What happened
On 11 February 2009, a Boeing 777-268ER, registration HZ-AKC, arrived at Stand 319 at London Heathrow Airport. Following standard procedures, the commander engaged the parking brake and confirmed the system was set via the Engine Indicating and Crew Alerting System (EICAS). Because the required wheel chocks from the ground handling agent had not arrived due to previous stand changes, the commander agreed to the ground engineer's request to disembark passengers without chocks in place.
During the disembarkation process, after approximately 10 passengers and the full crew of 14 remained on board, the aircraft began to roll backwards. The movement lasted for approximately two metres, during which time the jetty structure made contact with the side of door L2, resulting in minor damage to the door surface.
An engineer on the jetty alerted the maintenance manager, who entered the flight deck. The manager switched the right hydraulic system to AUTO, which activated the AC electric pump. This action restored hydraulic pressure and successfully stopped the aircraft's movement.
The investigation
Investigators examined the hydraulic systems and the parking brake mechanism. It was determined that pressure in the parking brake system had dissipated, leading to a loss of brake application. The investigation focused on the right hydraulic system, which provides the pressure necessary for the parking brake.
Maintenance inspections using the Boeing Fault Isolation Manual identified a defective parking brake valve as the source of the pressure loss. A subsequent pressure decay test confirmed that the replacement of this component resolved the issue. The investigation also noted that the flight crew, engaged in post-flight duties, were unaware of the gradual loss of pressure or the initial movement of the aircraft.
Findings
- The failure of the parking brake valve caused the loss of hydraulic pressure in the parking brake system.
- The absence of wheel chocks, which were not provided by the ground handling agent, left the aircraft without a secondary means of immobilization.
- The flight crew did not observe the loss of accumulator pressure or the aircraft's movement because they were occupied with post-flight activities.
Safety action
Following the incident, the operator implemented a new requirement ensuring that their engineering vehicles carry a set of chocks to be used in conjunction with those provided by ground handling agents when attending to aircraft at a stand.