It looks as if either the controllers made a catastrophic mistake, or one of the aircraft - which were in the most vulnerable phases of their flight, taking off or landing - did not do as it was told or misunderstood instructions.
Two years ago, an Uzbek aircraft crashed at Delhi after confusion caused by language difficulties, and it could be that communication difficulties were again the problem.
Whatever the cause, the crash will renew concern about the risk of mid- air collisions close to airports in Britain, where there have been a number of "near misses" which could have killed hundreds of people. The Indian crash will be the subject of minute investigation, but a technical cause looks unlikely. Precise details, such as the height at which the aircraft were flying when they collided, were not initially available.
"Under air-traffic control they are told what to do. Provided they did what they were told to do, it was an air-traffic control error," said David Rider, editor of Jane's Air Traffic Control. "Otherwise, it was pilot error - if an aircraft was told to turn right and turned left, for example."
The Saudi Arabian Airline Boeing 747 taking off from New Delhi's Indira Gandhi airport collided with a Ilyushin Il-76 of Kazakhstan Airlines close to the ground, according to initial eyewitness reports, which spoke of "fireballs".
A report by the Press Trust of India (PTI) cited officials of the Civil Aviation Directorate as saying that the Saudi plane, SV 763, had been cleared to climb to 14,000ft and the Kazakh airliner, KZA 1907, had been cleared to descend to 15,000ft on the same route. Such a separation of 1,000ft is perfectly normal. PTI said that the radar blips of both aircraft disappeared seven minutes after the Saudi airliner took off. The aircraft crashed about 60 miles west of Delhi.
Flights over most of India are not radar-controlled from the ground, and they use "procedural control" - strict separation by time, distance and altitude.
Delhi airport is one of a number at present undergoing a massive modernisation programme. It now has radar which reaches out to about 50 miles, and which is normally used only to monitor aircraft approaching Delhi, according to Peter Quaintmere, the technical director of the International Federation of Airline Pilots' Associations. Departing aircraft follow a standard path to link up with known airways.
The crash therefore probably occurred outside ground radar control, but why the two aircraft, which should have been widely separated in time, space and distance, flew into each other remains a mystery.
In Britain, by contrast, the air-traffic control tower normally hands over a departing aircraft to the regional air-traffic control centre at about 10 to 12 miles out. As in India, the airport spends more time monitoring inbound aircraft than outbound.
Indian airspace is extremely crowded because aircraft flying from the Far East to Europe are funnelled through a corridor over the country.
European airports are not open at night and therefore aircraft tend to congregate over Asia, where air-traffic control, in one expert's words last night "leaves something to be desired".
INDIAN AIR DISASTER
At about 18.40 local time the two planes collide above Charki Dadri, a town of about 50,000 people, 60 miles west of New Delhi. Debris from the crash was spread over six miles
Kazakh Airways KZA flight 1907, a cargo flight from Shymkent with 39 passengers and crew. It is cleared to descend to 15,000 feet. The Ilyushin 76T, which has a crew of seven and first entered service in 1970, can carry up to 90 passengers
Saudi Airways flight SV 763 takes off from Indira Gandhi International Airport at 18.33 for Dhahran and Jeddah, Saudi Arabia, with 312 passengers and crew. It is cleared to climb to 14,000 feet.