What happened
On 12 July 2007, an Aerospatiale AS350B1 helicopter, registration EI-IHL, was performing a scheduled safety inspection of underground gas pipelines near Ballynacally, Co. Clare. The flight, operated by Irish Helicopters Limited, involved a pilot and an observer conducting a low-level survey of the infrastructure.
While flying at approximately 400 feet near Lisheen, the aircraft performed a left-hand inspection turn. During this maneuver, the engine suffered a sudden and total loss of power. The pilot attempted to establish an autorotation to manage the descent over the undulating terrain. Despite efforts to steer the aircraft toward a hedgerary to avoid a stone wall, the helicopter struck farmland. The impact resulted in one fatality (the observer) and one serious injury (the pilot).
The investigation
The AAIU investigation focused on the sudden engine stoppage and the mechanical state of the engine's accessory gearbox. Investigators examined the wreckage and conducted laboratory testing on various engine components, including the starter-generator and the fuel control unit (FCU).
Technical analysis centered on the 41-tooth bevel gear, a critical component within the engine accessory gearbox. Laboratory examinations, including hardness tests and macrographic observations of the fracture surface, were used to determine the nature of the component's failure. The investigation also reviewed the aircraft's maintenance history and the operational parameters of the flight, including the low-flying permissions in effect at the time.