Dassault Falcon 900B, Gatwick, UK, Jan. 20, 2007–The UK’s Air Accidents Investigation Branch (AAIB) has ruled the probable cause of the uncontained engine failure was the fracturing of a stage-two low-pressure (LP) turbine blade due to a casting defect, with the possibility that this was caused by a third-stage blade failure.
The trijet departed Farnborough Airport and was forced to make an emergency landing at Gatwick Airport when it experienced an engine problem. While approaching FL150, the crew heard a loud noise coming from the rear of the aircraft, accompanied by the engine fire aural warning and the illumination of the number-three engine fire warning light. The interstage turbine temperature (ITT) warning light for the number-three engine was lit as well, and the ITT indication was fluctuating “wildly.” Indications for the other two engines were normal.
The airplane landed safely at Gatwick, and inspection revealed the number-three Honeywell TFE731-5BR engine had suffered an uncontained breach in the combustor case and ejected debris from the turbine assembly, causing slight damage to the horizontal stabilizer. While many of the fractured parts were lost before landing– precluding definitive conclusion as to the cause of the turbine assembly break-up– detailed examination of the remaining turbine parts showed that one of the LP stage-two blades displayed evidence of damage before the accident, consistent with a defect known as a “hot tear” formed during the casting process. Honeywell estimated that the operating time before complete fracture of a blade with such a defect would be less than 1,000 hours. The LP stage-two blades in the engine had accumulated 624 hours.