David Learmount, operations and safety editor at Flight International magazine, assesses the issues behind the near-catastrophe.

How close a near miss was this?

This near miss was very close, and if the captain of the British Airways Boeing 747 on approach had not already begun a go-around manoeuvre about a second before the controller told him to, it might have been a collision.

That said, it is not surprising that the pilots in the 747 had taken their own action, because the safety of the aircraft is ultimately their responsibility and, late on the approach, they could see that the two aircraft were too close.

An indication of how seriously the authorities took this event is that the level of investigation carried out by the Air Accident Investigation Branch (AAIB) was the same as that normally applied only for full scale accidents.

Do we know why it happened?

The AAIB says that this occurred because the supervising controller misjudged the developing situation and intervened much later than he should have done to correct it.

A contributory factor is that, although National Air Traffic Services (Nats) clearly have a formal procedure for selecting controllers for specific jobs according to their abilities, there was no formal procedure for vetting controllers for their suitability as instructors - or mentors - for junior controllers being familiarised with their new task.

This is what the senior controller was doing in this case.

The AAIB reports that Nats has since introduced a formal procedure for vetting potential training controllers for their abilities in the training role.

Are trainees routinely asked to take control of flights like this, and if so, should they be, however well supervised?

The junior controller was not a trainee in the sense of being new to air traffic control, but she was new to Heathrow and was being familiarised with one of her tasks there.

This is clearly essential, to replace retiring controllers. However, she was completely cleared of blame for this near miss.

Although a trainee has to be given decisions to make, the mentor has the responsibility for monitoring them and reversing them if necessary, or for taking over completely at any time that the trainee controller is faced with a combination of tasks which is beyond his/her ability at that stage of their training.

Is it ever possible to remove all risk of air collisions?

It is not possible to remove the risk of collision absolutely, but there has not been a mid-air collision involving airliners in UK airspace since the late 1940s, and statistics indicate that the number of risk-bearing near-misses has been reducing over the last three years despite an increase in the amount of air traffic.

What difference would it have made if the air traffic control operation had been in private hands?

Air traffic control privatisation is not an issue in this case, because air traffic control at airports always has always been privatised.

Nats carries out the air traffic control for Heathrow because it won the competitive contract to do so when airport operator BAA last put the task up for bids.

The part of air traffic control due for imminent part-privatisation is en-route ATC, which is control of aircraft once they are clear of airports on departure, and before they call the airport shortly before their arrival there.