What happened
On the night of 10 January 2008, an Airbus A380-800, registration 9V-SKA, was being prepared for a scheduled passenger service to Sydney. During a pushback maneuver from Bay A4 at Singapore Changi Airport, the aircraft's right-hand wing landing gear and part of the right-hand body landing gear drifted off the paved surface of Taxiway WA. The aircraft moved onto the grass verge, where the weight of the plane caused the ground to sink over a concrete airfield lighting transformer box.
In an attempt to recover the aircraft, the air tug driver engaged reverse gear to pull the plane back onto the taxiway. During this recovery effort, the force applied caused the fuse pins connecting the towing pin to the nose landing gear to shear, resulting in the tow bar detaching from the aircraft. While the aircraft sustained no structural damage, the incident was classified as an occurrence by the Air Accident Investigation Bureau of Singapore.
The investigation
The investigation examined the maneuvers of the pushback team, the visibility conditions at the time, and the mechanical integrity of the towing equipment. Investigators noted that the air tug driver was performing his first pushback of an A300-800 and his first operation from Bay A4. The investigation also looked into the maintenance of the tow bar, specifically the torque applied to the shear bolts and the spacing between them.
Findings
Several contributing factors were identified during the inquiry:
- The driver failed to maintain the aircraft's position relative to the yellow lead-in lines.
- The taxiway centerline was difficult to see because it was not reflective and the area was unlit.
- The headset man, acting as the team leader, was not sufficiently assertive in directing the driver to stop when the aircraft deviated from the path.
- The tow bar shear bolts were improperly tightened, likely due to maintenance personnel using imperial-calibrated torque wrenches without proper metric conversion tables.
- The spacing between the shear bolts on the tow bar was wider than the manufacturer's specifications, which altered the shearing characteristics of the hardware.
Safety action
Following the incident, the air tug operator implemented several changes, including inspecting all tow bars to ensure bolts were tightened to the correct torque and providing metric torque meters to maintenance staff. The tow bar manufacturer also redesigned the shear bolt spacing to align with Airbus specifications. Additionally, the aerodrome operator introduced new pushback lines at Bay A4 to assist drivers in navigating the complex maneuver.