Airline spokeswoman Megan McCarthy said 96 aircraft were being kept on the ground because the carrier determined it had not completed operational checks after updating air data computers following a 2004 federal directive. She said all of the computers are fully functional.
The issue could affect United's schedule into Wednesday. Travelers are urged to check their flight's status before heading to the airport.
McCarthy said the airline discovered the oversight Tuesday during routine quality assurance checks. An FAA spokeswoman, Laura Brown, said United's move was voluntary.
The checks were taking about an hour and seemed to be going smoothly, a United spokesman told NBC Chicago, adding that the checks would continue through the overnight hours, .
It only affected United's fleet of Boeing 757s. McCarthy said Continental's fleet of 62 757s weren't grounded.
The two airlines combined last year to create the world's largest airline. They will continue to operate separately under the parent holding company until they receive a single operating certificate from the Federal Aviation Administration, which they expect to receive by the end of 2011.
The airworthiness directive required a modification of the air data computer system on certain Boeing aircraft.
It involved installing new circuit breakers, relays, and related components, and making various wiring changes in and between the flight deck and main equipment center.
According to the published directive, the actions were necessary to ensure that the flight crew is able to silence an erroneous overspeed or stall warning. A persistent erroneous warning could confuse and distract the flight crew and lead to an increase in the flight crew's workload. Such a situation could lead the flight crew to act on hazardously misleading information, which could result in loss of control of the airplane, according to the directive.
The directive was effective June 22, 2004.