Updated 02/16/11 – 10:55 a.m.

CHICAGO (CBS) – The message, “Your flight is delayed for a safety check,” is not exactly a comforting one to hear as you’re getting on the plane, but that’s exactly what happened for some United passengers.

As CBS 2’s Susanna Song reports, United said Tuesday afternoon that many of its customers saw flight delays or cancellations, while crews perform maintenance on “air data computers” on board its fleet of Boeing 757 aircraft.

A United spokesman told CBS News that as of Wednesday morning, more than 75 percent of the checks on 757s were complete. The airline expects to “operate as normal” today with “minimal to no impact” on the flight schedule.

There were a handful of delays at O’Hare International Airport on Wednesday due to the computer issues, but for the most part, things were going a lot smoother on Wednesday than on Tuesday evening.

“So far so good,” Sarah Bowcott said. “We just found out we’re 20 minutes delayed, I think, but nothing bad.”

Wolcott said she was on her way to Las Vegas to get married. She said she was thankful that United’s planes were being checked and that her flight wasn’t cancelled as a result.

United spokesman Mike Trevino said that the airline was performing “unscheduled maintenance” on all 96 of its Boeing 757s starting Tuesday afternoon, to perform “a modification to the 757 air data computers.”

The computer equipment, which measures everything from speed to air pressure, was replaced following Federal Aviation Administration orders back in 2004.

Trevino said workers discovered the issue Tuesday afternoon and maintenance was being performed on all of its 757s while they are on the ground.

LISTEN: Newsradio 780’s Steve Miller Reports

News of the computer issues had spread quickly on aviation blogs and Twitter, citing some sort of unconfirmed software glitch.

“The reason why it was so sudden is they probably found out there was a malfunctioning system – an initial navigation system, or the autopilot which links to the INS, on one particular 757, they decided to check others,” said CBS News Travel Editor Peter Greenberg, “and when they found at least one other, they realized, ‘Uh oh’ – that was their ‘uh oh’ moment – and they’d better check all the planes.”

It turned out that it was not actually a glitch, but rather just an update that was not performed according to regulations.

The update was performed properly, but not by the book, as per a 2004 Federal Aviation Administration airworthiness directive.

The United spokesman said that during routine quality assurance checks, it “discovered that those checks had not been completed and at that point made the decision to go ahead and complete them”.

LISTEN: Newsradio 780’s Lisa Fielding With Industry Experts Weighing In On Decision

Trevino said the maintenance work takes 60 to 90 minutes for each plane and officials expected it would take 12 to 24 hours to provide the maintenance on all of its 757s.

Travelers at O’Hare International Airport were none too happy about the news.

“I came to the airport a few hours ago,” one man said. “All it said was my flight was delayed.”

Joe Rodriguez also found himself stranded. He spoke to WCBS-TV in New York.

“It makes for a pretty long day,” Rodriguez said. “We’ve been here for about 4 hours, 5 hours, and it’s pretty frustrating.”

While United is the only airline that’s been affected, passengers could feel the ripple effect across the country.

“The problem right now is Chicago; I guarantee the San Francisco maintenance base is working overtime as well tonight, and the guys at Kennedy (International Airport) as well, because every place you’ve got a 757 on a transcon, or making a stop in Denver, you know you’ve got mechanics in your cockpit right now,” Greenberg said.

Greenberg also said other airlines likely have been checking their own 757s in case there’s a similar issue with their computer systems.

“If this is a system used by other airlines – whether it’s American, which operates 757s as well – you’d better believe they’re checking it right now as well,” he said.

Rodriguez said it’s better to be safe than sorry.

“I prefer for them to ground the planes then something happen,” he said.

The 757 is used primarily for domestic flights, Trevino said. Boeing 757s make up less than a third of United’s fleet of about 360 planes. The maintenance issue does not impact any planes owned by Continental Airlines, which is in the midst of a merger with United.

Travel experts say there’s a possibility other airlines use the same air data computer that United does. In that case they could be checking as well.

They say United is lucky to have found this early, because usually it’s discovered after an accident.

United customers are urged to check with their flight status with the airline before leaving for the airport. They can also find out if their flight is scheduled to be on a Boeing 757 by visiting the United website and checking their flight number.

United has been “working to provide additional aircraft where available,” Trevino said.

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