United Air Lines" href="http://www.chicagotribune.com/topic/economy-business-finance/transportation-industry/air-transportation/united-air-lines-ORCRP017350.topic">United Airlines has grounded its fleet of 96 Boeing Co." href="http://www.chicagotribune.com/topic/economy-business-finance/manufacturing-engineering/aerospace-manufacturing/boeing-co.-ORCRP017215.topic">Boeing 757s after determining that the planes' air data computer software did not comply with a Federal Aviation Administration" href="http://www.chicagotribune.com/topic/politics/regulatory-policy-organizations/federal-aviation-administration-ORGOV00000232.topic">Federal Aviation Administration airworthiness directive.
United began testing its 757s late Tuesday afternoon and may delay or cancel flights until the process is completed over the next 12 to 24 hours, said United spokeswoman Megan McCarthy. The software checks take approximately 60 minutes to 90 minutes per plane.
"We apologize for any inconvenience and ask customers to check their flights status on United.com before going to the airport," McCarthy said. As of 6 p.m., central time, United had only cancelled 7 flights on its Tuesday schedule, according to FlightStats.com.
United is scrambling to comply with a 2004 FAA airworthiness directive that spelled out software and hardware changes for air data computer systems in Boeing 757, 767 and 747 aircraft. "This action is necessary to ensure that the flightcrew is able to silence an erroneous overspeed or stall aural warning," the directive stated.
During a routine maintenance check Tuesday, United discovered that it hadn't followed all of the steps mandated by the FAA to address safety concerns with the 757 flight computers that measure air speed and monitor atmospheric conditions.
Chicago-based United acted voluntarily, and not at the behest of federal regulators, said FAA spokeswoman Elizabeth Isham Cory.
The action does not affect Boeing 757 aircraft flown by Continental Airlines" href="http://www.chicagotribune.com/topic/economy-business-finance/continental-airlines-ORCRP003883.topic">Continental Airlines, which merged with United in October. The two carriers continue to operate separately and won't combine fleets, flight crews and maintenance stations until they gain a single operating certificate from the FAA.
United's action is not related to electrical shorts that have caused some 757 windshields to crack, said FAA spokeswoman Laura J. Brown" href="http://www.chicagotribune.com/topic/politics/regulatory-policy-organizations/laura-j.-brown-PEPLT000007598.topic">Laura Brown, referring to another safety issue that has made headlines over the last year.