CHICAGO (AP) — United Airlines canceled about 18 flights this week when it realized that some of its Boeing 777-200 planes had not undergone required inspections of the front edges of the wings.

The airline canceled flights Monday night and Tuesday morning, and said it’s trying to rebook stranded passengers.

The Federal Aviation Administration said United grounded 25 of the jets after discovering it had failed to inspect slats on the wing edges that are used during takeoffs and landings.

The FAA said United reported the issue, and the FAA is reviewing circumstances that led to the missed inspections.

United said late Tuesday afternoon that it had inspected 10 of the planes and was working with the FAA to return the others to flying in the next two weeks without causing additional flight cancellations.

The matter was first reported by The Wall Street Journal.

United’s Boeing 777-200s had been grounded until earlier this year by the failure of a Pratt & Whitney engine on one that caused parts of the housing to fall on the Denver area.

The planes seat between 276 and 364 passengers, according to United’s website, and are mostly used on long international flights.

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