What happened
On 6 November 1998, a BAe 146-200, registration G-JEAR, was performing a scheduled public transport service from Lyon, France, to London Heathrow. During the takeoff roll, as the pilot began the rotation maneuver, a loud noise was heard and the aircraft experienced a failure of the No 3 engine. The crew executed the necessary flight deck procedures, and the aircraft successfully returned to land at Lyon without further incident. There were no fatalities and no injuries to the 5 crew members or 13 passengers on board.
The investigation
Initial inspections of the No 3 engine revealed that all stage 4 turbine blades had separated near their roots, causing fragments to penetrate the jet pipe and fan-stream duct. Following permission from the French BEA, the aircraft was ferried to the operator's maintenance base in the UK.
Detailed examinations at both the operator's facility and the manufacturer's overhaul centers established a complex chain of mechanical failures. Investigators found that the stage 4 nozzle bolts had sheared and the combustion liner had twisted. Crucially, the investigation revealed that the high-pressure (HP) shaft had actually severed. This was evidenced by the fact that the HP turbines could be rotated independently of the HP compressor. The area of the shaft failure showed signs of severe overheating caused by rotational contact with the outer diameter of the low-pressure (LP) shaft.
Findings
- The primary cause of the engine failure was a fatigue fracture in one stage 3 LP turbine blade.
- This initial failure triggered a sequence of secondary failures, including the loss of three adjacent stage 3 blades and all stage 4 turbine blades.
- The resulting imbalance in the LP turbine caused the LP shaft to run with a significant bow, leading to friction and overheating between the LP and HP shafts, which ultimately caused the HP shaft to separate.
- The fatigue was likely caused by excessive vibration resulting from increased clearances between the turbine blade tip platforms.
- The engine had been returned to service following a hot section inspection in September 1997 without the implementation of Service Bulletin ALF502R 72-279, which recommended a hard surface coating to improve wear resistance.
- The investigation noted that the Service Bulletin's description of the modification's purpose did not adequately warn of the potential for severe secondary damage following a blade failure.