BA said its Boeing 737 was nearing the end of its flight from Gothenburg in Sweden to Gatwick airport when it was alerted to the presence of another plane. According to one report, the two aircraftwere just four seconds apart when its onboard Traffic Collision Avoidance System (TCAS) instructed the BA plane to "climb - climb now", while the system in the United aircraft said: "Descend - descend now."
It is understood that the BA aircraft was ordered to circle while waiting to land and was somehow put on the same flight altitude as the United plane. Neither the Civil Aviation Authority nor the airlines were able to confirm that.
A BA spokeswoman said: "As we were about to start the descent, the pilot received instructions from air traffic control to turn right off course and descend. At the same time our TCAS gave us advance notice that there was another aircraft in the vicinity." She said the pilot landed safely at Gatwick.
"The passengers would have been unaware that anything had happened," she said.
United Airlines confirmed that a Boeing 777 carrying 19 passengers from Amsterdam was involved in the near-miss or "airprox" as it was flying into Heathrow en route to Los Angeles. The CAA said the incident would be fully investigated by the independent UK Airprox Board.
The CAA said there were an average of 41 incidents in controlled UK airspace every year, of which 80 per cent were shown to have involved no risk.
In February, a Boeing 737 came within 100ft of a Gulfstream IV business jet over Essex. In July 1997, a Boeing 747 and a Gulfstream IV were involved in a near-miss over Lambourne, Essex, while a BA Boeing 737 and a Virgin Express Boeing 757 came within 200ft of each other in dense fog over Heathrow in August 1997.
The main air traffic control centre at West Drayton, west London, is due to be replaced by a pounds 340m centre at Swanwick in Hampshire, to increase capacity. The transfer should have taken place in 1996, but computer software problems have put the opening date back to 2001-02.
The Public and Commercial Services Union, which represents controllers, called on the Government to put on hold its plans to sell of 51 per cent of National Air Traffic Control Services. Barry Reamsbottom, the union's joint general secretary, said: "The latest near-miss shows that the current system is dangerously overstretched."
The near-miss has highlighted the importance of Airborne Collision Avoidance System (ACAS), of which TCAS is one version. From 1 January 2000, ACAS will become obligatory in European airspace.
There was some doubt about the wisdom of introducing the system, but the CAA said it strongly backing the system now that the software had been improved. "The worry was that if the system was faulty it would alert the pilot to take action that would itself cause an accident," said a CAA spokesman.
Kieron Daly, the editor of Air Transport Intelligence, said: "TCAS is very much a last-ditch system - to be used when all else fails. The system has worked very well and is generally accepted in the aviation industry."Reuse content