Computer Glitch Grounds United’s 757 Fleet

United Airlines said it is in the process of temporarily grounding its 96 Boeing 757 jetliners Tuesday and into Wednesday so it can perform checks on earlier modifications it did to the air-data computers on those planes.

The unit of United Continental Holdings Inc. discovered it hadn't properly complied with steps required by the Federal Aviation Administration in checking the work on those computers, which take air speed, air pressure and other parameters in flight and input them into the auto-pilot and other critical cockpit functions.

As a result of not being in compliance with all aspects of an FAA airworthiness directive, United began canceling 757 flights today, an airline spokeswoman said. The 757 fleet amounts to more than a quarter of all the mainline jets United flies.

The checks, which take 50 to 90 minutes, are being performed on airplanes already on the ground and will continue when planes in the air land at their destinations, she said. No planes are being diverted and the air-data computers "are fully functional," she said.

United uses the 172-seat 757s on domestic routes. Its fleet is about 18 years old on average. The company said it is in the process of informing customers traveling Tuesday and Wednesday on 757s that they may experience delays or cancellations due to unscheduled maintenance. If United had continued to fly the aircraft knowing it was out of compliance with an airworthiness directive, it would have been liable for FAA fines of $25,000 per flight.

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