What happened
On 12 February 2000, a Boeing 747-436, registration VB-HOZ, was positioned at stand K17 at London Heathrow Airport's Terminal 3. During the arrival or departure process, the airbridge (K17B) experienced a sudden malfunction. While the bridge was positioned against the aircraft's L1 door, the structure dropped approximately one metre. This unexpected descent resulted in the top of the aircraft's left-side L1 door being ripped.
The investigation
Investigators examined the bridge's automatic height adjustment system and its safety mechanisms. The bridge is designed to use a drum and reed switches to maintain the bridge floor level with the aircraft door sill. The investigation revealed that the first and second line reed switches had their contacts welded together. This failure was attributed to the installation of a new, high-current klaxon-style horn, which had likely caused over-current damage to the switches.
Furthermore, the investigation found that while the bridge's alarm horn activated during the descent, the bridge continued to move downward even after the hydraulic dump valve had been de-energised. Electrical testing confirmed that in automatic mode, the down valve remained incorrectly energised despite the signal to close. This-continued descent occurred even after the hydraulic dump valve was replaced.
Findings
- The primary cause of the damage was the uncontrolled descent of the airbridge due to electrical and hydraulic failures.
- The reed switches in the height adjustment system were damaged by excessive current from a newly installed horn.
- An electrical fault caused the down valve to remain energised during automatic operation, overriding the de-energisation of the hydraulic dump valve.
Safety action
Following the incident, a relay was installed in the circuit to protect the reed switches from future over-current damage. Additionally, the wiring for the up and down valves was rerouted through a secondary relay linked to the dump valve relay to ensure both valves de-energise when the timer times out. All eight bridges of this specific design at Heathrow have since undergone similar modifications, and all are scheduled for full refurbishment.