What happened
On 13 November 2008, during the night, a Boeing 737-8AS, registration EI-DLR, was undergoing a pushback maneuver at London Stansted Airport. The aircraft was operating with a deferred defect, meaning the Auxiliary Power Unit (APU) was unavailable, necessitating a ground-air start of the engines on the stand.
During the 'dogleg' pushback maneuver, the ground crew provided clearance to start the second engine using a cross-bleed method. To prevent previous issues with low pneumatic pressure, the commander increased the thrust on the No 1 engine. This increase in power generated more thrust than the tug could counteract, especially as the tug was positioned at an angle that offered little resistance.
As the engine power surged, the aircraft began moving forward, causing the towbar attachment to fail. The aircraft's No 1 engine cowling struck the side of the tug, pushing the vehicle across the wet apron. The tug driver, seeing the approaching engine, exited the cab to avoid injury. The commander applied the brakes to stop the aircraft after the collision had occurred.
The investigation
The AAIB examined the aircraft, the tug, and the towbar, alongside cockpit voice and flight data recordings. The investigation found that the aircraft's No 1 engine reached 51% N1 during the event. The investigation also reviewed the ground handling company's training and operations manuals. It was noted that while the headset operator was trained to prevent starts during pushback, the company's manual lacked specific instructions prohibiting cross-bleed starts while the aircraft was still being moved.
Findings
- The aircraft was performing a cross-bleed engine start while the pushback maneuver was still in progress.
- The commander increased thrust on the No 1 engine to ensure sufficient duct pressure for the start.
- The increase in thrust on the No 1 engine exceeded the resistive force provided by the tug, leading to the forward movement of the aircraft.
- The towbar attachment failed due to the overload caused by the aircraft's forward motion.
- The headset operator provided clearance for the start despite the ongoing pushback, likely due to a lack of specific prohibitive guidance in the operations manual.
Safety action
Following the accident, the operator updated its Operations Manual to explicitly state that cross-bleed starts must not be attempted until pushback is complete, the parking brake is set, the tug is disconnected, and ATC clearance has been obtained.