What happened
On 2 September 2018, a Grumman AA-5 Traveller, registration G-BEZF, was involved in a ground collision at Turweston Aerodrome, Buckinghamshire. The aircraft was being operated on a private flight, traveling from Turweston to Coventry Airport, with two pilots on board.
After completing a walk-around inspection and starting the engine, the pilot received taxi clearance and released the brakes. As the throttle was opened to begin the taxi, the aircraft suddenly veered toward the left. The pilot attempted to correct the movement by applying full pressure to the right brake pedal, but when the aircraft failed to respond, he applied pressure to both pedals. To avoid a collision with a parked aircraft, the pilot closed the throttle and the passenger selected the mixture to off. Despite the engine stopping, the propeller remained in motion when G-BESB struck the stationary aircraft. There were no injuries to the occupants, though the collision caused scratches to the propeller and damage to the elevator of the parked aircraft.
The investigation
The investigation focused on the cause of the uncontrolled taxi and the subsequent failure of the braking system. The pilot suggested that a check of the brake pressure before engine start might have revealed the issue. Following the incident, a member of the aircraft syndicate tested the brakes while the aircraft was stationary and the engine was off. This test revealed that the left brake pedal felt significantly softer than the right pedal, which felt normal.
However, a subsequent inspection by the aircraft's maintainer did not identify any mechanical faults. The maintainer concluded that the pedal feel was within the expected range of operation.