How it works: Climb and dive

Well, that, plus a whole lot more.

Unlike the ADS-B In traffic alerts called TIS-B that many general aviation pilots know, use, and love, TCAS goes beyond simple traffic alerts and can give pilots guidance on the best action to take to avoid a collision. Better still, your TCAS coordinates its advice with the TCAS in the other guy’s flight deck, with the two units mutually agreeing on a plan of action. In less time than it takes to press the mic button, the two TCASs “talk” to each other and negotiate what’s called a “mutual avoidance maneuver.” In our airplane we might be advised “climb, climb” while the pilots in the other airplane are advised to “descend, descend.”

How does it work? The system uses secondary surveillance radar transponder signals (Mode C or Mode S) of airplanes in its vicinity to build a 3-D map of the surrounding traffic in an airspace bubble that varies in size depending on the speed, altitude, and heading of the airplane. It tracks its own movement and the movement of other aircraft in the vicinity using what’s called “interrogation-response cycles” that occur several times per second; it then uses this data to predict the future location of all airplanes in the vicinity (including the one it’s installed in) all the while constantly calculating collision risks in the near future.

TCAS works independently of air traffic control; and unlike our ATAS alerts, works independently of ground stations. TCAS is dependable enough that TCAS alerts trump air traffic control instructions.

If it’s so great, why don’t we all have one? With TCAS systems costing anywhere between $25,000 and $150,000 you’re not likely see one in a Cessna trainer near you any time soon.

But back to lives saved, while it’s impossible to calculate how many collisions might have happened without TCAS, we do know how many “resolution advisories” are issued by the systems. One in every 1,000 hours of flight time for medium and short-haul carriers, and one every 3,000 hours on long-haul flights.

That’s a lot of potential collisions avoided.