In the minds of even seasoned air travellers, the phrase "near miss" suggests airliners hurtling past one another just inches apart, their pilots straining at the controls to avert disaster.

Air traffic controllers aim to keep airplanes safely apart

Chris Yates, an aviation safety expert at Jane's publications, says passengers are usually oblivious to the few near misses which do occur.

"It's highly unlikely you'd ever get close enough to another aeroplane that you could see the pilot's face."

Mr Yates says near misses are usually measured in many hundreds of feet.

Distant threat

"Near misses are reported with aircraft in level flight, if they come within 1,000 feet, vertically or horizontally, of each other. That's quite a distance."

However, there are no hard and fast rules about what constitutes a safe distance between aircraft.

Pilots report if they feel a near miss has occured

Visibility, weather conditions, an aircraft's proximity to an airport and the type of aircraft can all determine the safety buffer pilots and air traffic controllers place around an aeroplane.

"If you're in the landing phase in a small aircraft coming in behind a jumbo jet, its jet wash can cause problems for you. That wouldn't be the case if you were in a larger aircraft," says Mr Yates.

Just as they decide the suitable distance between aircraft, pilots and air traffic controllers judge when that space has be breached and a "near miss" requires investigation.

Close call

Chris Mason, from the CAA, says once a "subjective assessment" is made that air safety may have been compromised, evidence is gathered.

This job is usually undertaken by the CAA Air Proximity Board, though the recent incident at Heathrow - involving a 747 and an Airbus - has been assigned to the Air Accidents Investigation Branch.

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If a near miss is confirmed, it is classified under one of four categories.

Category A is for incidents where investigators believe the actual risk of a collision existed, with all the risks to human life such an event would entail.

In the first six months of 1999, only three near misses were deemed to pose such a risk.

Small risk

All of these involved small light aircraft passing close to larger commercial flights.

In the most dramatic near miss, a novice microlight pilot took his aircraft to 11,000 feet, into the path of a 767 airliner leaving Manchester airport. The two came within 100 feet of colliding.

As the Heathrow incident demonstrates, near misses are not confined to private aircraft or inexperienced aviators.

Small aircraft can represent big danger

Last December, a 737 taking off from Gatwick flew close to a 757 waiting to land at Heathrow.

Though an investigation is yet to be concluded, "minimum separation" is said to have been lost between the large passenger aircraft.

Military traffic has also caused its share of near miss scares.

Up to trouble

Last year, a low-flying RAF Tornado jet near Edinburgh was forced to make an emergency ascent following a "bird strike". Climbing to 15,000 feet, the aircraft came into "close proximity" with a British Midland jet.

Mr Mason stresses the majority of the near miss stories reported are not rated as category A, or even category B - where there is no actual risk of collision, but aircraft safety is compromised.

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Categories C and D are reserved for incidents where there is no risk of collision or the risk cannot be determined.

Mr Mason says Britain's air safety record remains excellent. There are just 1.5 fatal accidents per million commercial flights in Europe, compared to 5.1 in Asia and 8 per million in Africa.

"Although the skies are busier than they were 50 years ago, thanks to regulation they are also much safer."