THE STORY of the disastrous mid air collision in India is, like many tragedies, surprisingly simple. The primary issue is not one of technology or lack of it, but of a simple human mistake.

Press reporting of it all over the world has been a revelation of people's basic belief that aviation is some form of magic; so when India's Civil Aviation Authority chief, Mr Yogesh Chandra, gave the simple clues to the cause at a press conference less than 24 hours after the event, no one recognised that he had told them what had happened. As a result, the rampant wildly misleading speculation continued.

What had happened, said the Indian CAA, was that the Delhi air traffic controllers had cleared the departing Saudi Boeing 747 to climb to 14,000 ft and level out at that height. ATC had cleared the inbound Kazakh Ilyushin 11-76 cargo jet to descend to 1,000 ft and level out.

The two aircraft were heading in opposite directions on the same track, the 747 heading west for Dhahran in the Gull, the 11-76 east to land at New Delhi. The intention was that the two aircraft would be seen on radar to have passed each other safely at their different heights before further clearance was given to climb or descend.

The pilots had each repeated back to the controller the height they had been cleared to, so ruling out misunderstanding. They had even been told to watch out for each other and how far apart they were at that moment.

But they then collided. Why? The obvious answer is that they were at the same height on the same track. Since they were supposed to be travelling in opposite directions on the same track, the only question remaining is why they were at the same height.

ATC had issued correct instructions for safety, so one of the pilots must have been flying at the wrong height. Possibly the pilot of the 11-76. Why would he have done that? There are three possibilities: he had set the autopilot to level out at the wrong height; could have misread or mis set the altimetre; or have simply been distracted momentarily while flying the aircraft manually, and have flown past the cleared altitude without noticing his mistake.

"Altitude busts", as pilots call them, are not rare. They have the potential to end in tragedy, but because aircraft are separated horizontally as well as vertically whenever possible, the system is very tolerant of mistakes. In this case they were not separated horizontally, so the room for error was reduced. This shows a weakness in New Delhi's ATC, not in terms of its equipment or people, but in terms of its approach and departure route layout.

Why did the controller not see the potential collision and stop it? ATC at Delhi has primary radar. That is the ordinary kind which shows dots on the display indicating where each aircraft is, but only in lateral or plan form. No height read outs are given next to the dots, so the controller has to trust that the pilots are at the heights they say they are at, or to which they have been told to climb or descend.

If New Delhi's new secondary radar had been installed to schedule, by summer this year, it would have been up and running. But installation problems mean it will not be operating until next year. If it had been in service on the day of the collision, the controller could have seen, next to each dot, the aircraft's identification, height, speed and destination.

Does that mean that the controller would have been able to stop the collision? It depends on how much he had to do at the time. If his attention was distracted by other aircraft, it takes a descending aircraft only 40 seconds to drop 1,000 ft to the level of the aircraft below, and less if the pilot is in a hurry. So even with secondary radar, it depends on how alert the controller is.

What about the future? Well a redesign of the New Delhi approach and departure paths, to separate climbing from descending aircraft laterally, would make the system more fault tolerant and make the controllers' jobs easier.

They have been campaigning for this for years, says the Indian Guild of Air Traffic Control Officers, but have been frustrated by the fact that the Indian air force controls much of the air space in Delhi's vicinity. The IAF denies that it has blocked the changes requested.

The final safety net for the ATC system is equipment on the aircraft, called TCAS - Traffic Alert and Collision Avoidance System. Aircraft equipped with it can see others with the same or similar equipment and if a collision looks imminent, the pilots are guided into a climb or descent to prevent a crash. If both have full TCAS, their manoeuvres are co ordinated so that one climbs and the other descends.

This equipment is mandatory only in the US. Europe will mandate it in a few years - it has been waiting for a more reliable version that does not give false warnings, which can be dangerous. The software for a system to warn controllers of possible conflicts has been difficult to develop, again to eliminate false warnings. Nothing could be more dangerous than for a controller to be constantly alerted to conflicts which do not exist.