What happened
On 18 May 2001, an Airbus A300B4-605R, registration G-MONS, was operating a scheduled passenger service from London Gatwick to Banjul, Gambia. While cruising at flight level 290 near Casablanca, the aircraft experienced sudden, intense vibrations and a loss of thrust from the number 2 engine. The crew observed the N2 vibration levels rising rapidly to 5.8 units.
In response, the pilot applied maximum continuous thrust to the number 1 engine and reduced the number 2 engine throttle to idle. Although the vibration levels fluctuated before stabilizing, the crew declared a PAN PAN and diverted to Faro, Portugal. The aircraft completed an overweight landing at Faro without further incident, and no injuries were reported. Following the landing, it was discovered that debris from the engine had breached the cowls and struck the aircraft's wing.
The investigation
The investigation focused on the mechanical failure of the CF6-80C2A5 engine. Physical examination revealed a large hole in the left-hand core cowl and a smaller hole in the right-hand cowl. Fragments from the Low Pressure Turbine (LPT) had escaped the engine, causing minor damage to the starboard wing skin and the inboard aileron, though no aircraft systems were compromised.
Investigators found a significant opening in the LPT case, where the edges showed signs of heavy scoring and heat damage. While the LPT rotor blades remained attached, they had lost their tip shrouds, and several blades were bent. The engine was subsequently sent to a facility in Scotland for a detailed strip examination.
Findings
Technical analysis established that the failure originated in the High Pressure Turbine (HPT). An HPT Stage 2 rotor blade had separated from the engine just above its platform. The fracture started from a small notch on the blade's leading edge. This failure was linked to cracks found in several HPT Stage 2 nozzle segments. These cracks had weakened the segments, causing them to deform under aerodynamic loads and strike the leading edges of the Stage 2 blades.
While the engine manufacturer had previously issued a service bulletin regarding this failure mode, the specific inspection for the nozzle parts involved in this incident was not mandatory. The operator had not performed the recommended borescope inspections, which likely would have detected the nozzle cracking and allowed for corrective action before the failure occurred.
Safety action
Following the investigation, a safety recommendation was issued to the FAA and CAA, requesting the implementation of mandatory instructions for operators to perform borescope inspections of CF6-80C2 HPT Stage 2 nozzles to check for cracking and distress.