On May 28, 2012, a Boeing 777-333ER, C-FITW operated by Air Canada was involved in an aviation accident near Toronto-Lester B. Pearson International Airport, Ontario, CA. No fatalities were reported. Investigators recorded the probable cause as: The number 2 engine shut down during the initial climb-out due to a failure of the high-pressure turbine stage 1 shroud, which was caused by a variation in the shape and size of the shroud cooling holes resulting from a change to a higher-intensity laser… This summary draws on records from the Transportation Safety Board of Canada (TSB); 8 related events involving the same aircraft type or operator are linked below.
An Air Canada Boeing 777-333ER (C-FITW) experienced a number 2 engine failure during initial climb from Toronto. The crew secured the engine, jettisoned fuel, and landed safely with no injuries.
Incident Overview and Immediate Response On 1425 Eastern Daylight Time, Air Canada Flight 001, a Boeing 777-333ER with registration C-FITW and serial number 35298, began its takeoff from Runway 23 at Toronto-Lester B. Pearson International Airport. The aircraft, carrying 16 crew members and 309 passengers, was en route to Narita International Airport in Tokyo, Japan. During the initial climb-out at approximately 1590 feet above ground level, the number 2 engine (GE 90-115B, serial number 906-456) failed. The flight crew followed Quick Reference Handbook procedures to secure the engine, notified air traffic control, and declared an emergency. After jettisoning fuel to reduce the aircraft weight to the maximum landing weight, the aircraft made an uneventful landing at Toronto-Lester B. Pearson International Airport. Emergency services stood by and escorted the aircraft back to the gate. No injuries were reported, though the underside of the right wing sustained minor damage from engine debris, and several automobiles on the ground were also damaged by falling debris. ## Flight History and Engine Failure Details The aircraft had arrived at Toronto-Lester B. Pearson International Airport (CYYZ) after a seven-hour flight from Frankfurt, Germany, with no reported engine defects. Air Canada line maintenance completed the pre-departure checklist and added 1.5 litres of oil to the number 2 engine, while normal oil consumption for this engine model is 0.34 litres per hour. At 1427, with the aircraft at 2160 feet above sea level and a computed airspeed of 206 knots, the number 2 engine lost power. Engine Indicating and Crew Alerting System (EICAS) parameters showed rapid decreases in fan speed (N1), compressor speed (N2), fuel flow, and oil pressure, while exhaust gas temperature (EGT) increased and peaked at 1252°C within two seconds of the failure. Approximately six seconds after the engine failed, the crew received an EICAS engine fail warning when N2 speed decreased below idle. The captain selected the autopilot and levelled the aircraft at 3000 feet asl. The crew followed the engine severe damage checklist, securing the engine by moving the thrust lever to idle, the fuel control switch to cut-off, and pulling the engine fire switch. With no fire warning or indication, the crew did not activate the fire extinguishing bottles. The flaps were retracted, the auxiliary power unit was started, and the aircraft climbed to 7000 feet asl. After levelling off, the fuel jettison nozzle valves were opened for approximately 38 minutes, reducing the total fuel load by 86,600 kg to bring the aircraft to its maximum approved landing weight. The aircraft then climbed to and levelled off at 12,000 feet asl. ## Landing and Post-Incident Examination Upon returning to CYYZ, the crew reviewed landing procedures for one engine inoperative, selected Autobrake 4, and set the flap lever to the 20 detent. Shortly after touching down on Runway 23, the crew activated partial thrust reverser on the operating engine, and the aircraft came to a stop on the runway. Emergency services personnel chocked the main landing gear wheels, and the brakes were released to reduce temperature before the aircraft taxied to the gate. The aircraft had been airborne for approximately one hour and 26 minutes. After passengers deplaned, the aircraft was towed to a hangar for engine examination. The weather at the time of the occurrence was reported as a few clouds based at 4000 feet asl, wind 150° true at 7 knots, temperature 29°C, and barometric pressure 29.85 inches of mercury, and was not considered a factor in the occurrence. ## Preliminary Findings and Safety Actions The aircraft received minor damage to the underside of the right-wing flaperon, consisting of three punctures approximately ¼ to 1¼ inches in diameter in the carbon fibre surface. The engine exhaust cone contained pooled engine oil and small fragments of high-pressure turbine (HPT) and low-pressure turbine (LPT) hardware, with most LPT damage located towards the outer periphery, consistent with excessive heat and impact from debris originating forward of the LPT stage. The engine damage was contained, with no damage to the engine cowlings, though heat damage was present to the HPT module near the HPT active clearance control panels. Fragments that fell from the aircraft while airborne were retrieved by local authorities and found to be identical in appearance to those in the engine exhaust cone. The digital flight data recorder (DFDR) and cockpit voice recorder were downloaded and forwarded to the TSB Laboratory, with the DFDR providing approximately 27 hours of flight information. The number 2 engine operational data was compared to the number 1 engine data, revealing minor but acceptable operational differences that were not considered abnormal, with no indicators of deteriorating engine performance. The propulsor core was seized by several HPT stage 2 nozzle fragments jammed in different locations, preventing the core from rotating freely, with no indication of a bird strike or component failure forward of the HPT module. The engine's service and maintenance history indicated it had been maintained in accordance with current regulations and General Electric's (GE) procedures. Service Bulletin (SB) 72-0401 was issued by GE as part of its HPT stage 1 shroud control program, and engine serial number 906-456 was identified as one of the suspect engines listed in SB 72-0401, subject to repetitive borescope inspections (BSI). Suspect engines were fitted with shrouds part numbers 1847M52P10 and 1847M52P11, whose cooling holes had been drilled with a high-intensity laser during production, producing a variation in the shape of the cooling holes that accelerated shroud deterioration and reduced structural integrity. The deterioration rate was quicker in the area of shroud number 33 due to the unique gas flow pattern in this area. The engine teardown inspection revealed substantial damage and missing shroud material on shroud number 33 and its corresponding hanger, as well as damage to the number 26 and number 27 shrouds, with the extent of the damage suggesting it was likely present but misevaluated or overlooked during the previous BSI. Shroud number 33 of the HPT stage 1 was the common factor in determining initial shroud distress on engines with similar failures. From investigations into similar engine events, GE determined that due to the manufacturing issue with laser-drilled holes, the flow at the leading edge region was lower than design intent, and holes were created as the shroud distress progressed. Shroud 33 is located in a region with a unique upstream cooling condition that could, under the right conditions, allow for a region of fuel-rich air. It is believed that, as the hole in the shroud reaches a critical size, the fuel-air mixture is ingested behind the shroud and is introduced to a larger volume of air, thus creating a super-heated zone that leads to rapid distress of the stage 1 shroud hanger and stage 2 nozzle outer band, with the release of the stage 2 outer band causing subsequent downstream damage. During shroud production, a change to a higher-intensity laser resulted in a variation in the shape and size of the shroud cooling holes. Over a period of time in service, these cooling holes eroded, resulting in both degraded shroud cooling and a super-heated zone, which increased the rate of erosion until the shroud integrity was reduced to the point of failure. Damage to HPT shrouds and hangers, which was likely present during the last borescope inspection, went undetected prior to the occurrence, resulting in the engine not being removed from service. The number 2 engine shut down during the initial climb-out due to a failure of the HPT stage 1 shroud. Air Canada took a proactive approach to establish a baseline for all of its engines, performing all borescope inspections using a four-metre flex scope. After the fleet inspection, Air Canada identified 15 engines currently in service with shrouds that would qualify for the re-inspection program in accordance with SB criteria, with three engines displaying sufficient shroud deterioration and being placed on a reduced cycle inspection interval, and two other engines displaying unserviceable shroud damage and being removed from service to implement either SB 72-0348 or SB 72-0363. Air Canada performed its own risk assessment to evaluate the condition of the remaining engines affected by SB 72-0401 to assist with the prioritization of their removal and repair, considering engine time in service, current conditions of the HPT stage 1 shrouds, engine modification status, and the pairing of affected engines on an aircraft to reduce the probability of a double engine failure. After completing the risk assessment procedure and evaluating the results, Air Canada reduced the amount of cycles between inspections by 50% or more from the inspection schedule cycles documented in SB 72-0401, depending on its assessment of the shrouds. The United States Federal Aviation Administration (FAA) issued Airworthiness Directive (AD) 2013-17-07, Borescope Inspections of the Stage 1 HPT Stator Shrouds, effective 18 October 2013, applying to GE90-76B, -85B, -90B, and -94B engines fitted with stage 1 HPT stator shrouds part number 1847M52P14, as well as GE90-110B1 and -115B engines fitted with similar shrouds.
Probable cause
The number 2 engine shut down during the initial climb-out due to a failure of the high-pressure turbine stage 1 shroud, which was caused by a variation in the shape and size of the shroud cooling holes resulting from a change to a higher-intensity laser during shroud production, leading to degraded shroud cooling and a super-heated zone that increased the rate of erosion until the shroud integrity was reduced to the point of failure. Damage to the high-pressure turbine shrouds and hangers, which was likely present during the last borescope inspection, went undetected prior to the occurrence, resulting in the engine not being removed from service.
