FAA orders warning system check at U.S. airports

WASHINGTON (CNN) -- The Federal Aviation Administration on Monday ordered the testing of low-altitude warning systems at the nation's airports following last week's Korean Air Flight 801 crash that killed 226 people.

The administration said it was directing that the systems, known as Minimum Safe Altitude Warning systems (MSAWs), be tested within the next two days. There are some 200 such systems in use in the United States and at U.S.-controlled airports like Guam International Airport, where the crash occurred.

The FAA ordered the action after investigators discovered that an error was introduced into the warning system software on Guam. The error eliminated warnings that could have helped prevent the crash of Flight 801.

How the warning system works

MSAWs work by sending air traffic controllers electronic signals on a radar screen indicating the altitude of aircraft.

If a plane gets too close to the ground, the system alerts the controllers, who can then radio the pilots to climb to a safe distance.

But because of the computer error, the system at Guam worked only in a narrow band between 54 and 55 miles from the airport, well out to sea.

As a result, air traffic controllers had no way of knowing that Flight 801, approaching the airport, had already slipped below a safe altitude and was about to hit a hillside.

Warning systems checked monthly

Officials say the error in the Guam airport's altitude warning system is being repaired. Now they are trying to determine when and how it got there. Sources have told CNN that the problem may have been in the Guam system when it was installed 2 1/2 years ago.

The warning systems are normally checked on a monthly basis, officials said. They were at a loss to explain how the problem with the Guam system escaped detection until after the Korean Air crash.

"Whether it was operable or not doesn't necessarily dictate the outcome of this particular accident," said Greg Feith of the National Transportation Safety Board. "Yes, it would have helped, but this was not, as we know it right now, the cause of this accident."

Officials also hope to learn whether there were similar errors in radar software at other airports. They say they believe this is an isolated incident. But they are checking the system, which is in use at practically all major airports, to be sure.

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