What happened
While en route with a patient, the crew of a Lifeflight helicopter experienced a sudden loud "pop" followed by clattering noises and the activation of the horn alarm. Shortly after these sounds, the aircraft impacted a field on a southerly heading, approximately 70 feet south of tall trees. The impact left ground scars and debris along a path measuring roughly 150 feet by 80 feet. Witnesses at the scene reported that the wind was strong and gusty from the south.
The investigation
The aircraft was powered by a Turbomeca Arriel 1B engine featuring a TU-76 modification. At the time of the accident, the engine had accumulated 2,482 total flight hours, approaching its recommended overhaul interval of 2,500 hours.
Investigators determined that the engine lost power due to the failure of a labyrinth seal located in the second-stage turbine nozzle guide vane. Technical analysis suggested that thermal low-cycle fatigue could initiate a crack on the hub of the second-stage turbine nozzle guide vane. This cracking could subsequently cause contact between the inner diameter of the hub and the inner turbine labyrinth lips.
Findings
Records indicated that there had been seven previous engine failures of a similar nature, all associated with TU-76 standard nozzles. To address this issue, Turbomeca issued Service Bulletin 292 72 0153, which was intended to increase the mechanical strength of the internal envelope of the second-stage nozzle guide vane.