With those rules, it is no wonder European skies are said to be congested. However, technology is already available that would allow more aircraft to fly in the same space without increasing the risk of accidents.
Using GPS (global positioning system), the accuracy of an aircraft's position can be pinned down to a few metres. Simple 3D geometry is used to work out how far away the aircraft is from four different satellites orbiting the earth. The system was used by the US in the Gulf war to guide 'smart bombs', and Japanese electronics companies use it in gadgets that sell for pounds 640 and guide cars around the labyrinthine streets of Tokyo.
If a single integrated computer system was added to keep track of aircraft positions, we would have the aerial equivalent of a modern city's road network. The basis of air-traffic control would change - and the controllers on the ground would be left out of the picture.
This will not happen for a while. First, the controllers are unlikely to carry on operating the present system submissively while they are being gradually computerised out of their jobs. (They earn 'more than a lawyer, but less than a dentist', according to one of the coy specialists at Eurocontrol.)
Second, satellites are risky. At present, their use could change at the whim of the US Department of Defense. Nobody knows under what circumstances the Pentagon might pull the plug, nor whether or how the system could be jammed by sabotage. The Americans are said to have carried out tests in Siberia with the Russians to prove that their rival satellite systems can work together - but that is little comfort.
Then there is the software problem. Millions of telephones in the United States have been put out of action for hours on end through undiscovered mistakes in computer programs. With the complexity of system necessary, the same might happen with air-traffic control. But instead of not being able to
call home, you could be stuck in the clouds 35,000ft above Geneva.
'I have two souls,' Wolfgang Philip, of Eurocontrol, says. 'As an engineer, I say it's possible. Psychologically, I know people wouldn't like it.' But Mr Philip thinks the new system will overcome its obstacles and probably be in operation by 2020.Reuse content