DALLAS (AP/CBSDFW.COM) – Southwest Airlines canceled 70 flights nationwide Monday as it continues to inspect a number of older Boeing 737 aircrafts. None of the airline’s flights from Dallas Love Field were canceled, however, because many of the inspections are being done at Southwest’s headquarters, based at the Dallas airport.

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The airline canceled about 600 flights and grounded 79 planes over the weekend after a Boeing 737-300 jet sprang a 5-foot hole in the roof shortly after takeoff from Phoenix on Friday. The plane made an emergency landing. A Southwest Airlines spokesperson said that no one was seriously injured during this incident.

Southwest had inspected 67 similar planes as of Monday night and returned them to service. The remainder of the planes had their inspections completed Tuesday morning. As of Monday night, cracks had been discovered in three of the planes, all similar to that which took off from Phoenix on Friday. CBS 11 News learned that two more planes were found to have cracks in their fuselages early Tuesday morning, bringing the total to five planes.

At least the first three planes were discovered with cracks near lap joints, on a part of the roof towards the front of the aircraft. Lap joints are where two pieces of metal are joined together. This is the same place where the Phoenix jet had a hole tear open during flight on Friday. The airline said that the three planes will remain out of service until Boeing makes repair recommendations.

At a Monday afternoon briefing, investigators with the National Transportation Safety Board said that Southwest Airlines had no outstanding maintenance issues, and that it was in compliance with all mandatory inspections and airworthiness directives of its older model Boeing 737s. “It was not believed that this was an area that could fail until we see it now,” said NTSB spokesman Robert Sumwalt.

The New York Times also reported that Boeing is telling all airlines to inspect older 737s for similar fatigue cracks. CBS 11 News tried to call and speak with someone from Boeing, but those calls have not been returned.

The Federal Aviation Administration announced Monday that it will mandate inspections for lap joints on all early model Boeing 737s. The agency said that there are 175 early model 737s still in use worldwide, with 80 of those being in the United States. They said that most of those planes are operated by Southwest Airlines.

Boeing produced more than 1,100 of the 737-300 jets between 1984 and 1999.

In the release, the FAA said:

“The FAA airworthiness directive will require initial inspections using electromagnetic technology in specific areas of the aircraft fuselage on certain Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 flight cycles. It will then require repetitive inspections at regular intervals.

Last November, the FAA published a rule designed specifically to address widespread fatigue damage in aging aircraft. The rule requires aircraft manufacturers to establish a number of flight cycles or hours a plane can operate and be free from fatigue damage. The rule requires aircraft manufacturers to incorporate the limits into their maintenance programs.”

A Southwest Airlines spokesperson said that the airline does not operate any 737-500 planes.

The FAA requires airlines to regularly inspect planes for cracks, which may result from metal fatigue caused by pressurizing the cabin while climbing to cruising altitude and decompressing as they drop lower before landing.

Sources 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.

airplane hole jay11 Scrutiny Increasing Over Southwest Plane Inspections

(credit: Shawna Malvini Redden)

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.”

Southwest normally has about 3,400 flights each day. The Southwest Airlines jet from Phoenix was 15 years old and had made nearly 40,000 takeoff and landing cycles.

“As long as the aircraft is being maintained properly, you can get the lifespan up to 75,000 to 80,000 cycles. But you have to be inspecting these aircraft very carefully,” said Mark Rosenker, a former member of the NTSB.

According to aviation experts,  the more takeoffs and landings a plane has, the more potential there is for the metal joints to become fatigued, and susceptible to cracking. Mary Schiavo is a former Department of Transportation Inspector General. “Every time you take off and pressurize your plane, it’s just like blowing up a balloon,” she said, “and you stress that skin.”

Southwest Airlines said, now that it has completed inspecting all 79 of the older 737 jets, that the airline should comply with the FAA’s just announced directive.

(©2011 CBS Local Media, a division of CBS Radio Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)

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