The two aircraft, operated by British Airways, collided on a taxiway adjacent to London Heathrow Terminal 4. The Airbus A321, G-EUXH, had landed after an uneventful flight from Zurich, Switzerland and had taxied to Stand 431 under instructions from the Ground Movements Control 2 (GMC2) controller. As it did so, the crew of the Boeing 777, G-VIIK, were preparing to depart for Washington, USA from Stand 429. Another A321 in the same livery was parked on Stand 432, immediately to the left of G-EUXH. As the Airbus approached its stand, the crew realised that the electronic Stand Entry Guidance (SEG) system was not switched on. This was because the operator’s ground staff responsible for activating it had not yet arrived at the stand. The Airbus commander stopped his aircraft about 50 metres short of the intended parking position; it was aligned with the stand centreline, but with about half the aircraft protruding into the taxiway behind. He made a radio call to GMC2, to advise that the stand guidance was not illuminated, but the frequency was very busy and the call was not acknowledged. Whilst the commander informed the passengers and cabin staff that the aircraft was not yet on stand, the co-pilot attempted to contact his company on discrete frequencies to request that ground crew attend the stand. About a minute after the radio call from the Airbus to GMC2, the crew of the Boeing 777 called GMC2 to request pushback from Stand 429, which the controller approved. During pushback, the Boeing 777’s left wing collided with the Airbus’ fin. The tug driver reported that he had seen the Airbus moments earlier and had applied the vehicle’s brakes, but was too late to prevent the collision. The collision was felt on both aircraft. The Airbus crew made a further call to GMC2, stating that their aircraft had been struck, but it, too, was not acknowledged. They then twice broadcast a PAN-PAN call, which was acknowledged after the second broadcast. The Boeing 777 crew also made a PAN-PAN call. The GMC2 controller took the appropriate actions, and alerted the airport emergency services. The tug was equipped with a radio capable of receiving and making transmissions on the GMC2 frequency, but it was not switched on prior to, or during, the pushback. The Airbus remained stationary after the collision, but the Boeing’s pushback crew immediately pulled the aircraft forward again, back onto Stand 429. The passengers on both aircraft reportedly remained calm and were disembarked via steps. There were no reported injuries. Conclusion: The accident occurred primarily because the Boeing 777 pushback was not conducted in accordance with the aircraft operator’s normal operating procedures and safe practices. Organisational factors which may have contributed to the accident included: the withdrawal of recurrent safety awareness training for ground handling staff, late stand guidance system activation issues, and incomplete risk assessments for towing and pushback operations.