In visibility of 80m it would be difficult to see the taxi-way... operating would be challenging

David Learmount, Flight International

But the result was almost identical; both planes wrecked; scores of people dead; and questions as to how - with all the modern technology at pilots' disposal - two planes can end up on the same piece of tarmac.

The answer - especially in fog - is frighteningly simple.

If a pilot gets lost while taxi-ing around a complex airport system - as seems to have happened at Milan - the plane can simply wander on to the runway.

Commercial licences

In thick fog - and visibility at Milan was reportedly just 80m - even following the central taxi-ing or runway lights can be difficult.

All pilots using the airport, even at the controls of a small plane, would have to hold commercial licences and be fully instrument-trained. They would also be following charts of the airport's taxi-ing and runway systems.

The SAS pilot had no time to avoid collision

Nevertheless, the possibility of "lost" planes is seen as sufficiently likely that airports have systems in place to guide bewildered pilots back to safety, using the low-technology method of following a rescue vehicle.

Normal pilots' protocol is to admit if you are lost and the vehicles will come and find you. That of course, only happens if you know you are lost in the first place, experts point out.

The risk of pilots going astray is seen as greater at some of the world's most complex airports, says David Learmount of Flight International Magazine.

Chicago, Atlanta and Dallas are among the world's trickiest - vast areas of tarmac, lights and taxi-ing systems.

Milan's Linate is considerably easier - roughly equivalent in complexity to London Heathrow, said Mr Learmount.

If conditions had been better, a sighting by one or both pilots would have been sooner rather than later, and the tragedy might have been avoided

Richard Dawson, Guild of Air Traffic Controllers

In fog, the risk of getting lost is clearly greater - while the chances of any error being spotted disappear.

In the conditions reported at Milan, pilots or air traffic controllers would have had no chance of seeing the impending tragedy.

Everyone was operating "blind".

"If conditions had been better, a sighting by one or both pilot would have been sooner rather than later, and the tragedy might have been avoided," said Richard Dawson, president of the Guild of Air Traffic Controllers.

Lethal mix

To add to the lethal mix of circumstances in Milan, the airport's ground radar system was not working.

First reports said it had been turned off three days earlier for maintenance. But later it emerged that it was a year - possibly even two - since the radar had last been working.

Italian unions, including the pilots' group, said the accident could have been avoided if the system had been on.

But safety experts say the radar systems have their limits, and do not guarantee safety. Indeed, ground radar is not obligatory at airports.

Air traffic controllers operate "blind" in thick fog

Even if mistakes are spotted by air traffic controllers, it has to be in time for them to alert the pilots.

But the absence of radar removes a crucial chance for a pilot's mistake to be spotted.

"It should not be an accident waiting to happen, but the world being what it is, not having the system makes it easier for accidents to happen, especially at a busy airport," one industry source told BBC News Online.

The industry is cautious against rushing to judgment - many past accidents have turned out to have more complex causes than first appeared

If fog is a problem - which it frequently is at Milan Linate - taxi-ing speeds are lowered and extra caution is used, but take-off speed has to be as fast as ever.

The SAS pilot, accelerating towards take-off, appears to have swerved at the last minute as the Cessna suddenly loomed out of the fog. But by then a collision was unavoidable.

The Italians have been quick to blame the German Cessna pilots for the tragedy, insisting they should not have been taxi-ing on the foggy runway.

But the industry is cautious against rushing to judgment.

Many past air accidents have turned out to have more complex causes than first appeared.

In the tragedy at Tenerife, it was the PanAm pilot crossing the runway who turned out to be blameless. Instead, the crash was blamed on the KLM pilot , who had not checked he was cleared for take-off - and sped down the foggy runway to disaster.