Southwest said it expected to cancel 300 flights on Sunday, as the airline pulled Boeing 737-300 aircraft out of service to check the planes for fatigue in the fuselage skin.
The airline anticipates about 175 flight cancellations on Monday.
Sunday's tests revealed "small, subsurface cracks" in two of the planes inspected as of 4 p.m. Central time on Sunday, said the airline.
"Further evaluation and potential repairs will be necessary before those planes are returned to service," said Southwest.
Southwest said it canceled about 300 flights on Saturday, too, one day after a Phoenix-Sacramento flight made an emergency landing at a military base in Yuma, Ariz., 18 minutes after takeoff.
A section of the fuselage burst open, which caused a loss of cabin pressure. One flight attendant sustained a minor injury and no passengers were hurt, said Southwest.
Robert Sumwalt of the National Transportation Safety Board on Sunday said investigators found "pre-existing fatigue along the entire fracture surface" of the Southwest jet with a hole in the fuselage. (Watch press briefing below.)
Southwest on Sunday said it would inspect a total of 79 Boeing 737s, and that the inspections were expected to be completed by late Tuesday. "The 79 aircraft designated for the additional inspections were designed differently in the manufacturing process," explained Southwest.