What happened
On 20 January 2007, a Dassault-Breguet Mystère-Falcon 900B, registration G-HMEV, was climbing through FL130 after departing Farnborough for Tel Aviv when a loud noise was heard from the rear of the aircraft. Shortly after, the No 3 engine fire warning and engine bay fire warning activated. The crew immediately initiated the engine fire procedure, using the first extinguisher shot to suppress the fire, and declared a mayday.
The aircraft diverted to Gatwick Airport, where it landed approximately 12 minutes after the initial event. During the approach, the crew experienced several GPWS 'too low, flaps' callouts. There were no fatalities and no injuries to the two crew members on board.
The investigation
The AAIB investigation examined the No 3 engine and the aircraft structure. The investigation found a 6x6 inch hole in the No 3 engine cowl and a scratch on the underside of the right horizontal stabiliser, likely caused by ejected debris. The interstage turbine transition duct (ITTD) had been breached, allowing hot gases to escape and strike the firewire sensing element.
Detailed examination of the engine components revealed severe damage to the low pressure (LP) turbine assembly. The outer portions of the Stage 2 and Stage 3 blades had broken off, and the Stage 1 blades showed significant impact damage. Metallurgical analysis of a Stage 2 blade (No 5) revealed a casting defect known as a 'hot tear' which included a pre-existing crack.
Findings
- The disruption of the No 3 engine turbine assembly was likely caused by the failure of a Stage 2 or Stage 3 LP turbine blade.
- A casting defect in a Stage 2 turbine blade was identified as a primary factor in the failure.
- Debris from the turbine assembly breached the engine casing and the nacelle cowl, causing damage to the horizontal stabiliser.
- The engine manufacturer had implemented new production processes to detect such defects, but some blades produced under the revised process may still be susceptible to similar failures.
Safety action
- Safety Recommendation 2008-013: It is recommended that the FAA comprehensively review the measures proposed by the manufacturer to prevent LP turbine assembly failures in Honeywell TFE-731 engines, ensuring timely incorporation across the fleet.
- Safety Recommendation 2008-014: It is recommended that the FAA require the timely incorporation of an upgraded standard of Inter-Turbine Transition Duct (ITTD) on TFE-731 engines to eliminate the hazard of debris non-containment.