DALLAS — Southwest Airlines Co. canceled flights Wednesday and temporarily grounded 43 planes to examine if they were sound enough to carry passengers, the latest twist in the low-cost carrier's saga of missed safety inspections and civil penalties.

The groundings affected about 8 percent of Southwest's fleet, and came as the airline faces a $10.2 million civil penalty for continuing to fly nearly 50 planes that hadn't been inspected for cracks in their fuselages.

Southwest shares fell more than 9 percent before closing down 7.3 percent.

Since the Federal Aviation Administration announced the penalty last week, Southwest has endured a steady drip of bad publicity, which is unusual for the nation's most consistently profitable carrier and one that has never had an accident that killed passengers or crew members.

On Wednesday, word filtered out that the airline had taken 38 planes out of service, along with five others that were already in hangars undergoing routine maintenance. That's about 8 percent of Southwest's fleet.

Spokeswoman Linda Rutherford said Southwest took the action after getting clarification from manufacturer Boeing Co. on Tuesday night about the type of inspection — visual or magnetic, or a combination of both — needed for areas around the windows on some older Boeing 737-300 and 737-500 jets.

By late Wednesday afternoon, Rutherford said, 25 planes had undergone the 90-minute inspection at maintenance bases in Dallas, Houston, Chicago and Phoenix and returned to service.

Rutherford told the Deseret Morning News that none of the Southwest planes had been grounded in Salt Lake City.

Rutherford said the remainder of the 38 taken off tarmacs were expected to be back flying by Wednesday night. A 44th plane covered by the Boeing instructions had already been retired, she said.

Southwest had canceled 139 flights by late Wednesday afternoon, or about 4 percent of its scheduled flights for the day, according to Flightstats.com, which tracks airline operations.

The company said it had 520 Boeing 737 jets at the end of last year. Nearly 200 of them are older models, the Boeing 737-300, that were supposed to undergo extra inspections for cracks in the fuselage.

The FAA said Acting Administrator Robert A. Sturgell met Wednesday with Southwest Chief Executive Gary Kelly, who gave a briefing on the steps the airline is taking to comply with inspection orders. The FAA is conducting its own review.