What happened
On 7 May 1999, a Boeing 777-222, registration N768UA, was undergoing pushback from Stand J15 at London Heathrow Airport. The aircraft was being operated as a public transport flight with 8 crew members and 147 passengers on board. At the time of the event, the pavement was wet following earlier rainfall.
During the procedure, the aircraft was cleared to start its engines. Due to an incoming aircraft holding for the same stand, the tug driver attempted to move the aircraft forward to create sufficient clearance. As the driver began to straighten the tug and initiate the forward movement, the tug unexpectedly accelerated forward despite the driver's foot being off the accelerator. This acceleration was caused by the thrust from the aircraft's engines, which caused the tug to skid and subsequently sheared the head off the towbar.
As the aircraft continued its forward motion, the driver exited the tug. Along with two wingwalkers, the driver signalled for the flight crew to apply the brakes. However, the crew did not respond to the driver's initial request to set brakes and cut engines because the communication cable had become disconnected. The aircraft eventually passed over the tug, with the starboard main gear wheels coming to rest against the side of the vehicle. There were no injuries to the crew or passengers, though a wheel change was required.
The investigation
The investigation established that the incident occurred on a slight 0.6% slope. The investigators examined the sequence of engine start and the physical interaction between the aircraft thrust and the tug. It was determined that the disconnection of the communication cable prevented the flight crew from receiving the driver's urgent instructions to stop the engines and apply brakes.
Findings
- The primary cause of the incident was the uncontrolled acceleration of the tug caused by aircraft engine thrust.
- The wet pavement surface contributed to the tug skidding.
- A disconnected communication cable prevented the flight crew from responding to the driver's emergency instructions.
- The aircraft's movement resulted in the shearing of the towbar head and contact between the main gear and the tug.