ST. PETERSBURG, Fla (Reuters) - National safety inspectors have found evidence of "widespread cracking" and fatigue on the fuselage of a Southwest Airlines Boeing 737 that made an emergency landing in Arizona with a hole in the cabin, a government official said on Sunday.
"Was the aircraft well maintained and should it have been maintained better? That is exactly why we are here, to look at why this problem occurred," National Transportation Safety Board Member Robert Sumwalt said at a press conference broadcast from Yuma, Arizona, via Internet streaming.
As a result of the incident, Southwest grounded some of its fleet for inspections.
By late Sunday afternoon, the airline said 19 planes had been returned to service after "intense" inspections of the aircrafts' skin uncovered no problems.
Small, subsurface cracks found on two other planes will result in further evaluation and possible repairs before they are returned to service, the airline said.
Southwest said it expects to complete inspections of 79 Boeing 737-300 planes by late Tuesday.
Flight 812 from Phoenix to Sacramento landed safely at a military base in Yuma, Arizona, on Friday after the hole appeared suddenly at about mid cabin.
A Southwest official said the company remains committed to safety and was in compliance with structural inspection requirements for the diverted aircraft.
"What we saw with Flight 812 was a new and unknown issue," said Mike Van de Ven, Southwest's executive vice president and chief operating officer.
"We regret any customer inconveniences as a result of the inspections currently underway," he said in a statement.
"Delays and cancellations are never the preference, however we are taking every precaution we can to ensure that our operation is safe."
The airline canceled 300 flights on Saturday and said it expected to cancel another 300 flights on Sunday as the investigation continued into what caused the five-feet long, one-foot wide tear to develop.
The cancellations are likely to continue for the next few days, said Southwest Airlines spokeswoman Whitney Eichinger.
Airline mechanics will saw out the portion of the plane skin that fractured, and it will be shipped to Washington for further inspection, Sumwalt said.
It will take six to eight hours to remove the piece, which is expected to be about eight or nine feet long and two feet wide and weigh three to four pounds.
Sumwalt said there was evidence of what is known as "multi-site damage."
"We did find evidence of widespread cracking across this entire fracture surface," he said.
Sumwalt said the tear occurred in a concealed part of the plane and could not have been detected by the naked eye. He said the plane was most recently disassembled for heavy maintenance in March 2010, which would have been the last time such a fracture could have been detected.
Recorders from Flight 812 arrived at NTSB's headquarters on Saturday night. They indicated the decompression occurred approximately 18 1/2 minutes after takeoff, Sumwalt said.
The flight crew donned oxygen masks and declared an emergency and the plane descended from its cruising altitude of 36,000 feet to 11,000 feet in approximately 4 1/2 minutes, Sumwalt said.
An inspection of the oxygen generators that supply oxygen to passengers indicated the generators had all been activated, Sumwalt said.