TCAS / ACAS
Airborne collision avoidance. It interrogates nearby transponders and, if a collision is likely, issues a coordinated climb or descend instruction to both crews.
A traffic advisory alerts; a resolution advisory instructs. The instruction is coordinated between the two aircraft so they are never told to go the same way, and crews are required to follow it immediately — including in preference to a conflicting air traffic control instruction.
That last rule was written in blood: the Überlingen mid-air in 2002 involved one crew following the RA and the other following the controller.
Related terms
Two aircraft collide in flight. Investigations centre on separation: who was responsible for it, what each crew could see, and what the collision-avoidance systems did.
Terrain awareness and warning systems. They compare the aircraft's position against a terrain database and call "terrain, pull up" before impact.