What happened
On 5 February 2015, an A330-323, registration N273AY, was taxiing to Stand 325 at London Heathrow Airport for a passenger flight. Following the directions of the stand guidance system, the commander brought the aircraft to a stop and applied the parking brake. After completing the engine shutdown sequence, the pilot observed an "ok" message on the guidance system. Under the assumption that wheel chocks had already been secured, the pilot released the parking brake.
As the aircraft began to move, the ground crew attempted to place a chock behind the nose wheel but quickly retreated to a safe distance as the aircraft gained momentum. Upon hearing a warning that the aircraft was moving, the pilot applied the toe brakes and reset the parking brake. The aircraft had traveled approximately 0.4m before stopping. The movement caused the front cowl of the left engine to strike the airbridge, resulting in damage to the engine and the cowl. There were no injuries to the 12 crew members or 156 passengers on board.
The investigation
The AAIB investigation examined the stand guidance system, the physical gradient of the stand, and the operational procedures in place at the time. It was established that Stand 325 features a gradient of approximately 0.86% sloping toward the terminal building. The investigation also reviewed the airline's recent communications regarding ground handling procedures and the specific practices of the aircraft handling agent.
Findings
- The pilot released the parking brake because they believed the aircraft was already chocked, following the "ok" signal from the guidance system.
- The movement was facilitated by the 0.86% gradient of the stand sloping toward the terminal.
- At the time of the incident, the airline had not yet implemented a mandatory "chock in" signal requirement for all European stations, though a message had been sent to crews shortly after the accident advising them to keep the parking brake set until an audit of ground handlers was complete.
- The ground crew's standard practice of chocking the rear wheels first is common at Heathrow due to the typical outward slope of most stands, but the specific inward slope of Stand 325 contributed to the forward roll.