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FOR most pilots of commercial airliners the first sign of an impending mid-air collision comes when a synthesised voice in the cockpit calls out: 鈥淭raffic, Traffic鈥�.

This voice comes from a complex piece of equipment called a traffic alert and collision avoidance system (TCAS). Compulsory on European and American commercial airliners, TCAS is designed to prevent a mid-air crash even if a pilot or an air traffic controller has made an error.

The system is based around the transponders fitted into every airliner that air traffic control uses to determine an aircraft鈥檚 identity, height, course and speed. On most incident-free journeys, pilots barely need to give their TCAS monitors a second glance.

But if two planes are heading towards each other for some reason, the system sounds its first warning, highlighted in amber on the TCAS screen, when they are 40 seconds away from potential collision.

Then, if neither changes course, the TCAS system will tell one pilot to climb and the other to descend when they are 25 seconds apart.

It can do this because the commands issued by TCAS are geared to take account of any small differences in height between the two planes. So the higher plane will be told to climb and the lower to dive. Pilots are trained to follow the TCAS command first and inform air traffic control later. 鈥淥nce TCAS has taken over, our standing orders are not to interfere,鈥� says one British air traffic controller, who did not want to be named.