The Sikorsky S76C helicopter with two crew and ten passengers on-board was in a hover with the flight crew completing before takeoff checklist items. The pilot reported that while trimming the engines, a "pop" was heard. He then noted that the number-one engine gas generator outlet temperature (T4) was in excess of 1000 degrees C. The helicopter was then landed uneventfully. The crew reported that the only cockpit indication of imminent failure was the almost simultaneous illumination of the number-one engine chip (magnetic particle) detector advisory. There was no reported engine fire.
Examination of the helicopter revealed minor shrapnel damage to the engine exhaust extension and engine cowling. The engine was removed from the helicopter and sent to the engine manufacturer for technical disassembly inspection.
Engine manufacturer examination
The Arriel engine was comprised of five modules. The gas generator or high-pressure section was known as Module Three (MO3). The manufacturer's final report noted a separation of turbine blade number six of the gas generator second stage disc. The blade had separated above the `fir tree' attachment point but below the blade platform, and had punctured the second stage nozzle guide vane turbine ring. One adjacent blade (number seven) in the direction of turbine wheel rotation was also noted as cracked.
Metallurgical examination by the manufacturer attributed the blade failure to a low-cycle fatigue cracking mechanism. Dimensional inspections failed to reveal any sign of non-conformity that could have led to the development of the abnormal loads. However, the manufacturer stated that platform/disc interferences were a potential source. The manufacturer concluded that abnormal loading was the major contributing factor in the failure, given the reported absence of anomalous material features or evidence of high-temperature operation.
Engine background
At the time of the occurrence, the Turbomeca Arriel engine model 1S1, serial number 15522, had accumulated 4,737.4 hours in service and 4,471 cycles since new. It had accumulated 1,740.0 hours in service and 1,615 cycles since overhaul. Following overhaul, the engine had been installed on March 11, 1999.
Turbine disc life limit
Part number 0292253250 turbine disc was life limited to 10,000 cycles. The incident turbine disc had accumulated 1,615 cycles since new.