Cimber Sterling AT72 at Copenhagen on Sep 13th 2011, engine fire

Last Update: November 27, 2012 / 15:29:11 GMT/Zulu time

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Incident Facts

Date of incident
Sep 13, 2011

Classification
Incident

Cause
Engine fire

Airline
Cimber Sterling

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

The Danish Havarikommissionen (HCL) released their final report concluding:

The root cause of the engine failure and engine fire was found to be fatigue crack propagation caused by casting defects (shrinkage porosities) of one of the Power Turbine (PT) stage 1 blade, which led to fracture of one of the PT1 blades. The fractured blade caused a lot of fractured PT stage 2 blades and the PT Rotor came in unbalance and broke the bolts holding the Rotor shaft Bearing Housing No 6 & 7. This in turn put all the force on the Bearing Housing oil transfer tubes causing them to break, subsequently leading to an oil leak. The internal oil leak caused a fire. The leaking oil continued out to the bottom of the nacelle and the internal oil fire then spread outside to the nacelle area.

The technical investigation revealed that this event was not an isolated event. The AIB-DK concluded that at the time of the serious incident, there were unidentified PT1 blades from the same Batch (Heat) in circulation around the world as the fractured PT1 blade - with micro shrinkage porosity.

The HCL reported the first officer (34, ATPL, 2,613 hours total, no time reported on type) was pilot flying for the takeoff from runway 22R, the captain (32, ATPL, 3,874 hours total, 3,108 hours on type) was pilot monitoring. Following takeoff at 136 feet AGL the crew received a master warning with reference to the left hand engine's low oil pressure, at 256 feet AGL a master caution with reference to the left engine's ITT was issued, the crew noticed bad smell and observed smoke in the cockpit. At 564 feet AGL the crew initiated the shut down of the left hand engine by executing the memory items of the checklist, at that time cabin crew attempted to establish contact with the cockpit but were told to "standby", 5 seconds later a master warning occurred with reference to a left hand engine fire, at the same time cabin crew reported smoke in the cabin. The crew was satisfied that the aircraft was flying safely on the right hand engine, declared Mayday and requested to land on either runway 22, an offer of tower to fly a tear drop back to runway 04R was not taken up. Another flight crew waiting for departure from runway 22R reported they could see smoke and a bright light from the left side of the aircraft. The crew agreed to hand controls to the captain, tower advised landing would occur on runway 22L. The crew noticed the left hand fire indication and discharged agent #1. At 1305 feet AGL, about 2:28 minutes into the flight, the crew began to discuss landing on runway 30, discovered the fire indication was still active, discharged agent #2 and requested tower to report whether any fire was seen from the ground and checked with cabin crew whether they could see any fire from the left hand engine, tower responding they would take a look and report, cabin crew reporting that now there were no longer any signs of fire. Tower reported they could not see any signs of fire from the ground. The crew requested wind information which indicated a crosswind component of 22 knots for runway 30, the crew decided to land on runway 30. The flight crew decided an emergency evacuation would most likely be necessary, advised tower they would perform an evacuation onto the runway and had cabin crew on standby for the evacuation through the right hand doors. The aircraft touched down on runway 30 5:02 minutes after becoming airborne, at that time winds were from 250 degrees at 17 knots. The crew issued "Standby, standby, standby" on the PA, verified that the fire had been extinguished and decided to cancel the evacuation informing cabin crew about the cancellation via interphone. The captain briefed the passengers on the extinguished engine fire and announced they would disembark through the normal passenger door but leave the hand luggage behind as smoke was still present in the cabin.

The aircraft had accumulated 20,554 hours in 31,168 flight cycles. The left hand engine had accumulated 16,018 hours since new (no cycles given), the last overhaul had occurred in 2006 at 10,078 hours and 13,608 cycles since new, the last visit to the engine workshop had occurred in February 2011 at 15,832 hours/23,937 cycles since new for a hot section repair, that visit, like the other earlier visits, did not reveal any evidence of stage 1 turbine blade failures.

A first visual inspection of the engine showed 2 blades of the stage 2 turbine were severely damaged, the scavenge oil line from bearing #6 and #7 was found loose and leaking at the 6 o'clock position into the power turbine support case. The engine oil tank was drained removing 2 liters of oil, the tank has a capacity of 22 liters.

The engine tear down inspection revealed all stage 2 turbine blades were heavily damaged, several blades had fractured. The stage 2 stator was heavily damaged, the shroud was heavily damaged with parts of the outer shroud missing. The stage 1 turbine was removed from the engine, two blades were found fractured and separated at the root, all blades showing indications of overheat. The bearing baffle of bearings #6 and #7 was damaged with three bolts holding the baffle and bearing houses in place had sheared off, the bearing houses could be manually rotated indicating that the other 9 bolts below the baffle and three more bolts at the oil transfer tubes had also sheared off. After removing the assembly the turbine inlet case was observed wet with oil. The three oil transfer tubes for bearings #6 and #7 were found sheared off almost in flush with the bearing housing, the tubes fractured just above the threads.

In a review conducted in 2008 by the engine manufacturer there had been 68 blades of 90,000 identified, that suffered from porosity exceeding x-ray limits resulting in 12-16 engines removed from service prior to schedule. The manufacturer improved inspection of new blades by introducing a new x-ray inspection in 2008 as result of the survey, the number of blades suffering from porosity, which had peaked in 2008, then decreased again. A service bulletin released in March 2008 required to remove suspect blades, the incident engine and its blades however were not covered by that bulletin as the bulletin did not cover stage 1 turbine blades manufactured before 2007 and there was no life time limit set on those blades.

The HCL analysed: "The flight crew decision on landing on runway 30 instead of landing on runway 04R (tailwind component of more than 10 knots) or a visual approach to runway 22L was in the AIB-DKÂ’s opinion an optimum compromise between operational flight safety on one hand and reduction of the total airborne time on the other. By a reduction of the total airborne time, the severity of this incident (inflight fire) and the total risk were lessened."

The HCL continued: "Taking the severity of this serious incident into consideration and in the interest of flight safety, the flight crew decision of overriding the operational procedures including checklists (six emergency checklists) and only perform memory items was - under the actual conditions - optimum. This decision also contributed to a reduction of the total airborne time and the flight crew workload."

The HCL further analysed: "The AIB-DK regards this incident as an unprepared emergency, which made time a crucial parameter. For that reason, crew interaction and communication had first priority rather than continuous information to and briefing of the passengers. However, the passengers were briefed in flight on the engine problems and again on ground on the disembarkation by the commander. Besides safety related cabin crew duties (securing the cabin), the AIB-DK finds it likely that a cabin crew announcement shortly before landing on the engine problems and on safety preparations (seatbelts) might have been supportive to the commanderÂ’s briefing and thereby sustaining a positive passenger control."

The HCL analysed that immediately upon takeoff the left hand engine suffered a stage 1 blade fracture and separation, that in sequence led to an oil fire in the nacelle section.
Incident Facts

Date of incident
Sep 13, 2011

Classification
Incident

Cause
Engine fire

Airline
Cimber Sterling

Aircraft Type
ATR ATR-72-200

ICAO Type Designator
AT72

This article is published under license from Avherald.com. © of text by Avherald.com.
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