What happened
On 7 May 2004, a Fokker F27 Mark 500, registration G-CEXG, was preparing for a scheduled cargo flight from Bournemouth Airport to Jersey. During the pre-flight phase, the crew observed the aircraft's pneumatic system being charged in a manner that deviated from standard procedures. Although engineers explained that this was a fleet-wide measure following issues on another aircraft, the first officer remained concerned about a potential leak due to slightly low brake bottle pressure.
In an attempt to protect the system, the first officer pulled the pneumatic isolating valve pin. This action was contrary to the required checklist, which specified the pin should be pushed in. During the engine start sequence, the crew failed to identify that the valve remained in the isolated position. As the aircraft began taxiing, the crew noted low pneumatic pressures. Shortly thereafter, the aircraft lost directional control and veered to the left. Despite attempts to use the steering wheel and right brake, the aircraft left the taxiway and traveled 50 metres across the grass before stopping. The commander attempted to rectify the valve position during the excursion, but the aircraft had already departed the paved surface.
The investigation
The investigation focused on the state of the pneumatic systems and the crew's use of checklists. It was established that the pneumatic isolating valve pin, when pulled, disconnects the main system and storage bottles from the main supply. While this preserves pressure during a leak, it also prevents the operation of the landing gear, nose wheel steering, and wheel brakes from the main supply.
Investigators found that the crew's suspicion regarding the pneumatic system's integrity was fueled by a lack of prior notification regarding the new engineering servicing procedures. Furthermore, while the 'AFTER START' checklist included a pneumatic check, the crew failed to verify the specific position of the isolation valve pin, which was a critical component of that check according to the Operations Manual.