A United Airlines Boeing 747-400 crept forward 4 sec. after coming to a stop at its gate, causing its left wing to clip an aerobridge at Australia’s Melbourne International Airport on April 22, 2013, an Australian Transport Safety Bureau (ATSB) probe found.
The likely cause: an inadvertent disengaging of the parking brake by the flight crew before the aircraft’s engines were powered down far enough and the ground crew had installed nose-wheel chocks.
“The reason why the parking brake disengaged could not be determined,” the ATSB says. “However, recorded data showed rudder pedal movement when the aircraft started to move after the parking brake was initially set.”
The bureau says the incident “highlights the importance of flight crew remaining aware of the possibility of aircraft movement whenever the engines are running, as aircraft movement, particularly if it is slow, is difficult to detect. Additionally, ground support crew are reminded of the need to leave the parking guidance system in normal mode, unless an emergency stop is required.”
The ATSB concluded that the flight crew’s attention “was inside the cockpit,” focusing on shutting down the engines, and they did not realize right away that the aircraft had started to move. While this was routine, the parking brake’s disengagement was not.
The bureau also found that the ground crew activated the visual docking guidance system’s emergency-stop button when the aircraft first stopped, rendering it useless to signal the crew once the 747 began moving again. Two interim updates have shed little light on what happened.