DALLAS (AP/CBSDFW.COM) – Southwest Airlines canceled about 600 flights this weekend as the airline continues its inspection of a number of its Boeing 737 aircrafts, in the aftermath of a hole tearing open on one of its planes in mid-air on Friday afternoon.

The Dallas-based airline said Sunday that 19 of the 79 planes pulled following Friday’s incident are already back in the air. The airline said that two of the planes which were screened were found to have small subsurface cracks on them, and that their inspection should be completed by late Tuesday.

Sources later told CBS 11 News that a third plane has now also been pulled from service after small cracks were found.

Southwest Airlines found cracks on the roof of the first two 737s after conducting emergency inspections of all older 737 models. All three planes were discovered with cracks near the part of the roof towards the front of the aircraft, the same place where the jet had a hole tear open during flight on Friday.

Sources also told CBS 11 News that a part the roof, toward the front of the planes and just above the windows, was not covered by the recent airworthiness directive that required inspections of the top part of the roof near the tail section. That is where a hole tore open during flight on another Southwest Airlines Boeing 737 in July 2009.

Because of this latest incident, sources told CBS 11 News that the Federal Aviation Administration will issue an airworthiness directive in the next couple of days to inspect the skin on the roofs of older model 737s, toward the front of the plane just above the windows.

Aviation expert Denny Kelly said that the cracks are not a surprise, but they are something that always need to be dealt with. “All airplanes have cracks. They may minuscule and have no effect whatsoever but,” Kelly said, “eventually, if you don’t check them, they get bigger and bigger and bigger.”

Kelly and the airline both said that a federal investigation will try to determine what caused Friday’s crack. The airline operates 548 Boeing 737 airplanes, and said in a press release that what they saw on Friday’s flight was “a new and unknown issue.”

On Sunday, federal investigators examining the damaged plane in Yuma said that the entire length of a five-foot long tear in the skin of the aircraft shows evidence of pre-existing fatigue cracking.

The airline canceled 300 flights on Sunday after canceling the same number on Saturday. Southwest Airlines spokeswoman Whitney Eichinger says the airline still has not made a decision about Monday’s flights, but told CBS 11 News that the cancellations will not be as large as they were over the weekend. “We are working as diligently as possible to minimize any impact on (customers’) travel plans,” Eichinger told The Associated Press on Sunday.

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At Dallas Love Field on Sunday, most travelers said that they were expecting chaos following the cancellations, but found the airport running smoothly. “Everything’s kind of calm and nobody seems to be waiting,” said passenger Sarah Bartfield.

Southwest normally has about 3,400 flights each day, though it is slightly reduced on Saturday. That means that almost nine percent of the total number of flights were canceled each of the two days. No flights were canceled on Friday.

No one was seriously injured on Friday, as the Southwest Airlines aircraft carrying 118 people rapidly lost cabin pressure and made a harrowing but controlled descent from 34,500 feet, landing safely near Yuma, Arizona, 150 miles southwest of Phoenix. But passengers recalled the tense minutes after a hole ruptured overhead with a blast, and they fumbled frantically for oxygen masks as the plane descended.

In a press release issued Saturday, Southwest Airlines said that the passengers on board Flight 812 have received a full refund along with an apology and two complimentary round-trip passes on Southwest Airlines for future flights.

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