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Turbine blade separation

The failure mode of the gas generator (GG) rear bearing collapse was attributed to an imbalance condition of the GG second stage disc following the separation of a GG second stage turbine blade. That imbalance condition resulted in high vibration loads and damage to the rear bearing, resulting in module three failure. The damage to the centrifugal diffuser was determined to be a secondary failure and not considered a safety of flight concern.  

Modification TU204

During the period 6 September 1996 to 11 July 2001, there were five reported incidents worldwide of turbine blade separation failure possibly related to modification TU204. Of those five documented failures, all had modification TU204 incorporated. 

The engine manufacturer had identified a possible mass increase and resulting stress increase on the blade roots of turbine blades with the modification TU204 plasma coating applied. They addressed those concerns by discontinuing its incorporation and removing TU204 compliant blades from service during overhaul of number three modules. The recommendation by the manufacturer to remove all TU204 modified modules was not made a mandatory requirement by any airworthiness authority. Arriel 1S1 engine number three modules (and other Arriel variant engines with modification TU204 incorporated) that have not passed through an approved overhaul facility since July 1998 may currently have modification TU204 installed. Those engines and modules may be subject to abnormal blade loading stresses. The imminent separation of a turbine blade is not detectable by any on-board instrumentation or flight crew observations.

Engine fire

After activation by the crew, the engine compartment fire bottles successfully extinguished the fire that occurred following rear bearing collapse and subsequent fracture of the return oil pipe. The external oil pipes were exposed to the high vibration loads imposed by the out-of-balance GG turbine disc and, as a result, two fractured. Following the fracture of the return oil pipe, a heated flammable liquid (oil), was sprayed onto the heated external outer surface of module three. During normal operation, the outer surface of module three experienced surface temperatures within the auto-ignition range of the engine oil. The oil ignited causing an in-flight fire. The fuel source of the fire was the heated engine oil escaping from the fractured return oil pipe. The ignition source of the fire was the hot outer surface of module three. If the flight crew had not secured the engine, or the engine had not stopped rotating, the supply of flammables for combustion would have been limited only by engine oil system capacity.

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