Investigation task force identifies manufacturing defect leading to metal
fatigue of bolts as cause of air-bridge collapse incident
(HONG KONG, 28 June 2013) – Metal fatigue in the bolts holding together two rotunda flanges was the root cause of the rare air-bridge collapse at Gate W71 of Hong Kong International Airport’s Northwest Concourse on 7 April, the investigation task force announced today. The task force added that the fatigue developed as a result of uneven flange surfaces due to manufacturing defect.
The task force is chaired by independent veteran engineer Edmund Leung Kwong-ho, with John Chai Sung-veng, Executive Director, Projects, Airport Authority Hong Kong (the AA) serving as vice-chairperson. Dr George Greene and Dr Eric Lim from Safety Accident and Failure Experts Ltd. are acting as lead consultants of the task force’s technical working group.
The investigation included a comprehensive analysis of the facts and evidence related to the technical and operational aspects of the collapsed air bridge. The task force’s technical working group performed a series of scientific laboratory and on-site tests covering ultrasonic inspection, air bridge control record analysis, vibration and acceleration analysis, apron pavement survey, bolt-matching, laboratory testing for bolts, rotunda flange flatness measurement, stress analysis and calculations, and extensive document and records reviews. The operation working group interviewed over 15 witnesses, and collected and analysed environment data such as wind, humidity, rainfall and visibility.
"We have determined that the two rotunda flanges, which were connected at the rotunda column of the rear bridge, contained manufacturing defect that caused their mating surfaces to fit unevenly," explained Edmund Leung Kwong-ho. "The resulting stress imbalance on the rotunda flanges and bolts induced extensive cyclic load and substantially increased the tension of some of the bolts securing them, leading to the development of metal fatigue and fracture."
Based on these findings, Mr Leung concluded that the flange defect was the root cause of the collapse, which was triggered by the air-bridge passing over a hump in the pavement, causing a sudden jerking motion. The resulting load broke the fatigued bolts and forced the two rotunda flanges to split apart. He added that he considered the collapse to be a rare incident.
The investigation revealed that the AA invited the OEM manufacturer, Bukaka, to conduct a technical audit on six air bridges in late 2009 to review the robustness of maintenance. Two of the six bridges, located at the Northwest Concourse, had the same design as the collapsed bridges and Bukaka did not inspect the rotunda flanges or bolts during the audit.
The current bridge maintenance company contracted Bukaka in early 2012 to provide maintenance training for their staff. The maintenance training and training materials did not refer to a need to check the rotunda flange bolts.
However, during the investigation, it was found that a page in the air bridge's Operation and Maintenance Manuals refers to checking bolts and tightening torque, but was not included in the Maintenance Task Check Lists and contained inaccurate information about the size of the bolts and their torque setting. The task force is of the view that this was not a factor leading to the collapse of the bridge.
On the maintenance of the bridge, the investigation concluded that the AA and its service providers carried out preventive maintenance in accordance with the schedule outlined in the operation and maintenance manuals provided by the manufacturer.
With regard to the operation of the bridge, the task force has reviewed and examined a myriad of factors such as operating environment, procedure and staff competency and workload. No operational non-compliance was identified.
CK Ng, Executive Director, Airport Operations, said, "We appreciate the in-depth, professional inquiry conducted by the Investigation task force into this incident. We accept the investigation findings and are adopting all of the recommendations.
"We regret that the defect of the collapsed rear bridge could not be detected sooner, although every effort was made to ensure the quality of air bridge prior to its commissioning, and to adhere to the maintenance task check lists provided by the air bridge’s OEM manufacturer," he added.
The AA added that, in addition to following up with the manufacturer and other relevant entities, it would implement the task force's recommendation to review operational and maintenance manuals to ensure that there is no discrepancy. It added it will strengthen its maintenance regime by conducting ultrasonic inspections on the rotunda flange bolts every half-year, and by checking the structural bolts’ torque annually even though such measures are normally not specified in regular maintenance programmes and not commonly adopted for structural parts.
Following the incident, the AA suspended nine rear bridges of the same design and conducted flange flatness measurements and ultrasonic inspections on the bolts. Small gaps were found in the flanges of four of the suspended bridges, although no signs of similar uneven flange surfaces or weakening or metal fatigue were found. As a safety precaution, the four bridges will remain suspended until they are thoroughly examined, repaired or replaced, while the others would be returned to normal operation today.
The AA reiterated its commitment to the safety of its passengers, employees and business partners, adding that it would extend regular checks to the structural aspects of the air bridges. It has also earmarked more than HK$80 million to overhaul all air bridges at Terminal 1 – a refurbishment programme that is underway and scheduled for completion in phases by 2015 – and is planning an overall review of maintenance schedules for other airport facilities.
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