Flying with TCAS II

The Aeromexico DC-9 and the Piper Cherokee Archer that collided in midair over Cerritos, Calif., last August should have been visible to each other for at least a minute before the crash, experts believe. One if not both of the pilots probably saw the other plane coming. That chilling fact confirms what experienced flyers already know: simply spotting an oncoming plane is not enough to avoid it. The pilot must then gauge whether the other craft’s speed and bearing pose a threat. In crowded airspace, the risk of error is high.

For this reason, the FAA and the industry have been working since the late 1950s to develop an onboard electronic system that will automatically alert pilots to the danger of a collision. Piedmont Airlines first tested a prototype in 1981 and 1982, and is currently evaluating a more advanced one. Next month United will also begin testing the device, known as TCAS II (for traffic alert and collision avoidance system); Northwest and Republic will quickly follow. By 1991, says FAA Administrator Donald Engen, all U.S. commercial planes will be required to carry the TCAS II; eventually, foreign aircraft entering U.S. airspace will too. At a price of about $80,000 a plane, the system will cost upwards of $500 million for the entire U.S. commercial fleet.

Designed by the FAA and built by both Allied Bendix and Sperry/Dalmo Victor, TCAS II uses a transponder to interrogate as well as answer another plane’s radar beacon by sending out information on its position. When two planes are on a potential collision course, onboard TCAS computers alert the pilots with flashing lights, voice messages and a radar screen display showing the planes’ relative positions; the computers even indicate up or down evasive action. Following the Cerritos tragedy, the FAA ordered that no aircraft be allowed into the terminal control area above major airports without an altitude-signaling transponder. Although such transponders are now useful only to air traffic controllers, eventually they will be an integral component in the air-to-air warning system.

Still to be resolved are important questions about the system’s reliability. Under FAA rules, TCAS II cannot be certified for all weather conditions unless it has less than a one-in-a-billion chance of failing. There is concern as well that the system is too excitable: in busy skies it could send out false alarms that could lead a pilot into dangerous and unnecessary maneuvers. Both the airline industry and the FAA think such nuisance alarms can be solved by fine-tuning TCAS II antennas.

Although the airlines must pay for installing TCAS II, they support Engen’s decision to proceed. Many pilots, however, would prefer to wait for more advanced technology. TCAS II can tell a plane to go up or down to get out of danger, but not whether to swerve left or right, the escape maneuver considered safer by pilots. That will come in the FAA’s TCAS III system, which is at least two years off, and perhaps as many as five. But, says Engen, “wouldn’t you rather go down or up, and miss, rather than sit around for two more years to go left and right? I’ve told those guys, let’s get a TCAS II in operation, and let’s get on with it.”