On April 27, 2007, a Hiller UH-12C (registration N92993) was involved in an aviation accident near Plant City, FL. Investigators recorded the probable cause as: Failure of the right magneto impulse coupling, which resulted in impact on the stud used for starting, causing gear train stoppage and engine cessation. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During a discovery flight, a helicopter's engine violently shook and ceased operating at 600 feet. The instructor performed an autorotative landing, which resulted in damage. Investigation revealed failure of the right magneto impulse coupling.
Incident Summary
A flight instructor reported that while conducting a discovery flight with a student, the helicopter experienced a violent engine shake during cruise flight at an altitude of 600 feet. The shaking lasted several seconds, after which the engine ceased operating. The instructor immediately entered autorotation and attempted an autorotative landing. The helicopter incurred damage during the landing.
Investigation Findings
A postcrash examination was conducted by an FAA-licensed airframe and powerplant mechanic under the supervision of an FAA airworthiness inspector. The examination revealed that the right magneto impulse coupling had failed. This failure caused the impulse coupling to impact the stud used for starting. According to the inspector, this resulted in gear train stoppage, leading to the engine ceasing to operate.
Maintenance History
The helicopter's maintenance log indicated that the last annual inspection was performed on June 5, 2006.
Probable cause
Failure of the right magneto impulse coupling, which resulted in impact on the stud used for starting, causing gear train stoppage and engine cessation.