What happened
On 13 June 2003, an HS125-700A, registration N125XX, was performing a passenger flight from London (Luton) Airport. During the takeoff roll on Runway 26, as the aircraft reached approximately 60 kt, the crew experienced a loud bang accompanied by a sudden rightward yaw. The commander immediately aborted the takeoff, applying brakes and shutting down the engines.
The crew noted that the engine fire warning bell had activated and the oil pressure warning light for the right engine was illuminated. Smoke was also observed emanating from the engine by air traffic control. There were no fatalities and no injuries among the two crew members and five passengers on board. The right engine sustained severe damage, including a rupture in the cowling and the loss of a significant portion of the low pressure turbine disc.
The investigation
The AAIB investigation focused on the cause of the uncontained failure of the right engine. Examination of the engine revealed that the 1st stage Low Pressure Turbine (LPT) disc had failed. A segment of the disc was recovered from the runway, while the remainder could not be located. Metallurgical analysis of the recovered fragment identified areas of high-cycle fatigue.
The investigation traced the engine's history back to a major inspection performed approximately 107 hours prior to the incident at a repair station in the USA. During this overhaul, a new LPT 1 disc had been installed, but the nozzle assembly had been overhauled by an unapproved repair station. This station had adjusted the vanes of the nozzle assembly by manually deflecting their trailing edges to achieve a specific total area, rather than using the manufacturer's proprietary 'NAPOLI' computer programme and specific tooling.
Findings
- The engine suffered an uncontained failure of the 1st stage LPT disc due to high-cycle fatigue.
- The fatigue was triggered by a resonant condition in the disc caused by unequal nozzle throat areas.
- The repair station used an improper method for adjusting the nozzle assembly, simply tweaking all vanes by an equal amount.
- The repair station was unaware of the manufacturer's Service Bulletins regarding correct adjustment procedures and the use of the 'NAPOLI' programme because they were not an authorised Honeywell repair centre.
Safety action
- The FAA is urged to ensure that FAR Part 145 repair stations possess all necessary manufacturer documentation for their approved tasks.