It was reported that the poor weather had forced controllers to divert another aircraft attempting to land 30 minutes before the crash.

Triggered by a power failure, on Wednesday morning Pittsburgh's radar system failed - the eighth such occurrence in six months - forcing controllers to divert more than 100 flights.

Then a second computer also briefly failed, which forced controllers to hold Midwest-bound planes on the ground around the country.

The 3083's have broken down intermittently around the country, forcing controllers to spread planes more widely, and causing delays that have rippled through the system.

The hijackers then turned the plane eastward, forcing controllers to direct other planes out of the area to avoid a collision.

Some of the nation's busiest air centers rely on creaking computers and other ancient technologies that are prone to malfunction, forcing controllers to use less efficient backup systems.

And at the Seattle-Tacoma International Airport, the control tower was completely knocked out of commission, forcing controllers to work out of a makeshift center in a trailer, limiting the airport to roughly half its normal flight load.

In September, a USAir Boeing 737 slid off the end of a runway, killing two people and forcing controllers to close one of the two runways for five days, causing additional delays.