coupling and power turbine shaft, resulting in binding. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 2 related events involving the same aircraft type or operator are linked below.
During a sightseeing flight, a pilot detected a burning odor and prepared for an off-airport landing. Engine power decayed, leading to an autorotative landing on a dirt road. The main rotor blades struck and separated the tail boom. Investigation found a bound coupling due to oil overheating and coking.
Accident Summary
A sightseeing flight was interrupted when the pilot smelled a burning odor and began preparations for a precautionary landing off the airport. Shortly thereafter, engine RPM decayed, and the pilot performed an autorotative landing on a dirt road. Upon touchdown, the main rotor blades flexed downward, striking the tail boom and causing it to separate from the fuselage.
Investigation Findings
The investigation revealed that the turbine-to-compressor coupling was bound to the power turbine shaft that sleeves it. Evidence of thermal and mechanical distress was found on the components, caused by over-heating and coking of the oil. No other factors were identified as contributing to the event.
Probable cause
Over-heating and coking of oil caused thermal and mechanical distress to the turbine-to-compressor coupling and power turbine shaft, resulting in binding.