Air Transport World

TCAS: not-quite-perfect solution. (Traffic Alert and Collision Avoidance System for aircraft) (Technology: Avionics)

To the relief of some but the consternation of others, airborne collision avoidance technology has arrived at last. In 1990, 76 carriers operating in U.S. airspace began outfitting their aircraft with the Traffic Alert and Collision Avoidance System (TCAS). By the end of last year, more than 2,000 planes, both domestic and foreign-registered, were equipped.

During these first months, some dangerous midair encounters were averted dramatically but some spectacular snafus also occurred. TCAS may be operational but it is far from perfected.

TCAS II, the version of the system mandated by FAA, has the capability of issuing "traffic advisories" (Tas), which warn a pilot about the presence of other aircraft in the vicinity, as well as "resolution advisories" (RAs), which direct climb or dive maneuvers to avoid a potential conflict. In one recent incident documented by FAA, TCAS first gave a TA and then two RAs in rapid succession, all before the pilots of the air transport caught sight of the intruder, a Cessna 172 that eventually passed directly below. Reportedly, the small plane was not displayed on the screen of the controller working the scheduled flight.

In july, TCAS warned the crew of a Continental DC-10 about traffic directly ahead in the night sky over the Pacific which turned out to be a Qantas 747.

In December, 1990, an RA issued during final approach to Long Island MacArthur Airport left an airline pilot with this impression, reported to NASA'S Aviation Safety Reporting System: "Had it not been for TCAS, this could have had disastrous results." In its less-stellar moments, TCAS has directed pilots to evade aircraft that posed no threat, as well as aircraft that did not exist. At times, it has been similarly diligent warning about ships, bridges and even the pilots' own aircraft.

Both TAs and RAs are triggered by signals from nearby transponders. if a marine transponder is within range, or one mounted on a bridge, or one in an avionics repair shop, or one left on in a parked aircraft, TCAS doesn't know the difference and dutifully will sound the alarm. FAA is working with the U.S. Navy, among others, to develop means of eradicating these spurious warnings. …

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