United Continental Holdings says it's almost finished inspecting most of the nearly 100 Boeing aircraft it grounded late Tuesday afternoon.
The airline kept them on the ground to inspect computer systems. The voluntary move by Chicago-based United affected 96 Boeing 757s, which make up more than a quarter of United's fleet of 370 aircraft.
Aircraft operated by Continental, which recently merged with United, were not affected by the incident.
The grounding had to do with on-board air data computers, which measure air speed and atmospheric conditions outside the plane, said United spokeswoman Megan McCarthy.
The airline discovered late Tuesday afternoon that some inspections of the computers after a recent modification weren't performed correctly, McCarthy said. But she added that all computers have been fully functional since the federally-mandated upgrade in 2004.
The unscheduled maintenance caused 15 cancellations Tuesday, with minimal delays Wednesday, she said.
A spokeswoman for the Federal Aviation Administration says it's investigating the situation.