Tahiti AT72 at Papeete on Jan 12th 2013, engine fire
Last Update: June 8, 2016 / 15:54:40 GMT/Zulu time
The French BEA reported that fire fighters on the ground extinguished an engine fire. An investigation has been opened into the serious incident.
On Jun 7th 2016 the BEA released their final report in French concluding the probable causes of the serious incident were:
A blade in the Power Turbine 1 (PT1) or Power Turbine 2 (PT2) probably fractured during the take off roll causing unusual variations of engine parameters and resulting in fire during the initial climb. The strong vibrations as result of the fracture of the blade caused the rupture of the lubrication system, which resulted in fire.
The investigation could not determine the cause of the fracture of the blade. However, service experience shows that consecutive fracture of PT1 blades were caused by lack of control procedures during manufacture and maintenance. Experience further tells that fires as result of such occurrences only occur during takeoff or landing.
The indications on the engine instruments could have alerted the crew to an engine anomaly prior to reaching V1.
The BEA reported that the crew applied takeoff power and began their takeoff roll, when at 45 knots the InterTurbine Temperature (ITT) exceeded the maximum of 765 degrees C, the first officer (Pilot Monitoring) called however "power checked". Two seconds after the 80 knots call the ITT exceeded 800 degrees C which - in theory - triggered the amber ITT warning light. Without noticing these anomalies the crew continued takeoff, the aircraft was climbing through 500 feet AGL when an engine oil warning was issued for the right hand engine (PW127F). Climbing through 750 feet AGL the engine fire warning activated. The crew carried out the memory items for engine fire, shut the engine down and discharged the fire agent, the first officer radioed an emergency call and requested to return to Papeete runway 04. As the fire indication remained active the crew discharged the second bottle of fire agent, the captain alerted cabin crew to an engine fire and emergency return. Cabin crew reported seeing flames from the right hand engine. The crew advised ATC of having an engine fire which was not under control, they would stop on the runway. The aircraft landed on runway 04, emergency services confirmed presence of fire on the right hand engine and recommended an evacuation to the left hand side. The crew initiated the emergency evacuation. There were no injuries during the evacuation.
A post occurrence examination of the engine revealed that both stages of power turbine showed fractured blades, in addition the rotating parts incuding shaft and bearings were seriously damaged. At least one of the two fire loops was open. At least one blade fractures in PT1 or PT2 creating an imbalance on the spool that resulted in severe vibrations causing additional damage, which eventually caused the failure of the oil lubrication system causing oil to spill onto hot parts of the engine and catch fire.
The BEA reported that between 2002 and 2013 15 cases of PT1 blade failures had occurred and 11 occurrences of PT2 blade failures on the PW127 engines, one of which resulted in an engine fire. In response to the occurrence Pratt & Whitney developed a new improved x-ray inspection of the blades resulting in a service bulletin released in 2012 recommending the use of the new methodology to inspect turbine blades manufactured between 2005 and April 2008 and replace any such blade found defective. Following the SB Air Tahiti decided to replace all turbine blades irrespective of whether they were found good or defective.
Following the Air Tahiti occurrence another technology, a resonance test, was introduced for PT1 blades.
The BEA analysed that company procedures required the pilot monitoring to verify the engine had reached required torque, the autofeather system was armed, autobrakes are armed, the propeller rpms reach 100% and "all engine parameters are in the green". The aircraft manufacturer's procedures were slightly different including a specific check of the ITT values.
Pilots are not particularly trained to pick up differences in indications between left and right hand engines. Due to amount of data and indications to be processed this could be difficult for the pilot monitoring.
The BEA analysed that there were no guidelines and flight crew operation manual procedures for the case, that an engine fire indication continued indicating an active fire after both fire bottles had been discharged.
This article is published under license from Avherald.com. © of text by Avherald.com.
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