Cracks found in three grounded Southwest planes

US Inspectors have found small, subsurface cracks in three more Southwest Airlines planes that are similar to those suspected of causing a jetliner to lose pressure and make a harrowing emergency landing in Arizona, a federal investigator said.

Southwest said in a statement on Sunday that two of its Boeing 737-300s had cracks and will be evaluated and repaired before they are returned to service.

A US National Transportation Safety Board member told The Associated Press later Sunday that a third plane had been found with cracks developing.

The cracks found in the three planes developed in two lines of riveted joints that run the length of the aircraft.

Nineteen other Boeing 737-300 planes inspected using a special test developed by the manufacturer showed no problems and will be returned to service. Checks on nearly 60 other jets are expected to be completed by late on Tuesday, the airline said.

That means flight cancellations will likely continue until the planes are back in the air. About 600 flights in all were cancelled over the weekend after Southwest grounded 79 of its planes.

NTSB board member Robert Sumwalt said Boeing was developing a "service bulletin" for all 737-300 models with comparable flight cycle time as the Arizona jet, which was 15 years old and had about 39,000 take-off and landing cycles.

There are 931 such models in service worldwide, 288 of which are in the US fleet.

Boeing's bulletin would strongly suggest 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 FAA is likely to make them mandatory.

Friday's flight, carrying 118 people, rapidly lost cabin pressure after the plane's fuselage ruptured - causing a 1.5-metre-long tear - just after take-off 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 (10,485 meters) into a southwestern Arizona military base. No one was seriously injured.

The tear along a riveted "lap joint" near the roof of the Boeing 737 above the midsection shows evidence of extensive cracking that hadn't been discovered during routine maintenance before the flight - and probably wouldn't have been unless mechanics specifically looked for it - officials said.

"What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest executive vice president and chief operating officer, said.

"Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft."

Sumwalt said that the rip was 30 centimetres wide, and that it started along a joint where two sections of the plane's skin are riveted together.

An examination showed extensive pre-existing damage along the entire tear. Further inspection found more cracks in areas that had not torn open.

The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, 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 FAA declined to say if it was requiring other operators to check their aircraft for similar flaws.

The NTSB also could issue urgent recommendations for inspections on other 737s if investigators decide a problem has been overlooked. The agency's investigation has not determined that the cracks caused the rupture, but it is focused on that area.

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 aircraft 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 for the Arizona plane.

The records 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 ageing 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 pressurising, generally a count of take-offs and landings. Cracks can develop from the constant cycle of pressurising for flight, then releasing the pressure.

Southwest officials said it had undergone all inspections required by the FAA. They said the plane was given a routine inspection on Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.

The decompression happened about 18 minutes after take-off 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, 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 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 548 planes, but it replaced the aluminium 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.

Southwest said "based on this incident and the additional findings, we expect further action from Boeing and the FAA for operators of the 737-300 fleet worldwide".

Boeing did not immediately return messages left on Sunday.

US Airways operates 19 of the older-model 737-300s. Airline spokeswoman Liz Landau said they have not been grounded and no additional inspections are being done.

Air New Zealand, which has 15 Boeing 737-300s in its fleet, none of which are more than 15 years old, says it will only check its fleet if it gets a directive from the planes' manufacturer.

A spokeswoman for the airline said regular checks would continue to be carried out but further checks would only be done if a directive was sent from the relevant authorities - Boeing, the New Zealand Civil Aviation Authority or the Federal Aviation Administration.

In July 2009, a football-sized hole opened up in-flight in the fuselage of another of Southwest's Boeing 737s, depressurising 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.

A flight attendant was sucked out of the plane and plunged to her death, and dozens of passengers were injured.

- AP