Aviation safety investigations & reports

Sikorsky Aircraft S-76C, VH-EXN

Investigation number:
Status: Completed
Investigation completed


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.


The turbine disc and blades were well within the life limits established by the manufacturer. The gas generator second stage turbine blade sustained progressive fatigue cracking and subsequently separated causing rear bearing damage and engine failure.

Metallurgical examination by the manufacturer attributed the fatigue cracking and subsequent failure to the development of abnormal loads within the blade root region. The reason for the abnormal loading could not be conclusively established.

Safety Action

Local safety action

The engine manufacturer has implemented a platform/turbine disc interference check on all new MO3 assemblies.

General details
Date: 17 February 2001   Investigation status: Completed  
Time: 1300 hours ESuT    
Location   (show map): Longford, HLS    
State: Victoria    
Release date: 07 November 2001   Occurrence category: Incident  
Report status: Final   Highest injury level: None  

Aircraft details

Aircraft details
Aircraft manufacturer Sikorsky Aircraft  
Aircraft model S-76  
Aircraft registration VH-EXN  
Serial number 760423  
Type of operation Business  
Sector Helicopter  
Damage to aircraft Nil  
Departure point Longford, VIC  
Departure time 1300 hours ESuT  
Destination Kingfish A Tasman Sea  
Last update 13 May 2014