PHOENIX (AP) - The airliner whose roof ripped open 34,000 feet over Arizona has had a busy 15-year life: taking off and touching down more than seven times a day, on average, and possibly developing microscopic cracks in its aluminum skin each time.
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 a 5-foot-long 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 grounded nearly 80 Boeing 737-300s for inspections after its jet leaving Phoenix lost pressure Friday, forcing pilots to make an emergency landing 125 miles away in Yuma. By Monday evening, 64 were cleared to return to the skies, but three were found with cracks similar to those found on the Arizona plane.
Friday's incident, however, raised questions about the impact that frequent takeoffs and landings by short-haul carriers like Southwest put on their aircraft and the adequacy of the inspections.
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 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 18 1/2 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,