CHICAGO — United Airlines said Friday it is removing the grounded Boeing 737 Max from its schedule until early September, forcing it to cancel thousands more flights.

United, Southwest and American are bracing for a second straight summer without their Max planes, which have been grounded since last March after two crashes killed 346 people.

The result will be fewer flights than the airlines had planned to offer during the peak summer-travel season. That could result in slightly higher fares.

United had 14 Max jets when they were grounded, but expected to be flying more of them by now. The Chicago-based airline said it will scrap 80 flights a day — about 2,500 a month — over the summer.

By taking the planes out of the schedule now until Sept. 4, United hopes to avoid last-minute cancellations that could leave passengers stranded. United’s decision — its latest delay in returning the Max — came a day after Southwest took the plane out of its schedule through Aug. 10.

Boeing is testing updates to flight-control software that played a role in the crashes by pushing the planes’ noses down based on faulty readings from sensors. Boeing does not expect the Federal Aviation Administration to sign off on the changes until this summer, however, and airlines will need extra time to update planes and retrain pilots on the Max.

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