On June 11, 1985, a Bell UH-1B (registration N99676) was involved in an aviation accident near Juneau, AK. Investigators recorded the probable cause as: The spacer between the No. 5 compressor and the impeller was not properly seated during the last overhaul, and during the accident flight it slipped into its proper position, releasing the compressive torque load, causing the compressor assembly to uncouple… This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a log slinging operation in mountainous terrain, a rotor craft suffered total power loss, forcing an autorotative landing into trees. Investigation revealed an improperly seated spacer from the last overhaul caused a chain of failures.
Accident Scenario
While engaged in a log slinging operation over mountainous, densely forested terrain, the rotor craft experienced a total loss of engine power. This forced the pilot to execute an autorotative landing into high conifer trees.
Investigation Findings
Disassembly of the engine revealed evidence that the spacer located between the No. 5 compressor and the impeller had not been properly seated during the last overhaul. During the accident flight, the spacer slipped into its correct position, thereby releasing the compressive torque load that had been applied to the assembly during build-up.
Consequences of the Failure
The release of the compressive torque load caused the compressor assembly to become uncoupled. This forced the impeller forward into its magnesium housing. The impact resulted in an overtemperature condition and subsequent breakup within the turbine assembly, brought on by the combustion of magnesium particles.
Probable cause
The spacer between the No. 5 compressor and the impeller was not properly seated during the last overhaul, and during the accident flight it slipped into its proper position, releasing the compressive torque load, causing the compressor assembly to uncouple and resulting in overtemperature and turbine breakup.