What happened
On 30 June 1998, a Jetstream 31, registration G-LOVA, was performing a scheduled passenger flight from London (Stansted) Airport to Le Bourget, France. During the takeoff roll on Runway 23, the aircraft initially tracked the centerline normally. However, as the aircraft reached a ground speed of approximately 80 kt, the crew noted a slight vibration. Shortly thereafter, the aircraft began a rapid, uncommanded left turn.
Despite the commander applying full right rudder and right braking, the aircraft could not be maintained on the runway. The aircraft departed the paved surface at an angle of approximately 15 degrees to the left of the runway heading. The aircraft eventually came to a halt on the grass, roughly 145 meters from the point of exit. Following the excursion, the crew observed smoke and flames in the right engine intake, prompting an emergency shutdown and the evacuation of the 8 passengers and 2 crew members. No injuries were reported.
The investigation
The AAIB investigation focused on the nosewheel steering system after runway markings indicated a progressive, uncommanded left offset. While the aircraft's braking and hydraulic systems were found to be functional, investigators identified significant wear in the bushes of the torque link within the feedback mechanism.
Detailed examination of the steering control valve assembly at the manufacturer's facility revealed that the valve failed to return cleanly to the neutral position after a leftward steering command. Specifically, wear was found on the rocker return spring plungers and the contact areas on the underside of the rocker. This wear prevented the mechanism from centralizing consistently. Furthermore, the investigation identified 6 degrees of total rotational slack in the feedback linkage connecting the nose leg to the control valve assembly.
Findings
- The runway excursion was caused by a progressive uncommanded nosewheel steering offset to the left.
- Wear on the rocker return spring plungers and the rocker itself prevented the steering valve from returning to the null position after a leftward input.
- The mechanical slack in the feedback linkage masked the initial valve offset, allowing the deviation to increase progressively.
- Torsional shock loads, generated as the nosewheels struck runway centerline lights, likely provided the physical impulse necessary to displace the valve spool into a dead band region just left of neutral.
- The presence of smoke and flames in the engine intake was likely due to the back-flow of gases through the engines after shutdown rather than an actual engine fire.