Years ago, Mr. Cox said, he was descending in a Boeing 737 over Orlando International Airport when the system alerted him and his co-pilot to traffic nearby. The pilots looked around for another aircraft, but saw nothing. Then, TCAS provided an urgent instruction: “Climb.” Mr. Cox said they did so and later saw a smaller aircraft beneath them. A collision would have been catastrophic, he said.
After a series of collisions, culminating in the 1986 crash of a passenger jet and smaller aircraft near Cerritos, Calif., that killed 82 people, Congress instructed the Federal Aviation Administration to require TCAS on all large commercial airplanes. It is optional on military aircraft, though, and it’s not clear if the Army helicopter involved in the crash on Wednesday was equipped with it.
That crash occurred at a relatively low altitude, though, where TCAS’s most critical collision avoidance instructions would have been limited by design, Mr. Cox and other safety experts said. As jets approach an airport, they are likely to be closer to each other than at higher altitudes. As a result, the system limits the alerts it issues to avoid causing confusion and hindering safety.
The Automatic Dependent Surveillance-Broadcast
In recent years, the F.A.A. has required all passenger airlines and many other aircraft to use another system, called the Automatic Dependent Surveillance-Broadcast. The technology broadcasts the location, altitude, speed and other data of aircraft it is installed on every second. Those broadcasts can be picked up by other aircraft or equipment on the ground.
Air traffic controllers and many passenger plane pilots rely on the information received from ADS-B systems using devices that can provide detailed maps of aircraft in an area. That information is useful in visualizing what is happening in the air, but the system typically does not issue the kind of alerts that TCAS does. Still, ADS-B is helpful to pilots, especially when they are flying in poor weather conditions or at times when visibility is low, said Dr. Hassan Shahidi, the president of the nonprofit Flight Safety Foundation.