PHOENIX -- Three more Southwest Airlines jetliners have small, subsurface cracks that are similar to the ones suspected in the fuselage tear on another of its planes, prompting federal aviation officials to prepare an order Monday for emergency inspections.
The checks will apply to 80 U.S.-registered Boeing 737 jetliners like the one on which a piece of fuselage tore away more than 30,000 feet above Arizona last week. The order, to be issued today, is aimed at finding weaknesses in the metal in the fuselage, but virtually all of the affected aircraft will have already been inspected by the time the order takes effect.
Boeing said Monday that it will issue guidance this week on how airlines should do checks on the affected airplanes now in service.
The 5-foot-long hole tore open Friday in the passenger cabin roof shortly after the Southwest plane carrying 118 people left Phoenix for Sacramento, Calif. The plane made a rapid descent, landing at a military base in Yuma, 150 miles southwest of Phoenix. No one was seriously hurt.
Since then, Southwest grounded 79 other Boeing 737-300s and began inspecting them. The grounding caused about 600 flight cancellations over the weekend and another 70 on Monday. Nineteen inspected aircraft showed no problems and will be returned to service.
Checks on the remaining jets were expected to be completed by late today, the airline said.
Charleston County Aviation Authority spokeswoman Becky Beaman said Monday afternoon that no flights out of Charleston would be canceled. The Greenville News said one Southwest flight from the Upstate was canceled Sunday. The low-fare airline began serving South Carolina for the first time about three weeks ago.
The incident in Arizona raises new questions about the vulnerability of the nation's air fleet to fatigue cracks, more than two decades after cracks caused part of the roof of an Aloha Airlines Boeing 737 to peel open. A flight attendant was sucked out and plunged to her death.
There are 931 737-300 models in service worldwide, 288 of which are in the U.S. fleet. Boeing said its service bulletin also will require checks on some 737-400 and 737-500 models as well, depending on how many pressurization cycles they have gone through.
The new inspection would be a one-time check, Boeing spokesman Marc Birtel said.
The service bulletin Boeing was developing will target all similar 737 models with comparable flight cycle time as the Arizona jet, which was 15 years old and had about 39,000 takeoff and landing cycles, National Transportation Safety Board member Robert Sumwalt said.
The bulletin would outline extensive checks of two lines of lap joints that run the length of the fuselage. The NTSB has not mandated the checks, but Sumwalt said the Federal Aviation Administration is likely to make them mandatory.
The NTSB also could issue urgent recommendations for inspections on other 737s if investigators decide a problem has been overlooked. The FAA declined to say if it would require other operators to check their aircraft for similar flaws.
The cracks found in the three Southwest planes developed in two lines of riveted joints that run the length of the aircraft. The agency is focusing its probe on the area of the cracks but has not determined that the cracks caused the rupture.
Two planes have been found with cracks similar to those in the Phoenix aircraft and will be evaluated and repaired before they are returned to service, Southwest said Sunday in a prepared statement about its inspections. Sumwalt said a third plane had been found with cracks developing.
Previously, the riveted joints were not extensively checked because they were believed not susceptible to fatigue, Sumwalt said.
"Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary," Sumwalt said. That will likely change after Friday's incident, he said.
The tear along a riveted lap joint above the midsection of the plane shows evidence of extensive cracking that hadn't been discovered during routine maintenance -- and probably wouldn't have been unless mechanics looked for it -- officials said.
"What we saw with Flight 812 was a new and unknown issue," said Mike Van de Ven, Southwest executive vice president and chief operating officer. He said the airline had complied with federally mandated and Boeing- recommended inspections.
Federal records show cracks were found and repaired a year ago in the frame of the same Southwest plane.
A March 2010 inspection found 10 instances of cracking in the frame, which is part of the fuselage, and another 11 instances of cracked stringer clips, which help hold the plane's skin on, according to an AP review of FAA records of maintenance problems.
The records for the plane show the cracks were either repaired or the damaged parts replaced. Cracking accounted for a majority of the 28 problem reports filed as a result of that inspection.
It's common for fuselage cracks to be found during inspections of aging planes, especially during scheduled heavy-maintenance checks in which planes are taken apart so that inspectors can see into areas not normally visible.
The Arizona jetliner had gone through about 39,000 cycles of pressurizing, generally a count of takeoffs and landings. Cracks can develop from the constant cycle of pressurizing for flight, then releasing the pressure.
Southwest officials said the plane was given a routine inspection March 29 and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
The decompression happened about 18 minutes after takeoff from Phoenix Sky Harbor International Airport.
At a cruising altitude of some 34,000 feet, the pilots declared an emergency and briefly considered returning to Phoenix before the cabin crew told them of the extent of the damage, Sumwalt said. The plane's voice and data recorders were being examined in Washington.
Southwest operates about 170 of the 737-300s in its fleet of 548 planes, but it replaced the aluminum skin on many of the 300s in recent years, a spokeswoman said. The planes that were grounded over the weekend have not had their skin replaced.
US Airways operates 18 of the older-model 737-300s, but the company said Monday it has been told by Boeing that its jets aren't affected.
In July 2009, a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurizing the cabin. Sumwalt said the two incidents appeared to be unrelated.
With contributions from other AP staffers and The Post and Courier.