The crash warning system known as TCAS, which was installed on the Cathay Pacific plane involved in a near miss on Sunday, is common on aircraft worldwide.
It is hailed by major airlines as a vital device although it cannot detect planes without the special radar transponders.
The Traffic Alert and Collision and Avoidance System (TCAS) became mandatory for all commercial airliners flying over United States territory after a series of mid-air collisions several years ago, and it is now in every Cathay Pacific, Dragonair and Air Hong Kong plane.
Aircraft travelling in different directions are supposed to fly at different altitudes and air traffic controllers should keep planes a safe distance apart.
TCAS acts as an extra safeguard by monitoring the position, altitude and heading of other aircraft fitted with TCAS transponders, displaying this information on a screen inside the cockpit and issuing alerts where necessary.
Typically, other aircraft are shown as white dots where there is no danger, yellow for a potential threat and red where immediate action is required.
