History of Flight
On 8 September 2004, at 0114 UTC, a Fokker F27-500 (registration EI-SMF) was departing from Stansted Airport, Essex, for a non-revenue public transport flight. The commander was the pilot flying. The crew had noted a higher than normal vibration from the right engine, which was deemed acceptable after ground inspection. A deferred defect in the technical log stated that the nose wheel steering was 'very sensitive', requiring operation in accordance with the Minimum Equipment List (MEL).
The aircraft had flown from Paris to Exeter with a cargo load, then continued to Stansted. After a normal turnaround, the aircraft taxied to hold for a flapless, rolling, dry takeoff from Runway 05. The surface wind was 050°/09 kt, visibility 10 km, few clouds at 900 feet, temperature 13°C, dew point 12°C, QNH 1034 mb.
During takeoff, the commander advanced the power levers to full takeoff power. The aircraft accelerated normally but the nose wheel steering felt sluggish. At about 60 kt, the commander removed his left hand from the steering control to the control column. Shortly after, the aircraft deviated sharply to the right. The commander reacted by applying left rudder and reducing power, more on the left engine. After containing the yaw, he re-applied full power, but the aircraft then yawed sharply to the left, crossing the runway centreline. The first officer called 'STOP STOP', and the commander retarded the power levers and used braking to bring the aircraft to a stop.
As the aircraft slowed, the commander saw an orange glow over his left shoulder and believed the left engine was on fire. He moved the left engine fuel cock lever into the propeller feather gate, and the engine ran down with sparks reducing. The left engine jet pipe temperature indicated approximately 1,000°C instead of the normal 400°C. The Rescue and Fire Fighting Service attended the scene; no fire was found externally.
Engine Investigation
Subsequent examination revealed metallic debris in the left engine jet pipe, indicating an overheat condition. A large quantity of oil had flowed from the engine around the reduction gearbox but had not ignited. The engine was removed for strip examination. It could not be turned initially, but when the compressor and turbine were separated, the compressor turned freely while the turbine was seized. Dismantling showed that the intermediate pressure turbine blades had melted, with molten material causing impact damage to downstream components. The high pressure turbine blades had residue adhered to guide vanes. The low pressure turbine blades had impact damage. The fuel system components (fuel control unit, fuel pump, burners) were found satisfactory. The propeller control unit governor was slightly out of tolerance. The investigation concluded that the engine experienced a turbine burnout due to an incorrect fuel-air mixture ratio, but no significant defect in the engine's fuel system was found.
The oil leak was attributed to continued operation of the feathering pump after engine shutdown, causing overfilling of the reduction gearbox as the scavenge pump was no longer operating.
Nose Wheel Steering Investigation
During rectification, the follow-up control valve (FUCV) of the nose wheel steering system was found to be defective. The FUCV had an incorrectly functioning lever spring: the gap between the spring levers was much larger than expected, allowing considerable free play of the valve spool. This would cause maximum pressurised air to be applied at any slight steering demand, with a tendency for the mechanism to oscillate, resulting in vibration of the nose wheel steering. The spring appeared to have been deformed to fit inside a housing that was too small, preventing it from centering the valve spool. The FUCV was in a 'pre-mod 5' state but incorrectly fitted with a 'post-mod 5' spring, and a 'Mod 5' plate was engraved. It was not possible to determine when these modifications occurred.
Propeller Pitch Stop Issue
The source explains that turboprop engines with constant speed propellers have propeller pitch stops to prevent the propeller from going into fine pitch in cruise. On the ground, these stops must be withdrawn to allow sufficient air through the engine. If the throttle is advanced too rapidly while the engine is at low speed and the propeller is hung on a pitch stop, the fuel-air ratio becomes rich and turbine temperatures rise rapidly, leading to possible burnout. Rolls-Royce had re-issued a Notice To Operators (NTO 1106) in 1997 highlighting adherence to the Aircraft Flight Manual to avoid such burnout.