On 13 November 2008, a Boeing 737-800 with an unserviceable APU was being operated by Ryanair on a passenger flight at night was in collision with a tug after a cross-bleed engine start procedure was initiated prior to the completion of a complex aircraft pushback in rain. As the power was increased on the No 1 engine in preparation for the No 2 engine start, the resulting increase in thrust was greater than the counter-force provided by the tug and the aircraft started to move forwards. The towbar attachment failed and subsequently the aircraft’s No 1 engine impacted the side of the tug, prior to the aircraft brakes being applied.
An Investigation was carried out by the UK AAIB who found that there was no prohibition or specific caution concerning cross bleed starts during pushback in the Aircraft Operator’s Operations Manual but that the procedures of the Handling Agent involved did not permit cross-bleed starts during pushback under any circumstances and that their personnel, including the headset operator during the event, had been so trained.
The commander advised the Investigation that “he would not normally have intended to carry out a cross-bleed start while the aircraft was still being pushed back. However, when the headset operator said “CLEAR TO START”, (contrary to his employers’ procedures and his own training), it had triggered the start process in his mind and he had automatically begun”.
It was noted that the pushback had been complex, involving an almost ‘S’ pattern, that the apron surface had been wet and that after problems on earlier departures that day in obtaining sufficient cross bleed duct pressure for the second engine start when the aircraft had been stationary, a higher than previous engine thrust had been set on Engine No 1 to avoid a recurrence. It was found that soon after the thrust peaked on the No 1 engine, the aircraft, with its fore-aft axis at an angle to the tug and towbar, had begun to skid obliquely forwards against the tug / towbar pressure. The towbar attachment to the nose landing gear had then failed and the aircraft had contacted the tug with the nose cowl of the No 1 engine.
Damage to the aircraft was mainly to the No1 engine nose cowling and the left nosewheel tyre with some distortion to the towbar attachment on the front of the nose landing gear. The tug was found to have only superficial damage, the towbar itself was damaged beyond further use.
As a result of the accident, the aircraft operator introduced a prohibition on all cross bleed starts until pushback is complete, the park brake is set, the tug is disconnected and ATC clearance has been obtained. No Safety Recommendations were made