What happened
On 18 May 2001, an Airbus A300B4-605R, registration G-MONS, was operating a scheduled passenger flight from London Gatwick to Banjul, Gambia. While cruising at 29,000 feet near Casablanca, the aircraft experienced a sudden, intense vibration and a loss of thrust from the No 2 (right-hand) engine. The crew responded by reducing the No 2 engine throttle to idle and increasing thrust on the No 1 engine.
Following a distress call to Casablanca, the flight crew diverted to Faro, Portugal. The aircraft performed an overweight landing at Faro with the airport fire service in attendance. While no injuries were reported among the 262 passengers or 15 crew members, subsequent inspections revealed significant damage to the engine cowling and the aircraft' and wing structure. Debris from the engine had breached the nacelle, causing minor damage to the starboard wing, the inboard aileron, and various access panels.
The investigation
The AAIB investigation focused on the mechanical failure of the CF6-80C2A5 engine. Examination of the engine at a GE facility in Scotland revealed that a single High Pressure Turbine (HPT) Stage 2 blade had separated from its root. This separation was traced to a notch worn into the blade's leading edge.
Investigators found that a Nozzle Guide Vane (NGV) segment had shifted or 'sagged' rearwards. This movement was caused by thermally induced fatigue cracks in the NGV aerofoil outer fillet. As the NGV segment moved, it acted like a machining tool, cutting a 0.25-inch deep notch into the HPT Stage 2 blades.
Furthermore, the investigation examined the Low Pressure Turbine (LPT) module, discovering a significant breach in the LPT case. Debris from the initial HPT failure had traveled downstream, causing a cascade of damage through the LPT stages. This debris eventually caused a 'rub-through' of the LPT case, allowing fragments to be ejected with enough energy to penetrate the engine cowls and strike the aircraft wing.
Findings
- The primary cause of the engine failure was thermally induced fatigue cracking in the HPT Stage 2 NGV aerofoils.
- The failure of the NGV structural integrity allowed the component to sag, which subsequently notched and weakened the HPT Stage 2 blades.
- The HPT Stage 2 blades eventually failed due to fatigue and tensile overload.
- The LPT case was unable to contain the secondary debris generated by the initial failure, leading to the breach of the engine nacelle.
- The NGV segments involved had undergone multiple previous repairs, which increased their susceptibility to thermal distress and cracking.
- At the time of the incident, the operator's maintenance programme did not require specific inspections of the HPT Stage 2 NGVs, as the inspection was previously left to the operator's discretion.