What happened
On 18 June 2005, a Boeing 747-436, registration G-CIVB, was undergoing a scheduled pushback from Stand 408 at London Heathrow Airport. The flight was a passenger service bound for Boston, carrying 334 passengers and 17 crew members. Following an initial successful pushback and engine start, the aircraft was positioned facing north on the taxiway. At this stage, the parking brake was applied.
During the process, ground ATC issued a new instruction for the aircraft to move forward to clear an adjacent stand. The commander relayed this instruction to the headset operator. While the aircraft was positioned abeam Stand 441, the commander queried the status of the tug connection and the status of the parking brakes. Due to a series of verbal exchanges, the commander misinterpreted the headset operator's response. Believing he had received authorization to release the brakes, the commander released the parking brake while the towbarless tug was still partially connected and in the process of being disconnected.
As the aircraft rolled forward, it struck the tug. The collision resulted in no injuries to the crew or passengers, but caused damage to the right nose wheel tyre and the nose landing gear torsion links.
The investigation
The investigation focused on the communication sequence between the flight deck and the ground crew. Analysis of the cockpit voice recorder revealed that the commander had misheard the headset operator's instructions. Specifically, the commander interpreted the phrase "we are connecting" as an indication that the aircraft was still connected to the tug, and interpreted the instruction "hold on" as an affirmative response to his request to release the brakes.
It was also noted that the cockpit voice recorder had been overwritten because the circuit breaker had been pulled following the incident. The investigation also examined the ground crew's procedure, which permitted the disconnection of the tug without prior consultation with the flight crew.
Findings
- The primary cause of the incident was a breakdown in communication between the headset operator and the aircraft commander.
- The use of non-standard phraseology during the manoeuvre contributed to the confusion.
- The commander's misinterpretation of the headset operator's status updates led to the premature release of the parking brakes.
- The ground crew's operational procedure allowed for the disconnection of the tug without explicit consultation with the flight crew.