Introduction
On 13 November 2008, a Boeing 737-8AS (registration EI-DLR) was involved in an incident at Stand D61, London Stansted Airport, during which the No 1 engine cowling was damaged and the towbar attachment broke. The aircraft was operating a commercial passenger flight with 164 passengers and 4 crew; no injuries were reported.
Background
The aircraft had a deferred defect: the Auxiliary Power Unit (APU) was inoperative. This required the crew to perform a ground-air start of one engine on stand and a cross-bleed start of the other engine after pushback. The procedure had been successfully completed on the previous two sectors, but both times there was difficulty maintaining sufficient pneumatic duct pressure. On those occasions, the aircraft had been stationary with the parking brake set before the cross-bleed start.
Event Sequence
The accident occurred during darkness, with rain and a wet apron. The flight crew were starting their third sector of the day on the same aircraft and were about 25 minutes behind schedule. The co-pilot received ATC clearance to start on stand. The commander initiated the No 1 engine start sequence but could not achieve sufficient N2 rotation and aborted. He then asked the ground crew via headset to increase air supply from the ground-air cart, and the second attempt succeeded.
After ground equipment was cleared, pushback clearance was obtained. The pushback from Stand D61L required a 'dogleg' manoeuvre. As the tug began straightening the aircraft onto the taxiway centreline, the headset operator called "CLEAR TO START NUMBER TWO." The commander responded that he would increase thrust on No 1 engine; the headset operator acknowledged with "OK."
The commander increased thrust to achieve an initial duct pressure of about 40 psi, aiming to prevent a recurrence of previous slow starts. While monitoring N2 rotation, he became aware that the nosewheel was skidding. He then heard the headset operator say "STOP PLEASE EMERGENCY STOP," but this message did not make sense to him as he believed the aircraft was still under control of the tug. Nevertheless, he reduced No 1 engine thrust to idle. The headset operator repeated "shut it down now," and the commander advised there was no APU. He opened his window, saw the situation, and applied the brakes. The aircraft and the now-driverless tug stopped moving. Shortly after, engine No 1 was shut down.
Damage
Examination revealed that the towbar attachment to the nose landing gear had failed as the aircraft moved forward. The No 1 engine nose cowl contacted the tug and pushed it across the apron for a short distance.
Personnel
The commander, aged 53, held an Airline Transport Pilot's Licence and had 14,000 flying hours (2,500 on type). The co-pilot, qualified on type for about six months, had performed cross-bleed starts only in the simulator. The tug driver and headset operator were employees of the ground handling agent. The headset operator had successfully completed training that included a requirement that cross-bleed starts should not occur during pushback; however, she was not available for interview after the accident.
Procedures
The company Operations Manual contained procedures for pushback and engine starts, including cross-bleed starts. The manufacturer's Flight Crew Training Manual advised operators to develop specific pushback procedures and that engine start may be accomplished during pushback, with ground personnel watching for hazards. The ground handling agent's training material stated that cross-bleed starts should not be permitted during pushback and that the aircraft must be stationary with the parking brake set before start clearance. The tug driver confirmed this was normal practice.
The commander noted that had the Operations Manual contained guidance on the risk of a cross-bleed start during pushback, he would not have increased left engine thrust. He was aware of the manual's caution against using brakes during pushback. The co-pilot heard the headset operator say "STOP" and repeated it to the commander. The flight was running late, but the commander did not consider this a factor.
Recorded Information
Data from the Cockpit Voice Recorder and Flight Data Recorder confirmed the sequence: No 1 engine started on stand, pushback clearance coordinated, and the ground crew cleared the flight crew to start No 2 engine just after a reversal in direction during the dogleg. The flight crew responded that they would increase power on No 1 engine.