CHICAGO (CBS) — After inspecting 79 older planes, officials at Southwest Airlines have found five of them that have cracks in the aluminum skin, less than a week after a hole ripped open in the roof a Southwest jet, forcing an emergency landing.

As CBS 2’s Jim Williams reports, there were fewer cancellations at Chicago’s Midway International Airport on Tuesday as Southwest continued ongoing inspections of older Boeing 737s.

Only about four flights were cancelled at Midway for all airlines on Tuesday. The airport does not break down cancellations by airline.

Even though operations were returning to normal for Southwest on Tuesday, federal investigators have a lot of work to do now that they have a key part of the plane that cracked open last week.

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The fuselage skin from that Southwest Airlines plane was on display at the National Transportation Safety Board’s headquarters in Washington.

Even as investigators look at the piece up close, they’ve been at a loss to explain why the fuselage cracked open right after taking off in Phoenix last Friday.

“This situation here is what is a surprise to us,” NTSB chair Deborah Hersman said. “We do not expect aircraft in service to rapidly decompress in flight, and to have situation where the airplane fuselage is ripping open.”

That plane is 15 years old and has logged 39,000 takeoffs and landings. It might seem like a lot of wear and tear, but Hersman insisted its age and use alone should not have made it vulnerable to a crack.

“The safety board is not focused on the age or the cycles of the aircraft,” Hersman said. “We’re focused on the safety, and if we think that something needs to be done, whether an aircraft is 15 years old or 50 years old, we will address it.”

Southwest inspectors have finished examining the older jets and found cracks in five of the airline’s 737-300s. They were using an electro-magnetic device instead of a mere visual inspection to check for cracks.

“You have planes flying that, in fact, have been checked, but may have these cracks that caused the problem like we saw on the Southwest jet,” former FAA chief of staff Michael Goldfarb said.

The cracks found in the other Southwest planes were described as minor, but those jets were still being grounded for repairs.

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