Introduction
On 17 February 2008 at 1527 UTC, an Airbus A321-211, registered G-DHJH, was involved in an incident during pushback at Manchester Airport. The aircraft was operating a commercial passenger flight with 7 crew and 220 passengers on board. The commander, aged 55, held an Airline Transport Pilot's Licence and had 18,000 total flying hours, including 7,000 on type. No injuries were sustained by anyone on board.
History of the Flight
The pushback began normally, but as the aircraft turned left, the cockpit crew heard clonking noises from the nose landing gear area, likened to a loose pin on a tug-and-towbar arrangement. The noises increased until a loud bang occurred, and the pushback stopped. The cockpit crew observed the towbarless tug at an acute angle to the aircraft. The tug crew requested the aircraft's park brake to be set, informed the cockpit crew of significant damage, and asked for engineering assistance. Passengers and crew deplaned via external steps and were transported to the terminal.
Engineering Examination
Engineers found that the nosewheels had fallen from the tug's hydraulic-powered 'grab and retention' mechanism and were in contact with the apron surface. Both nosewheel steering motors, mounted on the nose landing gear leg, had been damaged by the tug's structure, necessitating a complete replacement of the nose landing gear before the next flight. The operator, in cooperation with the AAIB and a manufacturer's representative, examined and tested the tug but found no faults. The problem could not be reproduced.
Previous Occurrences
Airport authorities informed the AAIB of four previous nosewheel damage events involving the same tug model, each with different operatives. Two events were attributed to human error and equipment failure, while no reasons could be determined for the other two.
Design Considerations
An inspection of another manufacturer's towbarless tug revealed a safety feature that prevents nosewheel tyres from contacting the ground if the retention mechanism fails. Several technical standards exist for towbarless tugs, including BS EN 12312-7:2005, SAE ARP 4852, EU Directive 98/37/EC, SAE ARP 5283, and ISO 20683-1. These documents specify requirements for positive mechanical locking, prevention of interference with aircraft structure, and wheel retention under dynamic conditions. However, these standards are not always matched by national or international aviation regulatory requirements.
Conclusion
The investigation established that a number of technical specification documents define standards for the design, manufacture, operation, or maintenance of aircraft ground support equipment, specifically towbarless tugs. However, these standards are not necessarily enforced by regulatory requirements.