On October 10, 1986, a Hughes 500-D (registration N58239) was involved in an aviation accident near Seattle, WA. Investigators recorded the probable cause as: Loss of engine power due to an out-of-adjustment throttle linkage on the pilot's side, which prevented adequate power during recovery from a practice autorotation. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced an unexpected power loss during a practice autorotation maneuver. The pilot's attempt to recover failed due to a mechanical fault, resulting in a hard landing that severed the tail boom.
What happened
The incident occurred while the aircraft was engaged in a practice autorotation maneuver. During this phase of flight, the engine experienced an unexpected loss of power. The pilot immediately attempted to recover engine function but was unable to restore thrust. Consequently, the helicopter sustained a hard landing. The impact forces caused the main rotor blades to flex significantly, which in turn severed the tail boom structure.
The investigation
Post-accident examination of the aircraft revealed the mechanical root cause of the power loss. Inspectors found that the throttle linkage on the pilot's side had been out of adjustment. This misalignment prevented the engine from maintaining proper fuel flow during the recovery attempt, causing the power output to fall below idle.
Findings
The primary contributing factor was the improper adjustment of the pilot-side throttle linkage. This mechanical failure directly resulted in the loss of engine power necessary for a safe recovery from the practice maneuver. The subsequent hard landing and structural damage were direct consequences of this inability to regain thrust.
Probable cause
Loss of engine power due to an out-of-adjustment throttle linkage on the pilot's side, which prevented adequate power during recovery from a practice autorotation.