Incident Description
A LifeFlight helicopter was en route transporting a patient when the nurse heard a loud 'pop' followed by a 'clattering' sound and a horn alarm. Shortly thereafter, the helicopter impacted a field on a southerly heading approximately 70 feet south of tall trees. Ground scars and debris were located along a 150-foot by 80-foot path. Witnesses reported that the wind was strong and gusty from the south.
Engine Findings
The helicopter was equipped with a Turbomeca Arriel 1B engine featuring the TU-76 modification. The engine had accumulated 2,482 total flight hours, with a recommended time between overhauls of 2,500 hours.
Investigation determined that the engine lost power due to failure of the labyrinth seal in the second-stage turbine nozzle guide vane. Turbomeca personnel proposed that under thermal low cycle fatigue, a crack could initiate on the second turbine nozzle guide vane hub, leading to rubbing between the inner diameter of the hub and the inner turbine labyrinth lips.
Investigation and Service Bulletin
There were seven reported engine failures of a similar nature, all related to TU-76 standard nozzles. In response, Turbomeca issued Service Bulletin 292 72 0153 to improve the mechanical strength of the internal envelope of the second-stage nozzle guide vane.