TCAS and its limitations

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Traffic alert and collision avoidance systems (TCAS) are supposed to be the pilot's final line of defence against air traffic control (ATC) errors, but this year two Japan Air Lines widebodies, the pilots in visual contact with each other, almost collided following controller error. The Boeing 747 crew decided to ignore their TCAS instruction to climb and continued an established descent, while the DC-10 pilot followed their TCAS advisory and also descended.

Meanwhile the UK Civil Aviation Authority (CAA) has just given its final responses to a report about an October 2000 near-miss in oceanic reduced vertical separation minima (RVSM) airspace (Flight International, 19-25 June 2001). An Airbus A340 was cruising 1,000ft (300m) below an A330 when the A340 hit sudden clear air turbulence which, almost simultaneously, tripped out its autopilot and triggered the aircraft's envelope protection system. The A340 climbed so rapidly that the A330 crew did not have time to respond to their TCAS climb advisory, and the A340 shot up past the other aircraft. The CAA has recommended that, since satellite navigation's accuracy has reduced the natural lateral scatter along oceanic tracks, the international agencies should reconsider their requirement for pilots to tell ATC before offsetting laterally while overtaking aircraft above or below them.