ST. PETERSBURG, Fla (Reuters) - Evidence of widespread fuselage cracks and fatigue were found on a Southwest Airlines Co jet that made an emergency landing in Arizona with a hole in the cabin, a U.S. safety investigator said on Sunday.
The incident on Friday prompted Southwest, the largest domestic airline by passengers flown, to ground planes and cancel hundreds of flights over the weekend so it could inspect its older model 737-300s.
Small subsurface fuselage cracks were found on two other planes, which may require repairs, Southwest said. Nineteen had been inspected and returned to service without any problems by Sunday.
Southwest anticipated 175 flight cancellations on Monday and hoped to complete the remaining inspections of 79 planes by late Tuesday.
The 737-300 represents roughly 20 percent of Southwest's all-737 fleet, the most popular commercial aircraft ever and a workhorse globally.
So far, the problem has been limited to Southwest, which paid a $7.5 million Federal Aviation Administration (FAA) fine for operating 737s without required fuselage structural inspections in 2006/07.
US Airways Group flies 19 737-300s and a spokesman said periodic inspections have turned up no fatigue-related problems.
Continental Airlines, now a unit of United/Continental Holdings, Delta Air Lines and American Airlines, a unit of AMR Corp, all fly newer model 737 that are not linked to the problem.
United Airlines does not fly any 737s.
The FAA is assisting the National Transportation Safety Board (NTSB) in the Southwest investigation of Flight 812, which was heading from Phoenix to Sacramento at 36,000 feet when a 5-foot tear opened in the fuselage 20 minutes after takeoff.
The plane landed safely at a military base in Yuma, Arizona, minutes later.
NTSB member Robert Sumwalt said at a news conference in Yuma on Sunday that post-flight inspections revealed "widespread cracking across" the damaged area.
"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," he said.
A Southwest official said the airline was in compliance with inspection requirements for the plane.
"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.
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.
(Reporting by Colleen Jenkins and John Crawley in Washington; Editing by Greg McCune and Paul Simao)