YUMA, Ariz. (AP) -- 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. Federal aviation officials are considering an order for other airlines to inspect their aircraft.
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. It made a rapid descent, landing at a military base in Yuma, 150 miles southwest of Phoenix. No one was 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 are expected to be completed by late Tuesday, the airline said.
The incident 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 such models in service worldwide, 288 of which are in the U.S. fleet. But it is unclear how many might need inspections or repairs, because some are out of service and other 737-300 models were made using a different process.
Boeing was developing a "service bulletin" for all similar 737-300 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 on Sunday in a prepared statement about its inspections. Sumwalt said a third plane had been found with cracks developing.
The NTSB planned a press briefing at 1:30 p.m. PDT on Monday.
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 for the plane.
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 Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
The decompression happened about 18 1/2 minutes after takeoff from Phoenix Sky Harbor International Airport.
At 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 it's 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.
A fuselage failure, although extremely rare, can have deadly consequences. In 1988, cracks caused part of the roof of an Aloha Airlines Boeing 737 to peel open while the jet flew from Hilo to Honolulu. The flight attendant died, and dozens of passengers were injured.
Associated Press writers David Koenig in Dallas; Joan Lowy in Washington, D.C.; and Terry Tang, Walter Berry and Mark Evans in Phoenix contributed to this report.
(Copyright 2011 by The Associated Press. All Rights Reserved.)