What happened
On 12 June 2009, an Aero AT-3 with registration G-TGUN was involved in a ground collision at Old Sarum Airfield. Following a local flight, the pilot and passenger parked the aircraft alongside a fuel bowser to refuel. After the refueling process, the pilot returned to the cockpit, started the engine, and set the throttle to 25% power.
Upon engine start, the aircraft unexpectedly moved forward and veered sharply to the left. Despite the pilot applying significant pressure to the toe brakes, the aircraft failed to stop, eventually striking the fuel bowser with its propeller and right wing. The impact caused damage to the engine gearbox, the propeller, and the right wing, and resulted in the engine stopping. There were no injuries to the two occupants.
The investigation
The investigation established that the parking brake lever had been inadvertently moved to the 'on' position, likely by the pilot's leg or clothing while exiting the aircraft. The aircraft was equipped with a specific version of the braking system that lacked a bypass valve. Consequently, when the parking brake lever was engaged, the toe brakes were rendered completely inoperative.
Investigators found that the Flight Manual for G-TGUN contained inaccurate diagrams and failed to warn pilots that the toe brakes would lose functionality if the parking brake was engaged. Furthermore, the pilot's checklist did not include a specific check for the parking brake lever position during the engine start sequence.