On 26 June 2006 after an uneventful pre flight pushback of a British Airways Airbus A320-200 at London Heathrow Airport using a towbarless tractor, that unit was disconnected from the aircraft. After receiving taxi clearance from ATC, the aircraft started moving under its own power and, shortly afterwards, collided with the tractor that had just performed the pushback, damaging both the right engine and the tractor. The headset operator had given the ˜all clear™ signal to the flight crew before the tractor had been repositioned to a safe distance from the aircraft. The co-pilot did not see the tractor and a defect was found to have prevented the tractor from being driven away before the aircraft began to taxi.