A controller mistakenly ordered a United Airlines jet to descend into the path of a British Airways aircraft after becoming confused by overlapping labels on his radar screen. The labels show each aircraft’s call sign, its height and the airport to which it is heading.

The aircraft, both Boeing 777s, were bound for Heathrow but had been directed into a holding pattern over Chesham in Buckinghamshire, known as the “Bovingdon stack”. There were so many aircraft in the stack that the controller, based at West Drayton, near Heathrow, was unable to distinguish between them.

He mistook another aircraft at 12,000ft for the BA aircraft which was at 13,000ft. He then ordered the United Airlines aircraft to descend to 13,000ft into what he wrongly believed was empty airspace. Within 40 seconds, the vertical distance between the two planes had reduced to only 600ft, breaching the minimum safety gap of 1,000ft.

The aircraft would have come much closer if it had not been for the collision avoidance system on the BA jet which ordered the pilot to dive.

An alarm on the controller’s desk also began buzzing and he ordered the United Airlines aircraft to climb. But the close proximity of

 other planes in the stack meant there was little room in which to recover. Other aircraft received a series of collision alerts in a “domino effect” which rippled down the stack.

A report by the UK Airprox Board, which investigates near misses, found that the problem of overlapping labels had been previously identified by National Air Traffic Services (NATS) following an incident over Romford in Essex in which an aircraft could not be identified after vanishing from the screen behind another.

The board found that the problem of overlap in the incident over Chesham had been heightened by the size of the airspace being monitored by the controller. Normally, controllers cover only one sector measuring 45 miles across, but the area is widened if not enough staff are available. On this occasion the controller was covering 65 miles of airspace.

The report concluded: “The avoiding action by the controller did little to take the heat out of the situation. Normal traffic flow had been compromised to the extent that safety had not been assured.”

The incident, on December 1 last year, happened shortly after 6am when about 30 aircraft were being held in four stacks on the outskirts of London.

Air traffic controllers are under intense pressure between 6am and 7am because airlines try to squeeze in as many flights as possible

 immediately after the noise restrictions at Heathrow are lifted at 6am. Many aircraft arrive over London just before 6am and spend up to 45 minutes circling before being given permission to land.

John Stewart, chairman of ClearSkies, which campaigns on behalf of people living under flight paths, called for a system of penalties to deter airlines from arriving so early that they had to be sent to a stack. He added: “It is bad enough that people living under the stacks get woken up by circling aircraft in the early morning without the added risk of a mid-air collision over their homes.”

A spokesman for NATS said a recent software upgrade had reduced the problem of overlapping and the system at West Drayton would be further upgraded by December next year. The controller involved in the near miss had been suspended but was allowed to return to work after re-training.

The number of near misses involving aircraft in Britain’s skies last year fell to a record low of 181, compared with 221 in 2002. But the risk for commercial airliners rose slightly from 0.51 incidents per 100,000 flying hours in 2002 to 0.79 in 2003.

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