Evidence of widespread fuselage cracks and fatigue were found on a Southwest Airlines jet that made an emergency landing in Arizona with a hole in the cabin, a US safety investigator said.
After inspection of all the airline's older model 737-300s, small subsurface fuselage cracks were also found on two other planes, which may require repairs, Southwest said on Monday.
The incident on Friday prompted Southwest, the largest domestic airline by passengers flown, to ground planes and cancel hundreds of flights over the weekend.
Nineteen 737-300s 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 per cent 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 $7.5m 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.
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 to land at a military base in Yuma, Arizona.
The Southwest flight had 118 passengers and five crew members on board. One flight attendant and at least one passenger were treated for minor injuries at the scene, Southwest said.
Passengers described the harrowing scene to the CBS television affiliate in Sacramento.
"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 long. You could see the insulation and the wiring. You could see a tear the length of one of the ceiling panels."
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.