Two passenger jets 'within 10 seconds' of colliding above Houses of Parliament

Two passenger jets came within 10 seconds of colliding in mid-air above central London last month when a computer alerted air-traffic controllers to the potential disaster.

Two passenger jets came within 10 seconds of colliding in mid-air above central London last month when a computer alerted air-traffic controllers to the potential disaster.

The United Airlines Boeing 777 and Virgin Atlantic Boeing 747 were 600ft apart at their closest point. The computeractivated alarm averted what one pilot said could have been Britain's worst air disaster.

The two planes, carrying more than 700 people, were flying over Westminster on their final approach to Heathrow when the "conflict alarm" was triggered. The device sounds a warning when planes are on a collision course and is the last line of defence in air safety.

Air-traffic controllers at West Drayton had not spotted the danger but once alerted by the alarm a controller told the pilot of United flight 998 from Boston: "Take urgent avoiding action now. Turn on to a heading of 270-degrees to avoid airliner." The message was repeated three times.

The captain of the United aircraft calmly replied: "Understood. Immediate action taken. I have got the other aircraft beneath me in my sight."

When the danger had passed, the air-traffic controller told the United pilot that the Virgin aircraft, from Washington, was "going 600ft underneath you".

A pilot who listened in on the exchange between the controller and the pilot said the jets could have crashed above Westminster had they continued on their flight paths.

"This latest frightening incident is a sign of the times. It was a computer conflict alarm which saved the day and prevented what could have been the worst ever air disaster in Britain.

"Can you imagine the devastation if these jets had come down on the Houses of Parliament? The scare highlights the all too heavy workload of controllers," said the pilot, who did not want to be named.

A Civil Aviation Authority (CAA) spokesman said a radar reading suggested the aircraft were two-and-a-half nautical miles apart horizontally when the conflict alarm was activated. Industry guidelines say aircraft should be a minimum of four nautical miles apart.

The spokesman said: "The incident has been fully investigated and it has been concluded that the safety of the aircraft was not compromised."

Iain Findlay, aviation officer for the IPMS union which represents air-traffic controllers, said lessons must be learnt to ensure a similar incident would not happen again.

"Clearly, rules of aircraft separation have been breached here," he said.

A spokesman for United Airlines said: "There was an incident. The situation has been investigated and is now closed."

A Virgin spokesman said: "These incidents are not uncommon in the stacking system for aircraft waiting for permission to land at Heathrow."

* An 81-year-old pilot and his 11-year-old passenger escaped serious injury when their light aircraft crashed just after take-off yesterday. The planecollided with a wire fence before stopping in a ditch at the airstrip near Rothesay, on the island of Bute, Scotland.