The Last Line of Defense: How TCAS Keeps Airliners Apart

For most of aviation history, the only thing keeping two aircraft from occupying the same point in space was a controller on the ground and a pilot's own eyes. A string of catastrophic midair collisions through the twentieth century made clear that this was not enough, especially as skies grew crowded. The response was an onboard system that does not depend on either ATC or visual contact: the Traffic Collision Avoidance System, TCAS. TCAS works by interrogating the transponders of nearby aircraft, building a real-time picture of surrounding traffic independent of ground radar. When two equipped aircraft close on each other, their TCAS units do something remarkable — they communicate directly and coordinate. One is told to climb, the other to descend, so the two resolutions never conflict. The system first issues a traffic advisory to alert the crew, and if the threat develops, a resolution advisory: a direct command such as "CLIMB, CLIMB" that the crew is trained to follow immediately, even if it contradicts an air traffic control instruction. That last point was written in blood. In 2002 two aircraft collided over Überlingen, Germany after one crew followed a controller's instruction while their TCAS commanded the opposite; the resulting investigation reinforced worldwide that the TCAS resolution advisory takes precedence. Modern TCAS II is mandated on airliners across the world and is one of aviation's most effective safety nets — a quiet, automated negotiation between two machines in the few seconds when human reaction time alone would not be enough. It is a last line of defense, not a substitute for separation, and the pilot in command remains the final authority per 14 CFR §91.3.