Southwest canceled about 4 percent of its flights Wednesday after deciding to perform new inspections on 43 Boeing jets, including 38 that had been carrying passengers Tuesday.
Southwest spokesman Whitney Eichinger said 34 planes had been returned to service as of Thursday, when the airline returned to a regular schedule.
Eichinger said the airline also inspected five other planes that had been undergoing unrelated maintenance Tuesday when the decision to conduct a new round of inspections was made. No cracks were found on those jets, she said.
The company offered no other details on the repair work.
The repair of four planes was the latest development in the airline's struggle to comply with safety directives issued by the Federal Aviation Administration and instructions from Boeing Co. The latest round of groundings occurred after Southwest decided it couldn't tell whether it had followed Boeing's steps for visual and mechanical inspections of the skin of the aircraft.
Last week, the FAA announced a $10.2 million penalty against Southwest for missing inspections on 46 planes last year and continuing to fly the planes after realizing that they had not been properly examined for structural soundness.
Southwest officials have said that six planes were repaired for cracks last year.
The events have raised questions about Southwest's maintenance program and the FAA's ability to regulate carriers. Both have come under fire from members of Congress.
Eichinger, the airline spokeswoman, said the events did not seem to be affecting ticket sales on Southwest, which carries more domestic passengers than any other U.S. airline.
"We are continuing to see strong bookings," she said. "We've had questions from our customers ... but there hasn't been any noticeable drop in bookings."
Southwest has said it is conducting an internal investigation into why it failed to perform inspections that were required under a 2004 FAA order called an airworthiness directive. It also suspended three employees with pay.
Dallas-based Southwest has said it will appeal the FAA penalty, the largest ever imposed by the agency. Chief Executive Gary Kelly said Southwest, which has a fleet of more than 500 Boeing 737 jets, is a safe carrier.
Southwest shares rose 21 cents, or 1.8 percent, to $11.70 Thursday after falling 7.3 percent Wednesday.