"Descend, descend, descend"

That cryptic message was uttered from their onboard TCAS collision avoidance system shortly after the British Airways (Speedbird) 777 left Tampa for London. The warning triggered a 600 foot freefall, leaving four cabin crew members injured and most of the passengers in need of a change of underwear.

TCAS (traffic alert and collision avoidance system) has been available on commercial jets for several decades. It provides a proximity radar sweep of each equipped airplane's immediate airspace to determine other traffic and to warn of possible collisions. In critical moments airline captains are instructed to abide by TCAS rather than air traffic controller instructions.

The most famous TCAS-related crash occurred in Europe several years ago. In that case one of the pilots heeded TCAS while the other listened to ATC. In a macbre twist the father of one of the victims later hunted down the air controller and killed him.

For an interesting fictional read on this stuff try Paul McElroy's "TRACON". It's one of the more realistic books on ATC and focuses on near-misses. Not a good flight time read, though.