French investigators have determined that an ambiguous taxiing instruction prompted a Bulgaria Air Airbus A319 to cross an active Paris runway while an A320 was rotating to take off.

The Turkish Freebird Airlines A320, departing Charles de Gaulle airport’s runway 08L, had been travelling at 139kt and was some 1,500m away when the A319 crossed 08L from the S6 taxiway.

Investigation authority BEA says the A320 rotated at 142kt and its pilots believed they overflew the A319 at around 100ft – although analysis later indicated that the S6 intersection was overflown at 500ft.

The event took place nearly five years ago, on 25 November 2014, but BEA has only just published its findings.

It states that the Bulgaria Air A319 had landed on the parallel runway 08R and, while it was taxiing along exit V5, was told to hold short of 08L at point S6.

The crew correctly read back this instruction.

Eight seconds later the Freebird A320 was cleared for take-off from 08L.

Charles de Gaulle was operating with reduced separations and the controller also had to deal with an Air France flight which was vacating 08R for taxiway S6 and threatened to conflict with the Bulgaria A319.

This prompted the controller to transmit an instruction to the A319 crew to “keep on taxiing”, which was read back by the Bulgaria crew but misinterpreted as a clearance to cross runway 08L.

The Air France crew sought clarification as to whether they were cleared to cross the runway, but this conversation took place in French.

BEA says the Bulgaria crew was unable to understand this exchange and the controller “did not connect” the Air France pilots’ query with a possible ambiguity over the instruction to the Bulgaria crew.

The Freebird A320 had reached the V1 decision speed before its pilots saw the Bulgaria jet, and they expected it to taxi to a halt.

“Shortly after, they realised that it was crossing the holding point,” says BEA. “As the rotation speed had been reached, the captain decided to take off.”

Although a runway incursion alarm sounded, just 1s after the A319 entered the runway, the alert was too late for controllers to intervene.

None of the occupants of either aircraft involved in the conflict – LZ-FBB and TC-FBJ – was injured.

Paris Charles de Gaulle’s air navigation service subsequently reminded controllers over the use of clear phraseology. The airport has also since installed a runway-status light system, which indicates to departing or crossing aircraft that the runway is occupied.

Source: Cirium Dashboard