In February 1984 the Federal Aviation Administration introduced a new computer device at its Air Route Traffic Control Centers that enraged many controllers:

It sounded an alarm every time two planes got within five miles of each other.

Before the device appeared, the only people who could tell if planes came too close were pilots and air traffic controllers.

Quickly dubbed the "snitch machine" by controllers who learned that a computer sent a message to their supervisors if planes violated the FAA traffic separation rules, the computer has been a source of dispute ever since.

"In the past people have accused us of hiding our errors," said Charles R. Reavis, air traffic manager at the Washington Air Route Center in Leesburg.

"Now it is very clear when there has been any violation. The problem was that we never gave the controllers a tool to determine where exactly the five-mile limit was, and that upset many of them."

So in June the FAA introduced another device that imposes a circle that appears on controllers' radar screens to let them know just how far five miles is. Two objects showing inside the same circle, called a halo, is one too many.

Officials say the halos have caused the number of errors to drop significantly.

Last month, one of the busiest in the Washington center's history, there were four occasions when planes flew too close to each other. In September 1984, the figure was 27.

"The controllers still look at it as Big Brother," said Reavis, "but at least now they have a chance."