What happened
On 21 October 1997, a Boeing 737-436, registration G-DOCG, arrived at Stand B8 at London Heathrow Airport. Following the arrival of the scheduled passenger flight, most passengers disembarked via the airbridge. As a passenger mobility assistant was preparing to board the aircraft, the airbridge suddenly began to retract and lower without any command from an operator.
The movement caused the airbridge canopy to catch on the open forward passenger door, creating a significant gap between the bridge floor and the aircraft door sill. The descent of the bridge end created a dangerous slope, prompting the mobility assistant to warn remaining passengers to evacuate the bridge immediately. The movement ceased when the bridge came to rest approximately 5 metres away from its correct position and 2 to 3 metres below the aircraft door. The incident resulted in no injuries to the 105 passengers or 7 crew members, but caused damage to the aircraft's forward passenger door and its mechanism.
The investigation
Investigators examined the hydraulic and electrical systems of the airbridge, which is maintained by the airport operator. While functional tests initially showed no electrical defects, specialists discovered that the bridge would raise incorrectly when a raise valve was operated manually, as the corresponding return valve failed to operate. Further testing revealed that the unit would undergo uncommanded motion whenever other valves were activated.
Technical examination of the unit revealed that the hydraulic fluid was heavily contaminated and the filter was partially blocked. Additionally, the valve assembly covers were missing, leaving the valves exposed to corrosion. It was also noted that a similar malfunction had occurred two days prior at the same stand, involving a different aircraft, but the reported issue had not been formally recorded or acted upon by the airport operator.
Findings
- The primary cause of the uncommanded movement was a dormant fault where the hydraulic return valve was stuck in the open position.
- This fault was caused by heavy contamination of the hydraulic fluid and a partially blocked filter.
- The existing maintenance and inspection regime was insufficient to detect dormant faults that do not manifest during standard functional checks.
- There was a lack of formal communication and record-keeping regarding a similar near-miss event occurring 48 hours before the accident.
Safety action
Following the investigation, the airport operator implemented a system of checks for all similar airbridge equipment and modified maintenance intervals to include hydraulic valve inspections every three months. The BAA also initiated a group to investigate potential failure modes in bridge control systems and updated maintenance standards to define clearer checking requirements.