Aeronautics: Avoiding Collisions

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Flying high over Baltimore last week, two Martin 404 airliners hurtled toward each other on what seemed like a sure collision course. Seconds before disaster, the planes suddenly veered apart. One swept upward; the other dipped slightly. So coolly had the maneuver been executed that it seemed as if an invisible electronic hand had guided the controls.

Both pilots, in fact, did have a helping hand. The "near miss" was the first public demonstration of a promising new collision-avoidance system (CAS) that may reduce some of the risks of flying in the nation's increasingly crowded skies. Last year the U.S. had 38 aerial collisions, a 46% rise over 1967. In the years ahead, the risks will increase, as more planes—including jumbo jets and SSTs (see BUSINESS)—join the rapidly growing U.S. air fleet.

Successive Alarms. The new system was sponsored by the Air Transport Association as insurance against airborne chaos. Like the planes used in Baltimore last week, each aircraft equipped with CAS is, in effect, shielded by a huge protective electronic bubble. When one plane's bubble brushes another's, it triggers successive alarms in both cockpits. The first comes 42 seconds before the moment of collision as calculated by the CAS's onboard computers. If the planes are still headed toward each other at 30 seconds, a flashing red light warns the pilots to prepare for evasive action. Five seconds later, the computers issue their final command. Depending on the relative positions of their planes, one pilot may be ordered to climb, the other to dive or stay level.

The secret of the system is timing. To form the electronic bubbles, each participating plane must send brief radio pulses—none longer than a tiny fraction of a second—in an assigned order of rotation at exact three-second intervals. The system demands such accuracy that all the planes must carry atomic clocks, which are precisely synchronized to a master timepiece on the ground or aboard one of the planes. Theoretically, CAS is so fast and efficient that it can safely handle as many as 2,000 planes over an area of more than 61,000sq. mi.

The ATA, as representative of the nation's airlines, would like to install the system (estimated cost: $24,000 to $50,000 per unit) aboard all commercial aircraft by 1974. But there is one serious drawback. Unless CAS is also carried by private planes, it will not prevent such collisions as the one between a big passenger jet and a small private plane near Indianapolis last month that killed 83 people. Many aviation men feel that the only long-range protection against more aerial tragedies lies in an all-encompassing, new air-traffic control system that would keep tabs on every plane in U.S. skies.