DORVAL, Canada – In its investigation report released today, the Transportation Safety Board of Canada (TSB) found that the failure of an engine oil feed tube seal led to the turbine rotor failure, and a subsequent fire, during Bombardier engine ground tests at the Montréal International (Mirabel) Airport, Quebec.
On 29 May 2014, two pilots and four test engineers onboard a Bombardier CS100 were conducting engine ground runs at the Mirabel Airport; this was part of testing processes prior to aircraft certification by Transport Canada. During the test, the left engine, manufactured by Pratt & Whitney Canada, experienced an uncontained turbine rotor failure and sudden power loss. The crew immediately shut down the engine and declared an emergency after being advised of smoke and fire on the engine. All personnel on board evacuated the aircraft safely, but the engine and aircraft sustained substantial damage.
The investigation determined that “heat soaking,” as a result of insufficient cooldown, caused the seal of a bearing oil feed tube to fail. It was determined that the engine had been shut down after high power operation, without sufficient time for its internal temperatures to reduce at lower power. As a result, when the seal failed, it allowed engine oil to mix with the turbine rotor’s cooling air flow. The resulting air/oil mixture ignited due to high ambient temperatures, and the ensuing combustion caused the entire turbine rotor stage to fail. This resulted in major damage to the engine, nacelle and wing.
The investigation identified that Pratt & Whitney had issued a Restriction and/or Special Instruction (RSI) with cooling procedures for their engines before shutdown, with alternate solutions for hot shutdowns. Bombardier interpreted the alternate solutions in the RSI as an alternative equal to the other shutdown options contained in the RSI. This resulted in the engine being exposed to one or more hot shutdowns, which led to heat soaking beyond the design criteria of the bearing oil feed tube’s seal.
The investigation also found that, while Bombardier ground personnel successfully extinguished the fire, the engine’s fire extinguishing system had not been activated. There is an increased risk that fire may spread if nacelle fire bottles are not deployed in the event of a fire, and/or if ground fire extinguishers are not located in a way to permit quick access.
Following the occurrence, Bombardier grounded the C Series test aircraft fleet until the cause of the occurrence could be clearly established. For its part, Pratt & Whitney proposed a plan to return to flight which included an enhanced seal, a revised cool-down procedure, and other measures to monitor engine temperatures and prevent hot shutdowns. Further, production engines will feature an enhanced oil supply tube and a cooling airflow configuration that will physically separate the turbine rotor airflow from the bearing compartment to eliminate the possibility of recurrence.