YUMA, Ariz. — Inspectors have found small, subsurface cracks in two more Southwest Airlines planes that are similar to the cracks that caused a jetliner to lose pressure and make a harrowing emergency landing in Arizona, the airline said Sunday.
The two planes will be evaluated further and more repairs will be undertaken before they are returned to service, Southwest said in a statement.
Friday's flight carrying 118 people rapidly lost cabin pressure after the Boeing 737-300's fuselage ruptured — causing a 5-foot-long tear — just after takeoff from Phoenix.
Passengers recalled tense minutes after the hole ruptured overhead with a blast and they fumbled frantically for oxygen masks. Pilots made a controlled descent from 34,400 feet into a southwestern Arizona military base. No one was seriously injured.
The tear along a riveted "lap joint" shows evidence of extensive cracking that hadn't been discovered during routine maintenance before Friday's flight — and probably wouldn't have been unless mechanics had specifically lookedfor it, officials said.
National Transportation Safety Board investigators on Sunday were in Yuma to oversee the removal of the top section of the jetliner's roof around the tear. The structure will be sent to Washington, D.C., for analysis.
Southwest said it cancelled about 300 flights for the second day in a row Sunday as it inspected 79 planes in its fleet similar to the one in Friday's incident. By Sunday afternoon, 19 planes had undergone the intense inspection with no findings and had been returned to service, the airline said.
NTSB board member Robert Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the 737's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.
But Mr. Sumwalt noted that the extensive cracking, known in the industry as "multi-site damage," could not have been spotted during routine maintenance.
The NTSB could issue urgent recommendations for inspections on other 737s if investigators decide there is a problem that has been overlooked. The type of riveted joint involved is not normally subjected to extensive checks for wear or fatigue.
Federal records show cracks were found and repaired a year ago in the frame of the same Southwest plane.
An Associated Press review of Federal Aviation Administration records of maintenance problems for the 15-year-old plane showed that a March, 2010, inspection found 10 instances of cracking in the aircraft frame, which is part of the fuselage, and another 11 instances of cracked stringer clips, which help hold the plane's skin on.
The records show the cracking was 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 jetliner had gone through about 39,000 cycles of pressurizing, generally done for takeoffs and landings. Cracks can develop from the constant cycle of pressurizing for flight, then releasing the pressure.
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.
The decompression happened about 18½ minutes after takeoff from Phoenix Sky Harbor International Airport after the pilots reached their cruising altitude. They immediately donned their oxygen masks, declared an emergency, and briefly considered returning to Phoenix before the cabin crew told them of the extent of the damage, Mr. Sumwalt said.
"They discussed landing in Phoenix, but quickly upon getting the assessment decided to divert to Yuma because it was the closest suitable airport," he said.
The plane's voice and data recorders were being examined in Washington, and Mr. Sumwalt said they worked well and showed no sign of a problem before the incident.
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, a spokesman said. The planes that were grounded Saturday have not had their skin replaced.
A total of 288 Boeing 737-300s currently operate in the U.S. fleet, and 931 operate worldwide, according to the FAA. It declined to say Sunday if it was requiring other operators to check their aircraft for similar flaws.
A similar incident happened in July, 2009, when a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurizing the cabin. Mr. Sumwalt said the two incidents appeared to be unrelated.