How TCAS collision prevention system works

HONOLULU (HawaiiNewsNow) -- The traffic collision alert that helped avoid a crash between two jets approaching Honolulu International Airport in January triggered the highest, most urgent warning level that required immediate action by the pilots of both aircraft.

The Traffic Collision Avoidance System or Traffic Alert and Collision Avoidance System, known as TCAS, was designed to reduce the incidence of midair collisions between aircraft.  Operating independently of air-traffic control, TCAS monitors the airspace around a plane for other aircraft with corresponding active transponders.  The system warns pilots of other aircrafts with TCAS transponders that might present the threat of a midair crash.

Both the Japan Airlines and United Parcel Service planes in the Jan. 14 incident near Honolulu have TCAS II technology onboard, which is required internationally in aircraft with more than 30 seats or weighing more than 33,000 pounds.

The JAL and UPS pilots received what's called a "Resolution Advisory" from their TCAS systems, the highest alert level.  Audible and visual alerts went off in both cockpits, with the TCAS system telling the JAL pilots to "descend, descend, descend," while the UPS pilots were instructed to "climb, climb, climb."

When pilots receive a TCAS alert of that urgent nature, they must disengage autopilot immediately and manually fly their plane, disregarding instructions they have received from air-traffic controllers.  Then the pilots must quickly notify their controller they received an alert and are changing course.

"TCAS resolution advisories are rare and they're designed to protect the airplanes and the public.  And they work quite well," said Bob Gould of Kaneohe, who was a pilot for Northwest Airlines for 33 years and retired in 1998 as the airline's senior captain in Honolulu.

TCAS systems also feature a less serious alert called a "Traffic Advisory."  Such alerts also set off aural and visual advisories in the cockpit, warning the pilots of "traffic, traffic."  While a traffic advisory is not the highest alert level, its purpose is to call attention to a possible conflict of another plane nearby.

The 1986 fatal collision of an Aeromexico jet and a single-engine plane in Cerritos, Calif. spurred Congress and other regulatory bodies to require collision avoidance equipment on some aircraft.  The Aeromexico DC9 collided with a Piper Archer plane that had crossed into controlled airspace without authorization.  All 64 people aboard the Mexican jetliner were killed in the crash along with the three people onboard the Piper and another 15 people on the ground.

Smaller aircraft are not outfitted with TCAS systems mainly because of the high costs, anywhere from $25,000 to $150,000.  Many smaller business jets, for example, are not required to have TCAS systems installed, even though they fly in the same airspace as larger aircraft that must have TCAS systems onboard.

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