What happened
On 5 January 2005, a DHC-8-402, registration G-JEDI, was taxiing towards Runway 15 at Birmingham International Airport. While moving along Taxiway A, the flight crew attempted to centralise the aircraft after passing a metal plate by using the nosewheel steering tiller. However, the initial steering inputs had no effect. Despite further attempts using the tiller and applying right rudder and braking, the aircraft failed to turn and eventually came to a halt with its left main landing gear positioned on the grass adjacent to the taxiway.
During the incident, the crew observed that the 'nose steering' fault light had illuminated and noted a failure of the co-pilot's airspeed indicator. There were no injuries to the 4 crew members or 21 passengers, and the aircraft sustained no damage. Following the event, passengers were transferred to a bus, and the aircraft was moved to a stand for inspection.
The investigation
Investigators analysed the flight data recorder (FDR), which revealed that the 'weight-on-wheels' (WOW) signal had been lost for one second during the early stages of taxiing and again at the moment the steering failure occurred. The aircraft is equipped with two separate WOW switches on the nose landing gear. If the signal from either sensor is lost, the Steering Control Unit (SCU) removes steering commands from the hydraulic actuators. The loss of steering control was found to be consistent with the intermittent loss of one of these signals.
Subsequent checks on 6 January 2005 at Glasgow Airport identified a problem with the wiring harness connected to the WOW2 sensor, which was subsequently repaired. The investigation also found that the aircraft had been released for service following the incident based on a fault code in the SCU, as engineers were unaware that this specific code could indicate a loss of the WOW signal. This specific diagnostic detail was not present in the existing maintenance manual.