The crack that tore open the side of a Southwest Airlines (LUV) Boeing 737 on Friday and forced an emergency landing presents inspectors with a new potential malfunction to watch for, an industry analyst said Monday.

“This is a brand new failure mode,” said Robert Mann, president of R.W. Mann & Co. in Port Washington, N.Y.

While what’s known as “fatigue cracking” occurs on all planes beginning with their first trip down the runway, the cause of the hole several feet wide in the fuselage of the Southwest plane will “add to the encyclopedia of what should be inspected,” said Mann.

Southwest said it cancelled an additional 70 flights on Monday as inspectors checked similar models of Boeing 737-300s for metal fatigue. The airline said cracks were discovered on three other planes.

The airline cancelled 600 flights over the weekend, or 9% of its overall schedule.

Meanwhile, the National Transportation Safety Board continued to investigating the cause of the crack.

James M. Higgins, an airline analyst at Soleil Securities, said in a research report, “Southwest’s high utilization of its aircraft and short average flight length means that its planes accumulate more takeoffs and landings than do most airlines’ fleets in a given time frame, thereby placing more stress on Southwest’s equipment.” 

Southwest uses only Boeing (BA) 737s and 171 of them, or one-third of its overall fleet, are Boeing 737-300s. Of those171, a total of 92 previously had renovations to repair and prevent further fatigue.

The 79 aircraft grounded by the airline were models that have not been renovated for structural upgrades.

The aircrafts in question were valued by Southwest in the mid-1990s at about $32 million, or $47 million in today’s dollars. The airline has since begun purchasing the more modern Boeing 737-700 models, buying 63 such aircraft in 1997.

The NTSB said Sunday there were signs of fatigue cracking near the hole which opened in the plane’s fuselage and forced the place into an emergency landing in Yuma, Arizona. Flight 812 was heading from Phoenix to Sacramento when a tear opened 20 minutes after takeoff.

Southwest’s fleet of 737-300s averages 19 years in age as of 2010. But Mann noted that a plane’s age is not measured by years but rather by takeoffs and landings -- known as cycles -- and by air hours.

Mann said the NTSB and Federal Aviation Administration will likely use Friday's incident to determine whether more inspections are needed and what form those inspections should take.

“Managing fatigue cracking is a learned art and science,” he said.  

Shares of Southwest were down 2.1 percent, or 26 cents, at $12.41 in midday trading on Monday, while Boeing (NYSE: BA) was off 15 cents, or .2% percent, at $73.86.

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