Southwest Airlines was sending all of its 737-300s to Boeing for detailed inspections.
The new small, subsurface cracks are similar to the one suspected of causing one plane's fuselage to tear last Friday while it was in the air.
Flight 812, from Phoenix to Sacramento, was forced to land from 34,000 feet in the sky at a military base in Yuma, Ariz., due to the hole, which measured about 5-feet long in the roof of the passenger cabin. The pilots declared an emergency and brought the plane down 25,000 feet in less than five minutes. No one was hurt but many of the 118 passengers on board were startled by what happened.
Since then, Southwest grounded all 79 of its 737-700 aircraft for inspections. In all, 600 flights were canceled over the weekend.
The Federal Aviation Administration was investigating the mid-flight rupture. A piece of the fuselage will be examined, along with flight recorders.
Inspectors looking at the damaged plane found fatigue cracks in the metal near the area where the plane ripped open. It was not clear how those cracks got there or what role they played. Another concern was that the plane was damaged while being painted.
Southwest said it followed all the required safety checks, but the National Transportation Safety Board said the area around the tear shows cracks that wouldn't have been discovered during routine maintenance.
Passengers say the flight cancellations during the safety inspections are annoying but understandable.
"It's an inconvenience, but as I said, I'd rather have Southwest be safe than sorry, you know, and their equipment be flying well. Nobody wants to crash," said Southwest passenger Debby Martin.
"Obviously they're searching for the right thing, so I think they're doing an OK job. And they landed the plane just fine, so I'm not too worried about it," said Southwest passenger Brandon Bevans.
The incident raises new questions about the vulnerability of the nation's air fleet to fatigue cracks. Federal aviation officials are considering an order for other airlines to inspect their aircraft.
So far, nineteen inspected aircraft showed no problems and will be returned to service. Checks on the remaining jets are expected to be completed by late Tuesday, the airline said.
If you have plans to fly the airline this week, make sure you check your flight before heading to the airport. For flight information, call 1-800-I-FLY-SWA or check travel alerts online at http://www.southwest.com/.
Meantime, a separate problem emerged for a Southwest flight headed from Oakland to San Diego.
The flight had to make an emergency landing in Los Angeles Sunday night after reports of a burning electrical smell. That issue was being investigated.
Passengers were put onto another plane and flown to San Diego without any problems.
The Associated Press contributed to this report.