Stress fractures in Boeing 737s
Commercial airlines and military aircraft receive very high use as designed. However, stress fractures from the constant expansion and contraction from changes in pressure during take-off, flight, and landings wears the external skin just like bending a piece of metal or plastic back and forth with your hands. Eventually, the material will break.
Preventative inspections and maintenance are intended to predict, find, and repair before such breaks happen during use. Something went wrong here. Since the planes had undergone repair a short while ago, the repair must have been defective, I suspect.
“Southwest Grounds Dozens Of Jets After Mishap
by THE ASSOCIATED PRESS
April 3, 2011
Federal records show cracks were found and repaired a year ago in the frame of the Southwest Airlines Boeing 737-300 that made an emergency landing at an Arizona military base after a hole was torn from the passenger cabin.
No one was seriously injured Friday as the aircraft carrying 118 people rapidly lost cabin pressure and made a harrowing but controlled descent from 34,500 feet, landing safely near Yuma, Ariz., 150 miles southwest of Phoenix.
But passengers recalled tense minutes after a hole ruptured overhead with a blast and they fumbled frantically for oxygen masks as the plane descended.
Federal investigators arrived in Yuma Saturday and said they were cutting a piece from the fuselage of the stricken plane to determine how the fuselage rupture occurred.
Southwest grounded 80 similar planes to carry out inspections.
An Associated Press review of Federal Aviation Administration records of maintenance problems for the 15-year-old plane showed that in March 2010 at least eight instances were found of cracking in the aircraft frame, which is part of the fuselage. The records showed that those cracks were repaired.
It's not uncommon for fuselage cracks to be found during inspections of planes that age, especially during scheduled heavy maintenance checks in which they are taken apart so that inspectors can see into areas not normally visible.
The National Transportation Safety Board worked to determine what caused part of the fuselage to rupture.
NTSB member Robert Sumwalt said investigators will study the cut-out fuselage piece for fracture patterns and examine the plane's black box and flight recorders.
Southwest officials said the Arizona plane had undergone all inspections required by the FAA. They said the plane was given a routine inspection Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
Meanwhile, the airline grounded the 80 planes resulting in about 300 canceled flights Saturday, airline spokeswoman Linda Rutherford said.
Southwest operates about 170 of the 737-300s in its fleet of about 540 planes, but it replaced the aluminum skin on many of the 300s in recent years, Rutherford said. The planes that were grounded Saturday have not had their skin replaced, she said.
"Obviously we're dealing with a skin issue, and we believe that these 80 airplanes are covered by a set of (federal safety rules) that make them candidates to do this additional inspection that Boeing is devising for us," Rutherford said.
Julie O'Donnell, an aviation safety spokeswoman for Seattle-based Boeing Commercial Airplanes, confirmed "a hole in the fuselage and a depressurization event" in the latest incident but declined to speculate on what caused it.
A total of 288 Boeing 737-300s currently operate in the U.S. fleet, and 931 operate worldwide, according to the Federal Aviation Administration. "The FAA is working closely with the NTSB, Southwest Airlines and Boeing to determine what actions may be necessary," the FAA said Saturday.
The 737-300 is the oldest plane in Southwest's fleet, and the Dallas-based company is retiring 300s as it takes deliveries of new models. But the process of replacing all the 300s could take years.”