Investigators have determined that a manufacturing defect, leading to metal fatigue, caused the collapse of an airbridge at Hong Kong in April.
The collapse of the rear airbridge at gate W71 resulted in damage to the door of a Cathay Pacific Airbus A330.
Two flanges connected at the rotunda column of the airbridge contained a manufacturing defect which meant their mating surfaces did not fit evenly, says engineer Edmund Leung Kwong-ho, who chaired the investigating task force.
This created a stress imbalance on the flanges, causing "excessive" loading and increasing tension on some of the securing bolts, he says, leading to metal fatigue. As the airbridge passed over an uneven part of the apron, the weakened bolts fractured and caused the rotunda flanges to split.
Airport Authority Hong Kong says six bridges on the concourse underwent a technical audit in 2009 by manufacturer Bukaka, two of which had the same design as the collapsed bridge. But the rotunda flanges and bolts were not inspected.
The investigation, however, found that the airport authority carried out preventative maintenance in accordance with the manufacturer's manuals.
"No operational non-compliance was identified," the authority states, referring to the way the airbridge was handled.
Airport operations executive director CK Ng says: "We regret that the defect of the collapsed rear bridge could not be detected sooner."
He says "every effort" was made to ensure the quality of the airbridge before it was commissioned and to stick with the maintenance schedule from the manufacturer.
"We accept the investigation findings and are adopting all of the recommendations," he adds. The authority will reinforce its maintenance regime with regular ultrasonic inspections on the rotunda flange bolts.
After the accident Airport Authority Hong Kong suspended the use of nine rear airbridges of the same design to conduct inspections. It says "small gaps" were found in the flanges of four airbridges but no sign of uneven surfaces or metal fatigue.