Investigators have been unable to explain why the crews of two Boeing 747s each apparently followed instructions meant for the other aircraft, resulting in an airprox event over Scotland.
While the 23 June incident was assessed by the UK Airprox Board as bearing no collision risk, its analysis has been unable to determine the reason for the joint error.
The crews of both aircraft – a British Airways 747-400 and a Lufthansa 747-8, according to archived automatic dependent surveillance data – had been preparing for the transition to oceanic airspace for transatlantic crossings but were on converging flightpaths at 34,000ft.
Following activation of a short-term conflict alert, a controller at the Prestwick air traffic centre attempted to redirect the jets onto diverging tracks, by instructing the BA aircraft to turn left and the Lufthansa aircraft right.
Simulations showed that, had the aircraft followed these instructions, they would have remained separated by at least 7.2nm during the event.
But the aircraft continued to converge, closing to 3.9nm with 100ft altitude difference in the vicinity of Loch Rannoch. The conflict was resolved with the assistance of collision-avoidance resolutions.
During the event the 747-400 turned slightly right, eventually crossing 1.6nm behind, and about 1,100ft below, the 747-8. Written submissions showed the crews had each wrongly interpreted the controller's directions.
“It was apparent that both crews had taken each other’s instructions,” says the Airprox Board, adding that it “found it hard to determine why this had occurred”.
Even though at least one crew had read back the avoidance instruction correctly, it says, all four pilots had “misheard or misinterpreted” the directions.
Callsign confusion was ruled out because the two flights had dissimilar identifiers, and the radio transmissions were found to be clear.
“It was possible that the crews may have been distracted,” the Airprox Board suggests, pointing out that the pilots might have been receiving oceanic clearances via datalink. It also theorises that the crew would not have expected the avoidance instructions and might have responded instinctively, without proper assimilation.