Incident Description
On 12 February 2000 at 1805 hours, a Boeing 747-436, registration VB-HOZ, was parked at Stand K17, Terminal 3, London Heathrow Airport. The aircraft, operated on a public transport flight with 21 crew and no passengers, was being serviced when the airbridge (K17B) dropped about one metre while positioned at the L1 door. The drop ripped the top of the left-hand door. No crew injuries were reported.
Investigation Findings
Examination of the airbridge revealed multiple faults related to its automatic height adjustment function. The normal procedure requires the operator to select Manual mode, position the bridge bumper 15–20 cm below the door sill, then switch to Auto after bumper contact. An automatic height adjustment device on the right side of the bridgehead uses a drum that contacts the fuselage; as the aircraft rises or falls, reed switches signal the hydraulic system to adjust. Safety features include an alarm horn that sounds if movement continues beyond six seconds (about 12 cm) or if the drum loses contact; a second-line reed switch activates after about 3.5 cm as a backup.
Inspectors found that the first- and second-line reed switches had their contacts 'welded' together, causing the horn to sound and the bridge to descend. This damage was attributed to the installation of a new, higher-current klaxon-style horn. Previously, a relay had been installed in older bridges to protect reed switches from overcurrent; no such relay was present on this occasion.
During testing, the bridge functioned correctly in Manual mode. In Auto, the bridge operated properly when raising; when lowering, the timer timed out and the alarm sounded, but the bridge continued descending despite the hydraulic dump valve being de-energised (signalled to close). The dump valve was replaced due to potential spring breakage, but further testing revealed the fault persisted. Electrical checks showed that in Auto mode, when the horn sounded and the dump valve was de-energised, the down valve remained energised; in Manual mode, no other valves were energised when the dump valve was de-energised.
Modifications
A modification was implemented that routed the up and down valve wiring through a secondary relay linked to the dump valve relay. This ensured that when the timer timed out in Auto mode, both the dump valve and the up/down valves would be de-energised, halting movement. After the modification, the bridge functioned correctly.
All eight bridges of the same design at Heathrow have since been similarly modified and are scheduled for full refurbishment during 2000/2001.