The government plans to order emergency inspections today of 80 older Boeing 737s similar to the Southwest Airlines jet that suffered a 5-foot tear in its roof on Friday in a search for possibly dangerous cracks. 
 
 The Federal Aviation Administration order will cover some 737-300s, 737-400s and 737-500s, models of the popular jet built in the 1980s and 1990s. The jets covered under the order include those built with a specific manufacturing process and with more than 30,000 flights, the agency said Monday. 
 
 There are about 175 of the aircraft worldwide, 80 of which are registered in the USA, the FAA said. Southwest operates 78 of the jets and Alaska Airlines has two. Boeing said it is also working on a bulletin to airlines suggesting inspections. 
 
 Although the number of jets affected by the order is small, some aviation safety experts said the incident is shining a new light onto aging aircraft safety. 
 
 Despite more than two decades of efforts to better understand and detect cracks, the tear on the Southwest jet occurred in an area that was not considered high risk. 
 
 "This could change the conversation regarding the risk of aging aircraft," said Bill Voss, president of the Flight Safety Foundation. The FAA last year toughened the standards for aging aircraft, but the latest discovery could lead to even broader requirements for costly inspections, Voss said. 
 
 "Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation," Transportation Secretary Ray LaHood said in a statement. 
 
 Southwest Flight 812 lost cabin pressure at 34,000 feet and had to make an emergency descent into thicker air after leaving Phoenix for Sacramento. A flight attendant was injured, but none of the 118 passengers was hurt. 
 
 National Transportation Safety Board investigators found additional cracks on the same seam in the skin where it ripped open, NTSB board member Robert Sumwalt said. Southwest also found similar cracks on three additional 737-300s and grounded the jets, the airline said. 
 
 The jet involved in the accident appeared to have suffered corrosion from moisture that got into the joint where two thin bands of aluminum skin were joined, Sumwalt said. Rivet holes had also become slightly enlarged, he said. 
 
 "It indicates that something is going on in that joint that should not be going on," Sumwalt said. 
 
 The FAA ordered the inspections be done with an electro-magnetic process that detects cracks invisible to the naked eye. 
 
 As of Monday afternoon, Southwest had returned all but 14 of its 737-300s to service. ||||| 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. 
 
 Fatigue Cracks Found in Three Other Southwest Jets 
 
 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. 
 
 Based on modeling and previous flight experience of Boeing 737-300s, it was believed that the cracks could not develop in this area of the plane. 
 
 Those cracks, which can develop after repeated takeoffs and landings, may well have been what caused the fuselage to fail, according to some experts. 
 
 "Now we may have to look at airplanes in places we never thought we would have to check before," said Ganyard, an ABC News consultant. 
 
 Other fuselage cracks were found and repaired on this particular aircraft during heavy maintenance in March 2010, according to The Associated Press. 
 
 Upon review of the records of the repair, ABC News discovered at least a dozen fuselage crack repairs. 
 
 Southwest now says that what was seen with Flight 812 was a new and unknown issue. 
 
 "Boeing has since identified an inspection program for this section of the aircraft," Southwest said in a statement. "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." 
 
 Southwest Flight 812 Drops 20,000 Feet in Four and a Half Minutes 
 
 On Friday's harrowing flight, the first 20 minutes all appeared normal as the plan climbed to 36,000 feet. Flight attendants had just taken drink orders when the plane's 118 passengers reported hearing loud pops. Then, with the plane at approximately 34,000 feet, the roof opened up near the luggage compartment and oxygen masks dropped in front of passengers. At least one flight attendant passed out. 
 
 Passenger Wade Allemand said he almost passed out. 
 
 "Your ears instantly start to hurt really bad. You feel like you're going to black out," he said. 
 
 The crew turned the jet sharply to the right, pushed the nose down, losing more than 20,000 feet in just four and a half minutes. 
 
 Passengers had a short amount of time to get their oxygen masks on. 
 
 "It depends on what the decompression is. Sometimes it's minutes, sometimes it's two to three seconds," Lauren Jarmoszko, a flight safety instructor, said. 
 
 Loss of oxygen impacts cognition, memory and reaction time. It can also lead to brain damage. 
 
 In a rapid decompression, like the Southwest flight, the oxygen mask drops quickly. There are also instances of slow decompression which is usually caused by a small leak in a window or door seal. Passengers and crew may not even notice the first effects: fatigue, mental confusion, and dulled reflexes. 
 
 The captain on Flight 812 had 19 years experience with Southwest, while his first officer had seven years of experience, and he received assistance as typical when declaring a emergency, according to the NTSB. 
 
 Some terrified passengers clearly thought it was the end. One woman whose husband was on the plane received a text from him -- "Plane going down. Love you." 
 
 However, the plane was able to land at Yuma Marine Corps Air Station/International Airport at 4:07 p.m. Friday, said Ian Gregor of the Federal Aviation Administration. 
 
 Passengers applauded the pilots upon landing, called loved ones and waited for a new Southwest jet to pick them up and take them on their way. 
 
 ABC's Jessica Hopper contributed to this report. 
 
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– The Federal Aviation Administration has ordered emergency inspections of 80 older Boeing 737s similar to the Southwest plane that suddenly cracked open last week during a flight. The order covers aircraft built with a specific process in the '80s and '90s and those that have more than 30,000 flight cycles, ABC News reports. Some 78 of the jets in question belong to Southwest Airlines—which has carried out its own inspections—and two are being used by Alaska Airlines. Inspectors must use an electro-magnetic process that detects cracks invisible to the naked eye. Neither the Federal Aviation Administration nor Boeing currently requires checks to be carried out on the section of aircraft that cracked open. The incident "could change the conversation regarding the risk of aging aircraft," and lead to tougher, more expensive inspections, the president of the Flight Safety Foundation tells USA Today.