DALLAS - Southwest Airlines Co. said Tuesday it finished inspecting its grounded older planes and five of them have cracks in the aluminum skin.
That's two more than the airline found by Monday afternoon.
Southwest said operations were returning to normal after nearly 700 flights were cancelled Saturday through Monday.
Southwest grounded 79 older Boeing 737-300 planes after a similar jet sprang a five-foot hole in the roof Friday on a flight from Phoenix.
Meanwhile, Federal aviation officials were preparing to issue an order Tuesday that calls for emergency inspections on 80 U.S.-registered Boeing 737 jetliners with histories similar to that Southwest Airlines jet, which had been pressurized and depressurized 39,000 times before the hole opened in its fuselage.
The order is aimed at finding weaknesses in the metal exterior, but virtually all of the affected aircraft will have already been inspected by the time the order takes effect.
The safety directive applies to about 175 aircraft worldwide, including 80 planes registered in the U.S., the Federal Aviation Administration said. Of those 80, nearly all are operated by Southwest. Two belong to Alaska Airlines.
Southwest said it had already complied with the new order by grounding and inspecting planes over the weekend. The airline said the five planes with "minor subsurface cracking" will remain grounded until Boeing recommends a process for repairing them and the airline completes the work.
The 737 is the workhorse of the skies. Because Southwest flies short hops, the planes take off and land several times a day, which takes a toll, reports CBS News correspondent Don Teague. Cracks can develop from the constant cycle of pressurizing the cabin for flight, then releasing the pressure upon landing.
Since there had been no previous accidents or major incidents involving metal fatigue in the middle part of the fuselage, Boeing maintenance procedures called only for airlines to perform a visual inspection.
But airlines, manufacturers and federal regulators have known since at least 1988 that planes can suffer microscopic fractures. That year, an 18-foot section of the upper cabin of an Aloha Airlines 737-200 peeled away in flight, sucking out a flight attendant.
The order is "certainly a step in the right direction," said National Transportation Safety Board member Robert Sumwalt, who is in Yuma with the board's accident investigation team.
"The problem is the technology to detect an aging fleet that came out of the crash in 1988 is no longer relevant," Michael Goldfarb, former chief of staff for the FAA, told "The Early Show" Tuesday.
The FAA's emergency order will require initial inspections using electromagnetic devices on some Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 takeoffs and landings. It will require repetitive inspections at regular intervals.
The 15-year-old Southwest jet in Friday's incident had logged 39,000 pressurization cycles, a measurement of the number of takeoffs and landings. That's 7.2 cycles every day for every year it has been in service.
Planes that have 30,000 cycles or have been in service for 15 years are considered about halfway through their useful life.
Boeing Co. said Monday that it will issue guidance this week on how airlines should do checks on the affected airplanes now in service. An estimated 1,800 airplanes, including -300, -400, -500 model 737s, are affected by the aircraft maker's service bulletin.
Southwest officials said the Arizona flight 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 18½ minutes after takeoff from Phoenix Sky Harbor International Airport when a gash opened above the overhead luggage bins in the middle of the plane. 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. No one was seriously hurt.
The voice and data recorders were being examined in Washington.
In its inspections, Southwest found two jets had small, subsurface cracks similar to the breached plane. Sumwalt said a third was also found with the problem.
The grounding that followed the accident caused about 600 flight cancellations over the weekend and another 70 on Monday. Southwest expected to have nearly all of its grounded planes back in the air by late Tuesday.
Some stranded passengers were angry and threatened to switch airlines, but others praised its decision to ground planes as a safety-first precaution.
Southwest appeared eager to shift blame to Boeing. The airline said it had never been alerted to a potential problem where overlapping panels of aluminum skin are riveted together on the 737-300.
"This is a Boeing-designed airplane. This is a Boeing-produced airplane," Southwest spokeswoman Linda Rutherford said. "It's obviously concerning to us that we're finding skin-fatigue issues."
Boeing officials declined to respond to Rutherford's comments.
Many of the planes that fall under the FAA order don't fall under U.S. auspices. FAA has authority only over U.S. operators, but government aviation agencies in most other countries usually follow FAA's safety directives with their own orders.
Germany's Lufthansa has a fleet of 63 737s, including 33 of the 300 series, but just three are from the same series as the Southwest jet.
The problem of what is known as "widespread fatigue damage" in aging planes has a long, well-documented history.
It became a major safety focus of the FAA and was the subject of congressional hearings after the Aloha Airlines 737-200 accident in April 1988. There were 95 people on board. A flight attendant and seven passengers were seriously injured.
Following the accident, the FAA instituted a new safety regime for older 737s for cracking that includes not only visual inspections, but the use of devices that employ electromagnetic currents to spot fatigue and corrosion.
The agency also began work in 2004 on a rule that would require more detailed inspections and maintenance procedures for other types of aging aircraft, not just the 737. Initially there was opposition from airlines to the new procedures because of the cost involved.
After over six years of work, FAA published a rule requiring the new procedures late last year. It went into effect in January.
It gives manufacturers 18 months to five years, depending upon the plane involved, to develop inspection programs. Airlines and other operators then have another two and a half to six years to implement the inspection requirements.
Bill Voss, president of the Flight Safety Foundation in Alexandria, Va., said an FAA safety order to be issued Tuesday is an acknowledgement that previous inspection procedures were inadequate. "There is no question this was a very serious safety event," Voss said.
That the skin peeled away shouldn't come as a surprise, said Paul Czysz, professor emeritus of aeronautical engineering at St. Louis University.
Czysz said fuselages are designed with a specific stress limit, based on the number of cycles a plane flies. When a fatigue crack emerges, he said, that means the limit is being pushed. The trick is to keep up a rigorous inspection program.
"It's not magic," he said. "It's just basic physics."