It was a crash heard 'round the country. In September, 1986, a single-engine plane plowed into an AeroMexico jet over Cerritos, Calif. Nearly 100 people perished in the air and on the ground. The Federal Aviation Administration concluded that the jet's crew never saw the small plane, whose pilot had veered out of his approved air space shortly before the collision.
Spurred mainly by this tragedy, Congress told airlines that by 1994 they had to install traffic-alert and collision-avoidance systems (TCAS) in the 4,300 commercial planes operating in the U.S. It has taken six years to hone the technology, and not all the bugs are out yet. "We're 80% of the way there," says Ross Beins, a United Air Lines Inc. manager who chairs an airline industry task force that is evaluating TCAS. But the system, which costs $200,000 installed, has already been put in half of all commercial planes--those that carry at least 30 passengers. And so far, the evidence shows that it may provide a crucial margin of safety as U.S. air traffic grows an estimated 56% by the year 2000.
Interest in a collision-avoidance system really dates back to 1956, when two jetliners crashed into each other over the Grand Canyon, killing 128 passengers. But it wasn't until the mid-1980s that advances in computer chips and software made it possible for the system's three manufacturers--Allied-Signal Inc.'s Bendix/King Div., Honeywell, and Rockwell International's Collins Commercial Avionics subsidiary--to produce sophisticated enough gear in a light enough unit.
What they've come up with, in a 48-pound package the size of a 19-inch TV, is a combination computer/receiver that can pick up the individualized radio signals emitted by the transponders that most planes carry. By locking in on the transponder signal of another aircraft, a TCAS computer can tell the other plane's altitude, speed, range, and bearing. Using software developed by Mitre Corp. in Bedford, Mass., and Massachusetts Institute of Technology's Lincoln Laboratory, the computer compares the planes' projected courses. If they come within three nautical miles and 1,000 vertical feet of each other, TCAS issues warnings that get more urgent as the time until potential impact decreases. It even tells a pilot to climb or dive.
CLOSE CALL. During 2,500 hours of tests in the past 18 months, pilots credit TCAS with helping avert 10 to 15 midair collisions, says Thomas Williamson, a TCAS expert for the FAA. In one case last October, a jet from now-defunct Midway Airlines Inc. had just taken off from Chicago's Midway Airport when TCAS warned that a Southwest Airlines Co. plane was in its path. The crew took evasive action--and even so, the planes missed by only about 1,000 feet at the same altitude. Now, says United's Beins: "You'd have a mass revolt if you tried to pull TCAS off" the planes that have it.
Not that TCAS doesn't have glitches. Its chief critics, air-traffic controllers, want to limit the system's ability to order planes out of an assigned air space without a controller's approval. Each controller tracks 15 to 20 aircraft at a time. And "we rely on knowing where each plane is because we're basing our next instruction on that," says Chicago controller Ray Gibbons, the TCAS expert at the National Air Traffic Controllers Assn. The system's backers, including the FAA, respond that it operates only in an emergency--and that when a collision is imminent, there's no time to radio the tower for instructions.
Still, there's no question that the system is sensitive. In tests on two of its planes, United reports that traffic alarms sounded every 2.2 flight hours, and an alert to dive or climb was issued every 28.1 hours. Complains John E. O'Brien, director of the Air Line Pilots Assn.'s air-safety department: "There are too many nuisance warnings, especially in dense areas."
Some of these shortcomings have been dealt with. For instance, Collins has fixed a software error that caused its system to pick up phantom planes. And all three manufacturers are revising their software to cure another headache: As planes prepare to land, the systems sometimes pick up aircraft on the ground and sound a verbal alert: "Traffic! Traffic!" The new software halts this close to the ground. Faulty transponders also can cause a plane to track itself and trip an alert. But as aging transponders are replaced, this should stop as well.
There is one potentially dangerous problem that seems particularly hard to solve. At some busy airports, including Dallas/Fort Worth and Detroit, airliners must take off or descend at very high speeds in close proximity to each other. TCAS often anticipates a crash and orders evasive action, even though these planes will soon be leveling off safely in their assigned airspace.
COMPROMISE. Some experts say that most of these alerts could be prevented by changing the software so that an alarm sounds when planes are within, say, 700 feet of each other's altitudeinstead of the current 1,000 feet. A jetliner can climb 700 feet in 30 seconds or less. So the idea of such a change makes United's Beins and others nervous. They would rather alter basic traffic patterns. But that would require an overhaul of the air-traffic control system, which is unlikely. As a compromise, the system's manufacturers are trying to develop software that will be able to infer when a plane is about to level off.
Even if that fix works, TCAS won't be omnipotent. No other country has required its airlines to install TCAS or anything like it. And American Airlines Inc. crews have noted 27 incidents where other aircraft, mostly foreign-operated, have put their transponders on settings that TCAS can't pick up. Over Newfoundland last fall, for instance, one American jet was climbing in response to a TCAS order and nearly overtook a third, foreign plane the system had failed to spot. The FAA can't force foreign airlines to alter their procedures outside the U.S., but it has advised them of the problem.
In the meantime, industry experts are eyeing the next-generation TCAS. It will direct pilots to move sideways, as well as up and down, to avoid collisions. That's better, because when banking left or right, "you get the best response from the plane, and there's less impact on passengers," says O'Brien of the pilots' union. That system may be ready by the end of the decade--and further ensure that it will take more than human error to cause two planes to collide.