The rules that control the flow of traffic are long established. Airspace is divided into control areas and air corridors. In the ;corridors across the Atlantic Ocean between New York and London, for example, aircraft are separated by 2000ft (610m) vertically and 60 nautical miles (110km) horizontally.

Over land,in Britain they are separated by 1000ft (300m) or more vertically and by 5 nautical miles (9km) horizontally.

Controllers must ensure that each aircraft is handed over from one section to the next during its flight – even over the mid-Atlantic.

Before takeoff, each aircraft files a flight plan, which is updated on computer print-outs during the flight. The air traffic controllers monitor the journey from the print-outs. Each plane emits an identification signal that shows up on the airport’s radar.

When an aircraft approaches a busy airport wishing to land, it is directed to a reporting point over a radio beacon, probably several miles away. It is then allotted its own flight path which takes it down to the runway. However, during busy periods more aircraft may wish to land at an airport than there is room for. In some countries they are instructed to form a stack, or holding pattern, by circling at different heights around the reporting point. In others they are not allowed to begin their flight until they have been guaranteed a landing space.

During heavy holiday congestion over Europe in July 1988, there were ten London-bound airliners circling one above the other at the same time over a beacon near Ostend in Belgium.

In the USA, some airports have commercialised the pressure, allowing those airlines that pay more to queue-jump.

In theory, air traffic control is a system of proven reliability. In Britain, for example, there has not been a midair collision between commercial flights in controlled airspace since air traffic control was introduced in the 1930s.

But as demand builds, problems multiply. Present computer systems are antiquated, and air traffic controllers, juggling dozens of flights, work under increasing pressure. A report on the crash of an airliner in a thunderstorm at Dallas/Fort Worth Airport, Texas, in 1985 revealed that air traffic controllers were dealing with one call every four seconds. They described the workload as moderate.



Across Europe

Across Europe, national computer systems are often not compatible with each other.

The chances grow that errors will be made, and not spotted. In April 1988, two flights bound for London – a Cyprus Airways Airbus and a Manx Airlines commuter jet carrying a total of more than 300 people – found themselves on a collision course at 8000ft, north of London. Both pilots took avoiding action and disaster was averted.

The way to preserve safety, and improve it still further, is with computerisation. The US Federal Aviation Administration is planning a revolution in air traffic control at a cost of nearly $20,000 million. The new system will quadruple capacity by using computers that have four times the capacity of the previous ones and are eight times faster. The system will suggest avoidance manoeuvres to the aircraft if it spots two planes heading towards each other. The radar screens will be in colour, and display weather information. Aircraft otherwise out of touch with air traffic control centres will be monitored by satellite. All flight plans and rescheduling will be updated automatically. It will warn of impending congestion.

Other improvements include microwave radio-guidance systems which will enable pilots to land even though they cannot see the runway because of bad weather.

A computer on board airliners will detect other aircraft in the vicinity and give avoidance instructions to the pilot in a synthesised voice. Another computer will cope with `windshear’, the sudden change of wind direction that can bring disaster to an aircraft as it comes in to land.

In this way, the skies may become more crowded. But they will be even safer – for a decade or two, at least.

Controlling the airways

Sitting before their complicated control panels, air traffic controllers at O’Hare Airport, Chicago, keep a painstaking check on the planes landing and taking off in the area – and maintain constant two-way communication with the pilots.