In its investigation report, the Transportation Safety Board of Canada (TSB) found that the failure of an engine oil feed tube seal led to ...
In its investigation report, 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 Bombardier's first prototype of the CS100 single aisle passenger jet, 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.
On the day of the occurrence, Bombardier and Pratt & Whitney planned to conduct engine ground runs on the Flight Test Vehicle 1 (FTV1) in 2 distinct phases.
The first phase was to run the right engine at various predetermined power settings in order for P&W engineering personnel to gather cabin air samples. This sampling was part of an ongoing effort to isolate the cause of an oil smell in the cabin and cockpit that was first noticed in early November 2013.
The second phase was to run the left engine, also at various predetermined power settings, in order to leak-check an oil pump assembly that had been replaced the previous evening as part of an effort to troubleshoot an oil consumption issue. This second phase was to be conducted in 2 parts.
The first part of the engine ground run would be carried out with the thrust reverser doors open, then the engine would be shut down and the thrust reverser doors would be closed and secured. The engine would then be restarted for the second part of the engine ground run.
Fuel tank inerting systems (FTIS) uses bleed air from the engine or auxiliary power unit (APU) and outputs nitrogen-enriched air (NEA) to reduces the chance of the fuel tanks igniting as a result of an in-tank ignition source. This non-flammable mixture is then routed to the fuel tanks, filling the ullage space with NEA.
During the occurrence, the engine fire detector loops were severed by rotor disk debris, and there was no fire warning in the cockpit. However, the EICAS generated several warnings as the engine spooled down, one of which was an L ENG FIRE DET FAIL indication on the cockpit EICAS screen. When this type of indication appears, the procedure calls for monitoring the engine instruments.
Following the occurrence, it was determined that the fire extinguishing system had not been compromised by the uncontained rotor failure and could have been activated to suppress the fire.
There was substantial damage to the left-engine outboard thrust reverser cowl as debris penetrated the carbon-fibre–composite core fairing section and the aft edge of the translating sleeve at approximately the 10 o'clock position (fuselage station). Some soot was also evident around the damaged area.
The inboard thrust reverser cowl also sustained damage, as debris exited outward in an arc between the 1 and 5 o'clock positions, breaking away most of the aft end of the core fairing and translating sleeve.
The left-wing structure sustained major debris impact damage when a segment of the first-stage LPT rotor disk 28 inches long penetrated the wing's centre fuel tank.
The impact created a span-wise gash 33 inches long and 3 inches wide in the carbon composite lower skin plank, inboard of the engine at wing rib No. 6 and extending to rib No. 5, just aft of the forward wing spar, with a total delamination area of 16 × 37 inches.
The turbine disk segment then partially exited through the upper wing plank, where it remained stuck, creating a hole approximately 13 × 7 inches, with a total delamination area of 21 × 10 inches. Signs of burning were found around the hole, although the fuel contained in the tank did not ignite.
The first stage disk of the LPT was missing, except for an ovalized, donut-shaped remnant of the bore resting on the centre shaft. The left side thrust link was severed in line with the LPT plane; some engine-air and -oil lines, as well as some electrical wires and fire detection loops, were damaged or severed in the vicinity of the LPT case breach.
Pratt & Whitney estimates that the subject rotor disk would take approximately 3 minutes to heat to the point of failure when subjected to a lean flame of 1900 °F, taking 145 seconds to reach 1500 °F and 180 seconds to reach 1600 °F.
This estimate was attained using a prediction model that accounted for volume, area, specific heat, density, and convective transfer parameters.
Once heated to the point of failure, the LPT1 rotor, rotating at high velocity, fractured from high strain at the inner web.
Bombardier's assessment concluded that, if the incident had occurred on the ground (before V1 [decision speed]) with a filled centre tank, it could have resulted in a fire hazard, and that if it had occurred on the ground (after V1), in the air or during landing, then the damage sustained would not have prevented continued safe flight and landing of the aircraft.
The high oil consumption first noticed on 26 May 2014 was likely the first apparent sign of severe oil leakage from the Teflon C-seal; however, the exact cause of the oil consumption had not yet been determined.
It is also possible that an oil fire may have been present, perhaps sporadically, in the bearing cavity during the 26 May 2014 engine ground run and that the disk integrity had begun to be compromised.
Source: Transport Canada
Two pilots and four test engineers onboard Bombardier's first prototype of the CS100 single aisle passenger jet, 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.
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| Left inboard thrust reverser cowl damage. Bombardier Photo |
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.
On the day of the occurrence, Bombardier and Pratt & Whitney planned to conduct engine ground runs on the Flight Test Vehicle 1 (FTV1) in 2 distinct phases.
The first phase was to run the right engine at various predetermined power settings in order for P&W engineering personnel to gather cabin air samples. This sampling was part of an ongoing effort to isolate the cause of an oil smell in the cabin and cockpit that was first noticed in early November 2013.
The second phase was to run the left engine, also at various predetermined power settings, in order to leak-check an oil pump assembly that had been replaced the previous evening as part of an effort to troubleshoot an oil consumption issue. This second phase was to be conducted in 2 parts.
The first part of the engine ground run would be carried out with the thrust reverser doors open, then the engine would be shut down and the thrust reverser doors would be closed and secured. The engine would then be restarted for the second part of the engine ground run.
Damage
Initial inspection revealed that the left engine sustained an uncontained failure in the low-pressure turbine's first stage area and that debris from the engine caused substantial damage to the airframe as well as lighter damage to various specific areas, including:- wing's lower surfaces
- wing-to-fuselage fairing panels
- wing leading edge slats
- flap fairings
- landing gear door panels and actuating mechanisms
- landing gear strut and braces
- fuel inerting equipment.
Fuel tank inerting systems (FTIS) uses bleed air from the engine or auxiliary power unit (APU) and outputs nitrogen-enriched air (NEA) to reduces the chance of the fuel tanks igniting as a result of an in-tank ignition source. This non-flammable mixture is then routed to the fuel tanks, filling the ullage space with NEA.
During the occurrence, the engine fire detector loops were severed by rotor disk debris, and there was no fire warning in the cockpit. However, the EICAS generated several warnings as the engine spooled down, one of which was an L ENG FIRE DET FAIL indication on the cockpit EICAS screen. When this type of indication appears, the procedure calls for monitoring the engine instruments.
Following the occurrence, it was determined that the fire extinguishing system had not been compromised by the uncontained rotor failure and could have been activated to suppress the fire.
There was substantial damage to the left-engine outboard thrust reverser cowl as debris penetrated the carbon-fibre–composite core fairing section and the aft edge of the translating sleeve at approximately the 10 o'clock position (fuselage station). Some soot was also evident around the damaged area.
The inboard thrust reverser cowl also sustained damage, as debris exited outward in an arc between the 1 and 5 o'clock positions, breaking away most of the aft end of the core fairing and translating sleeve.
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| Front face of the low-pressure turbine (Source: Pratt & Whitney, with TSB annotations) |
The left-wing structure sustained major debris impact damage when a segment of the first-stage LPT rotor disk 28 inches long penetrated the wing's centre fuel tank.
The impact created a span-wise gash 33 inches long and 3 inches wide in the carbon composite lower skin plank, inboard of the engine at wing rib No. 6 and extending to rib No. 5, just aft of the forward wing spar, with a total delamination area of 16 × 37 inches.
The turbine disk segment then partially exited through the upper wing plank, where it remained stuck, creating a hole approximately 13 × 7 inches, with a total delamination area of 21 × 10 inches. Signs of burning were found around the hole, although the fuel contained in the tank did not ignite.
Left Engine Damage
The main damage was found in the area of the first stage of the Low Pressure Turbine. The initial visual examination of the engine while still on-wing revealed that the LPT case was breached around 95% of its circumference. The sole unbreached area was located at approximately the 8 o'clock position.The first stage disk of the LPT was missing, except for an ovalized, donut-shaped remnant of the bore resting on the centre shaft. The left side thrust link was severed in line with the LPT plane; some engine-air and -oil lines, as well as some electrical wires and fire detection loops, were damaged or severed in the vicinity of the LPT case breach.
 |
| Thrust link breached |
Pratt & Whitney estimates that the subject rotor disk would take approximately 3 minutes to heat to the point of failure when subjected to a lean flame of 1900 °F, taking 145 seconds to reach 1500 °F and 180 seconds to reach 1600 °F.
This estimate was attained using a prediction model that accounted for volume, area, specific heat, density, and convective transfer parameters.
Once heated to the point of failure, the LPT1 rotor, rotating at high velocity, fractured from high strain at the inner web.
Bombardier damage assessment
At TC's request, a team of Bombardier engineers assessed the airframe damage to FTV1 resulting from the event. This damage assessment considered the structural and system damage induced by this rotor burst occurrence and evaluated the capability of the aircraft to continue safe flight and landing if the incident had happened during flight testing or operation.Bombardier's assessment concluded that, if the incident had occurred on the ground (before V1 [decision speed]) with a filled centre tank, it could have resulted in a fire hazard, and that if it had occurred on the ground (after V1), in the air or during landing, then the damage sustained would not have prevented continued safe flight and landing of the aircraft.
Effect of heat on Teflon C-seal
It could not be determined whether the heat damage to the Teflon C-seal was the cumulative result of several hot shutdowns or the sole result of the hot shutdown following the 21 May engine ground runs.The high oil consumption first noticed on 26 May 2014 was likely the first apparent sign of severe oil leakage from the Teflon C-seal; however, the exact cause of the oil consumption had not yet been determined.
It is also possible that an oil fire may have been present, perhaps sporadically, in the bearing cavity during the 26 May 2014 engine ground run and that the disk integrity had begun to be compromised.
Source: Transport Canada
