What happened
On 11 December 2019, a Bombardier BD-700-1A10 Global 6000, registration 9H-VJM, was performing a commercial passenger flight when it veered off the runway at Liverpool Airport. The aircraft, operated by a VIP charter service, had arrived from Bedford, USA.
During the landing roll on Runway 27, the aircraft experienced a nosewheel steering failure. As the crew attempted to maintain the centerline, the aircraft began to deviate to the right. The commander took control of the aircraft and applied full left rudder; however, the aircraft continued to veer right, eventually leaving the paved surface and traveling approximately 30 metres onto the grass. The aircraft came to a halt after the landing gear sank into the soft soil. There were no fatalities and no injuries to the three crew members or one passenger on board.
The investigation
The AAIB investigation examined the aircraft's flight data, cockpit voice recordings, and the nosewheel steering (NWS) system. Technical analysis of the NWS components revealed that the system had entered a "free caster" mode due to a fault. Specifically, the investigation found that the insulation resistance in both steering actuator LVDTs was below required limits, likely caused by moisture ingress.
Investigators also examined the cockpit ergonomics and pilot workload. Testing revealed that with the pilot's seat positioned forward, it was possible to inadvertently apply brake pressure when applying full rudder deflection. Furthermore, the investigation looked into the crew's training, noting that the existing procedures did not explicitly instruct the use of differential braking to maintain directional control during high-speed ground rolls following an NWS failure.
Findings
- The nosewheel steering system transitioned to a free caster mode shortly after touchdown due to a fault in the steering actuators.
- The commander inadvertently applied right braking while attempting to use full left rudder to correct the aircraft's deviation.
- The effectiveness of the rudder decreased as the aircraft's speed decayed, making it unable to counteract the effect of the unintended braking.
- The aircraft's flight crew lacked specific practical training for steering the aircraft at high speeds using differential braking during an NWS failure.
- The physical positioning of the pilot's seat allowed for the accidental application of brake pressure during heavy rudder input.