Canadian safety investigators with help from General Electric specialists are focusing on an unspecified turbine failure as a possible cause of a May 28 incident which led to the inflight shut down of a GE90-115B powering an Air Canada Boeing 777-300ER.

The event took place shortly after takeoff from Toronto Pearson International Airport and forced the crew of the aircraft, bound for Tokyo Narita International Airport, to dump fuel before returning to land. Local media reports indicated debris from the engine had fallen on surrounding neighborhoods, and air traffic control told the crew that part of one of the engine cowlings had been discovered on the ground.

Canadian transportation safety investigators also reported that an initial superficial examination of the GE90 showed no indications of damage to the inlet or fan. “The failure appears to have happened in the turbine section,” said an official.

Although the GE90 has established an impressive in-flight shut down rate of only 0.002 over a 15-year service record, the highest-rated thrust variant, which suffered the most recent failure, is currently subject to a number of turbine-related airworthiness directives (AD). One of these, issued by the FAA in 2009, relates to the Stage 6 low-pressure turbine (LPT) which sources close to the investigation indicate could be the focus for the Air Canada investigation.

The AD called for the repetitive inspections for shroud interlock wear of the Stage 6 LPT blades, and was issued after “eight reports of GE90-115B Stage 6 LPT single-blade separation events,” said the FAA.

The directive also required replacement of eligible blades at the next shop visit as a terminating action.

The FAA also issued an AD in November 2011 related to inspections of the GE90-110/115B high-pressure compressor (HPC) after “an aborted takeoff caused by liberation of small pieces from the HPC Stages 1-2 seal teeth and two shop findings of cracks in the seal teeth.” The compressor directive calls for eddy current inspection or spot fluorescent penetrant inspection of the Stages 1-2 seal teeth of the HPC stages 2-5 spool for cracks. The FAA adds the action was taken to “detect cracks in the HPC stages 1-2 seal teeth due to heavy rubs that could result in failure of the seal of the HPC stages 2-5 spool, uncontained engine failure, and damage to the airplane.”