A service bulletin will recommend checks of lap joints on “certain” 737-300s, 737-400s and 737-500s, Marc Birtel, a Boeing spokesman, said in an e-mailed statement today. Cracks were discovered on three Southwest 737-300s during inspections for metal fatigue following the April 1 incident over Arizona.
Southwest canceled 70 flights today, following 600 during the weekend, as it worked to complete inspections on 79 737-300s. Airlines must make regular checks of planes for metal fatigue, which can occur as jets endure the stress of takeoffs, landings and low outside air pressure of high-altitude flight.
“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,” James M. Higgins, an analyst for New York-based Soleil Securities, said in a report today. Higgins has a “buy” rating on Dallas-based Southwest.
National Transportation Safety Board investigators probably are looking at whether inspections are required often enough and whether the repetitive inspection process “actually contributes to accelerating the propagation of cracks,” said Bob Mann, president of consultant R.W. Mann & Co. in Port Washington, New York.
As of today, 60 planes had been inspected, and 57 returned to service, a Southwest spokeswoman, Linda Rutherford, said today.
Southwest fell 24 cents, or 1.9 percent, to $12.43 at 3:40 p.m. in New York Stock Exchange composite trading. Chicago-based Boeing fell 12 cents to $73.89.
The NTSB said yesterday that the jet showed signs of fatigue cracking near the hole in the hull after it was inspected following an emergency landing in Yuma, Arizona. Flight 812 passengers described the hole as being 1 foot (0.3 meters) wide by 3 feet long, Rutherford said. Southwest is the world’s biggest operator of 737s.
The cracks were “no longer than a quarter of an inch,” and difficult to see without magnification, Robert Sumwalt, an NTSB member, said in a briefing late yesterday.
“Cracking is wear and tear on an airplane,” he said. “From the first time an airplane flies, we see wear and tear on it. That is the reason our fleet of airplanes has redundancy built into it and that’s why we have maintenance and inspection requirements.”
NTSB investigators in Washington will study a section of the ruptured skin from Flight 812 that was removed from the plane yesterday, the board said yesterday in a statement.
Once Boeing’s service bulletin is released, the Federal Aviation Administration will decide “whether to make it mandatory for all similar 737 airplanes,” the NTSB said.
The Flight 812 aircraft will be 15 years old in June; its fuselage skin had been inspected on March 29 and Feb. 5, said Rutherford, the airline spokeswoman. It had flown 39,781 cycles. Each cycle is one takeoff and one landing.
Metal fatigue was blamed for an 18-by-12 inch rip in a Southwest 737 in July 2009 that was flying at 35,000 feet, an incident that also forced an emergency landing. In January 2010, the FAA ordered fuselage checks for metal fatigue on 135 737-300s, -400s and -500s in the U.S., after Boeing recommended such checks in September 2009.
Southwest said it operates 548 737s, the world’s most widely flown airliner. According to the airline’s website, it had 171 737-300s as of Dec. 31, 2010. The average age of those aircraft was 19 years as of the end of 2010.
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