What happened
On 20 June 2000, a BAe ATP, registration G-OBWN, was performing a public transport flight from Stansted Airport. During the takeoff roll, once the aircraft reached an estimated speed of 55 kt, it began an uncommanded deviation to the right. The commander attempted to correct the path by adjusting the tiller to the left; however, this action triggered a violent departure to the left accompanied by severe vibrations.
Despite the crew's attempts to use differential braking and engine power to maintain control, the vibration intensified. The commander eventually managed to bring the aircraft to a halt on the runway and shut down the engines. While there were no fatalities or serious injuries among the 4 crew members or 49 passengers, two passengers sustained slight injuries caused by detached interior trim panels that became loose during the oscillation. All passengers were eventually evacuated via bus to the terminal.
The investigation
Following the incident, the aircraft was moved to a maintenance facility for inspection. While the nose landing gear tyres showed signs of distress, they remained inflated, and no structural damage was found on the airframe or the landing gear itself.
Investigators examined the nosewheel steering system and discovered that the weight-on-wheels switch was unserviceable. A manufacturer's analysis identified green deposits on the wire attachments of the switch, which had caused a short circuit. This electrical fault caused the switch to continuously signal an "AIR" position, which in turn signaled the steering selector valve to the "OFF" position. This prevented the necessary hydraulic pressure from being supplied to the steering jack.
Analysis of the Flight Data Recorder (FDR) showed that the aircraft accelerated to 54 kt before decelerating, with data loss occurring below 45 kt. This loss of data was attributed to the intense vibrations of the shimmy affecting the tape speed of the recorder.
Findings
- The primary cause of the excursion was the loss of hydraulic damping and steering control due to a failure in the nosewheel steering system.
- The weight-on-wheels switch failed due to moisture ingress, which created green deposits that shorted the electrical connections.
- Because the switch incorrectly signaled that the aircraft was in flight, the steering selector valve remained in the OFF position, preventing hydraulic fluid from being retained in the steering jack.
- The absence of fluid in the circuit meant the system could not provide the necessary damping to suppress the torsional oscillation (shimmy).
- A known modification intended to prevent pressure loss in the steering valve supply line had not been implemented on this aircraft.