The federal government on Tuesday will order emergency inspections on 175 Boeing 737 airliners, and is rethinking its approach to plane inspections after a Southwest Airlines jet tore open in mid-flight Friday night, ABC News has learned.
Inspections will initially focus on 175 planes, used by airlines around the world, that make frequent takeoffs and landings. Eighty of the planes are in service in the United States, most of them for Southwest Airlines.
The government is particularly concerned about older 737-300, 737-400 and 737-500 jets that have taken off and landed more than 30,000 times. Jets that have accumulated many flight cycles are apparently more likely to develop the sort of fatigue cracks that may have caused the tear in the skin of the Southwest 737-300 last week.
As the nation's planes age, more jets could cause concern and require inspection for such fatigue cracks.
Inspectors use something called eddy current technology, passing an electric current through an aircraft's skin to look for small cracks. If any warning signs are detected, more sophisticated ultrasound and X-ray tools are then used for a closer examination. In some areas, a plane's skin can be as thin as a nickel.
Southwest said today that it has inspected some 90 percent of its 737-300s, which were removed from service after the weekend. So far, inspections have uncovered fatigue cracks in three planes, the airline said. 64 other planes have been inspected and returned to service, with a dozen other planes still requiring inspection. The airline believes its inspection process complies with the government's order.
Due to the voluntary grounding, Southwest has had to cancel at least 600 flights since Friday's emergency landing.
Southwest flight 812, enroute from Phoenix to Sacramento, Calif., was diverted to a military base at Yuma, Ariz., after a section of the plane's fuselage ripped open, depressurizing the plane and exposing the sky to passengers.
Last night, another Southwest flight was diverted. The flight, headed from Oakland, Calif., to San Diego, Calif. made an emergency landing because of a burning electrical smell.
Meanwhile, the five-foot section of the plane's fuselage that opened up Friday on flight 812 is headed back to Washington, D.C., for detailed microscopic analysis.
The jet came apart at a seam, where two pieces of metal are riveted together, which is an especially dangerous location because the plane can essentially unzip, experts said.
"We have clear evidence that the skin separated at the lower rivet line," Robert Sumwalt, a NTSB Board member, said.
The 15-year-old damaged aircraft had been through more than 39,000 takeoffs and landings -- each of which puts stress on an aircraft's skin.
"When airplanes take off and land, aircraft are pressurized. The internal cabin is expanded and contracts just a little bit," said Steve Ganyard, a former military pilot. "You're exercising the skin of that aircraft."
Neither the Federal Aviation Administration nor Boeing requires an intensive inspection to check for fatigue cracks on the section of passenger jet that tore open Friday, ABC News has learned.
After the incident Friday, the revelation calls into question the inspection program for aging U.S. aircraft.
The Southwest jet had what is known as a D-check in March 2010, the most comprehensive check for an airplane.