For decades, airlines, regulators, lawmakers and others have tried to make flying safer, creating layers of protection intended to prevent tragedies. But that apparatus failed on Wednesday, when a passenger plane crashed into a military helicopter over Washingtontmt play, killing 67 people.

Investigators have not yet identified a cause of the crash, but here’s what you should know about two of the technological systems in place to make passenger plane pilots aware of nearby aircraft.

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Ms. Harris was an assistant district attorney in the city. Ms. Guilfoyle was in discussions to join the office. Ms. Harris was calling, according to Ms. Guilfoyle, to suggest there was no job for her there.

The Traffic Collision Avoidance System

An important protection against a midair crash is technology known as a traffic collision avoidance system, or TCAS. The system, which is required on passenger airlines, alerts pilots when two aircraft come dangerously close to each other. At short enough distances, the system will alert pilots to a possible crash by instructing them to redirect their aircraft,ff777 casino either by climbing or descending.

TCAS has been in use for decades and experts say it has significantly reduced collisions. “I’m alive today because of TCAS,” said John Cox, a former airline pilot and founder of Safety Operating Systems, an aviation consulting firm.

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.

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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.

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