Neither the Federal Aviation Administration nor Boeing requires an intensive inspection to check for fatigue cracks on the section of passenger jets that tore open mid-flight Friday on a Boeing 737-300, ABC News has learned.
After the incident Friday, the revelation calls into question the inspection program for aging U.S. aircraft.
The 15-year-old Southwest Airlines jet was inspected in March 2010 with what is known as a D-check, the most comprehensive check for an airplane.
Based on modeling and previous flight experience of Boeing 737-300s, there was no feeling that cracks could 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.
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."
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.
"That is a very disturbing point because it's a place that can cause a great amount of damage to an airplane," former National Transportation Safety Board member and aviation maintenance expert John Goglia said.
After the incident Friday, Southwest Airlines ground 79 of its planes until they could under go further inspection.
The airline said today that small, subsurface cracks were found in three other airplanes, but as of 4 p.m. CT, 19 of the company's planes had undergone the intense inspection with no findings and were returned to service.
Tests of the remaining aircraft in the sub-fleet of 79 planes will continue for the next few days, Southwest said.
"This test is designed to detect any subsurface fatigue in the skin that is not visible to the eye," the airline said in the statement released earlier today.
As ABC News first reported, fatigue cracks were discovered when investigators got their first look at the break on the Southwest jet Saturday.
"We did find evidence of widespread cracking across this entire fracture surface," NTSB member Robert Sumwalt said today.
Sources told ABC News that they do not know why the cracks were missed during the D-check or couldn't simply be seen.
Sumwalt confirmed what ABC News first reported on "Good Morning America," that investigators examining the jet found "pre-existing fatigue along the entire fracture surface known as multisite damage" in the area that separated from the plane's roof.
At this point it is still unclear how those cracks got there, or whether they played any role in the ceiling of one of the airline's Boeing 737-300 jets tearing open in mid-air.
On Friday's harrowing flight, the roof opened up at 36,000 feet, oxygen masks dropped in front of passengers and at least one flight attendant passed out.
No one was seriously hurt in the incident and the plane, which was flying from Phoenix to Sacramento, Calif., made a rapid descent and an emergency landing at a military base in Yuma, Ariz.
The captain on the flight 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.
The 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."
Other fuselage cracks were found and repaired on this particular aircraft during heavy maintenance in March 2010, according to The Associated Press, which originally reported there were at least eight cracks in the fuselage.
Upon review of the records of the repair, ABC News discovered at least a dozen fuselage crack repairs. This sort of damage is not unexpected for a 15-year-old Boeing 737.
Airlines conduct major plane teardowns for these exact reasons -- in order to hunt for and fix damage such as fatigue cracks.
A Harrowing Incident at 36,000 Feet
The cracks do, however, indicate that the plane was showing its age.
Southwest said it is working with the NTSB and FAA to determine the cause of the "depressurization event," and the airline canceled approximately 300 flights on Saturday and another 200 today.
"The safety of our Customers and Employees is our primary concern. We are working closely with Boeing to conduct these proactive inspections and support the investigation. We also are working aggressively to attempt to minimize the impact to our Customers' travel schedules today," Mike Van de Ven, Southwest's executive vice president and chief operating officer said.
For the first 20 minutes of the flight, all appeared normal as the plane climbed to 36,000 feet. Flight attendants had just taken drink orders when passengers reported hearing loud pops.
Soon, the roof opened up. Astonished passengers described a gaping hole, which investigators now say is five feet long and a foot wide, right next to the luggage compartment.
The plane suffered a rapid decompression, oxygen masks popped out and the plane went into a dive, according to passengers and officials.
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 Southwest pilots radioed air traffic control, declared an emergency, and began a rapid descent -- quickly diving to a lower altitude so passengers would be able to breathe on their own.
The jet descended from 36,000 feet, cruising altitude, to 11,000 feet in four and a half minutes. It leveled off at 9,000 feet.
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.