Incident
On 13 February 2003, at approximately 1615 hours UTC, a Socata TBM 700B (registration N700VA) experienced a ground movement incident at Oxford Airport, Oxfordshire. The aircraft was on a private flight with only the pilot on board. After landing, the pilot taxied to a hangar and parked the aircraft outside. He shut down the engine and reportedly applied the parking brake, but the wheels were not chocked. The pilot left the aircraft and returned about 50 minutes later. As he approached, he noticed the aircraft rolling forward at a walking pace. He positioned himself in front of the aircraft and pushed against the propeller to try to stop its movement. He was succeeding in slowing the aircraft until he walked backward into a tree on the edge of the paved surface. The aircraft's momentum forced the pilot against the tree, trapping him. The force fractured one of his shoulder blades. People who heard his appeal for help arrived and pushed the aircraft away. Staff from a nearby office activated the crash alarm.
Aircraft and Braking System
The Socata TBM 700B is a single-engine turboprop powered by a PT6A-64 engine. The parking brake is activated by turning a control knob while applying pressure to both toe brake pedals mounted on the rudder pedals. The aircraft's Flight Manual notes: 'Operating the parking brake knob without applying pressure on rudder pedals does not cause the wheels to be braked.'
Investigation Findings
The maintenance organisation responsible for the aircraft checked the parking brake system after the incident and found no faults. They reported that when the parking brake knob is operated without applying full pressure to the brake pedals, only partial braking is activated. The pilot, experienced on the type, was certain he had applied the parking brake using full pressure.
The aircraft manufacturer was consulted but could not explain why the parking brake might start to slip after 50 minutes. They were not aware of any similar occurrences, although a few incidents had been reported where the pilot forgot to apply pressure while turning the knob, causing movement in windy conditions. The manufacturer discounted the possibility that applying the parking brake with hot brakes might result in slippage after cooling. To confirm, the maintenance organisation performed a practical test: they applied the parking brake after a flight and left the aircraft overnight; a pull test the next morning confirmed the brake was still holding securely.
Analysis
In the pilot's assessment, the cause was a slipping brake. He also remarked that he was unaware of the tree's presence as it was getting dark. Although the cause of the brake slippage could not be determined, the investigation noted that the aircraft's average empty weight is 1,860 kg (almost three times that of a Cessna 172) and that it was parked on a slight slope without chocks.