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. 
 
 Click here to return to the "World News" page. ||||| (Updates with possible impact on Sunday, other details) 
 
 By Alex Dobuzinskis 
 
 LOS ANGELES, April 2 (Reuters) - Southwest Airlines (LUV.N) canceled hundreds of flights over the weekend as it inspected 79 aircraft from its Boeing 737 fleet after one of its planes with a gaping hole in the fuselage made an emergency landing, the airline said on Saturday. 
 
 The airline said was canceling some 300 flights on Saturday, a day after the emergency landing, and could cancel around that number again on Sunday. 
 
 "We don't at this time know what the impact will be, but it's possible that it could be in the 300-flight range again tomorrow," Southwest spokesman Brandy King told Reuters. 
 
 Passengers aboard Southwest Flight 812 from Phoenix to Sacramento on Friday heard a loud noise and the hole appeared suddenly at about mid cabin, forcing the pilot landed at a military base in Yuma, Arizona. 
 
 The emergency aboard the Boeing (BA.N) 737-300 prompted the airline to examine similar aircraft within its fleet, with a total of 79 inspections planned at five locations over the next several days, Southwest said in a statement. 
 
 Southwest normally has about 3,400 flights on Saturday, King said, so the cancellations accounted for nearly 9 percent of that total. 
 
 "We did our best to accommodate those passengers on other Southwest flights," King said. 
 
 A total of 931 Boeing 737-300s are operated by all airlines worldwide, with 288 of them in the United States, the Federal Aviation Administration said. 
 
 The Southwest flight that was forced to make an emergency landing had 118 passengers and five crew members on board. 
 
 Southwest and Boeing engineers will inspect the grounded aircraft, and the airline is working with federal authorities to determine the cause of the incident, Southwest said. 
 
 The pilot made a rapid descent from about 34,400 feet (10.3 km) to 11,000 feet (3.4 km), following standard practice to reach an altitude where supplemental oxygen is no longer required, the FAA said. 
 
 One flight attendant and at least one passenger were treated at the scene for minor injuries, Southwest said. 
 
 The Boeing 737 landed at 4:07 p.m. local time after declaring an emergency, said Ian Gregor, an FAA spokesman. 
 
 The airline said it arranged for another aircraft to take the passengers from the Yuma Marine Corps Air Station to Sacramento. 
 
 Passengers described the harrowing scene to the CBS television affiliate in Sacramento, detailing the damage to the plane. 
 
 "They had just taken drink orders when I heard a huge sound and oxygen masks came down and we started making a rapid descent. They said we'd be making an emergency landing," a woman identified as Cindy told the station. 
 
 "There was a hole in the fuselage about three feet (91 cm) long. You could see the insulation and the wiring. You could see a tear the length of one of the ceiling panels." (Additional reporting by David Schwartz in Phoenix and Lauren Keiper in Boston; Editing by Greg McCune and Vicki Allen)
– Fatigue cracking has been found along the entire 5-foot section of a Southwest Airlines jet that ripped open on Friday, forcing an emergency landing in Arizona. The NTSB says that mechanics will cut a 9-foot by 3-foot section of the plane and send it to Washington, DC, for testing. Southwest has responded by grounding at least 79 planes, reports the AP; at least 300 flights were canceled yesterday. And more cancellations were on the horizon today: "We don't at this time know what the impact will be, but it's possible that it could be in the 300-flight range again," a Southwest spokesman told Reuters.