UK investigators believe a tow-tractor instructor was focused on training another driver when the vehicle pushed a Boeing 737-800 into the blast fence at London Stansted.

The Malta Air aircraft – before engine start – was undergoing a long pushback from stand 63R, to the end of a cul-de-sac abeam of stand 61L, on 21 August last year.

According to the UK Air Accidents Investigation Branch says a trainee driver initiated the pushback but turned for the taxiway centreline too early.

After several corrective turns, which had been queried by the 737’s captain, the trainee driver stopped the pushback and asked the instructor to take over.

“After swapping seats the instructor recommenced the pushback, correcting the aircraft back onto the taxiway centreline,” says the inquiry.

“He continued to reassure and advise the trainee while performing the pushback, looking at her while doing so.”

But when the aircraft’s nose-wheel reached the ‘tug release’ marking on the ground, the tractor did not stop. The aircraft (9H-QAA) struck the blast barrier at the end of the cul-de-sac, and the collision caused substantial damage to the aft section including the auxiliary power unit which had been running.

“Realising what had happened, the tug instructor pulled the aircraft forward off the fence then stopped the tug,” says the inquiry. The clearance between the 737 and the blast fence, at the correct stopping point, was about 15m.

It states that, while the pushback was accompanied by a wingman and headset operator, who were surprised when the tractor did not stop at the marking.

“By the time they had wondered about [the instructor’s] reason for doing so – with one of them trying to get visual contact with the other – it was too late to signal that the tug instructor should stop,” says the inquiry.

London Stansted’s operator has since increased the size of the ‘tug release’ markings.