The tear along a riveted "lap joint" near the roof of the Boeing 737 above the midsection shows evidence of extensive cracking that hadn't been discovered during routine maintenance before the flight — and probably wouldn't have been unless mechanics specifically looked for it — officials said.
"What we saw with Flight 812 was a new and unknown issue," Mike Van de Ven, Southwest executive vice president and chief operating officer, said. "Prior to the event regarding Flight 812, we were in compliance with the FAA-mandated and Boeing-recommended structural inspection requirements for that aircraft."
[NTSB inspector] Sumwalt said that the rip was a foot wide, and that it started along a joint where two sections of the plane's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.
The riveted joints that run the length of the plane were previously not believed to be a fatigue problem and not normally subjected to extensive checks, Sumwalt said.
"Up to this point only visual inspections were required for 737s of this type because testing and analysis did not indicate that more extensive testing was necessary," Sumwalt said.
That will likely change after Friday's incident, he said.
NTSB: Cracks found in 3 grounded Southwest planes - Yahoo! News