DALLAS — Southwest Airlines grounded 41 planes overnight — about 8% of its fleet — in the wake of its recent admission that it had missed required inspections of some planes for structural cracks.

Southwest shares fell 4% in midday trading.

The move announced today comes as Southwest faces a $10.2 million civil penalty for continuing to fly nearly 50 planes after the airline told regulators that it had missed required inspections of the planes.

The Federal Aviation Administration, which announced the penalty last week, has also come under fire for failing to immediately ground the Southwest jets when it learned they had not been inspected for cracks in the fuselage.

A Southwest spokeswoman, Christi Day, said today that the move to ground 41 planes resulted in some flights being canceled, although she didn’t have a precise figure.

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.

Southwest Chief Executive Gary Kelly had said yesterday he was concerned by findings from an internal investigation into the missed inspections. He announced that the Dallas-based company had placed three employees on paid leave while it investigated the situation.

The acting FAA administrator, Robert Sturgell, called the events “a twofold breakdown in the aviation system” — first, Southwest’s failure to properly inspect its planes; and the FAA’s failure to ground the jets as “at least one FAA inspector looked the other way.”

The $10.2 million penalty is the largest the FAA has ever imposed on a carrier. Southwest has said it will appeal.

Its shares fell 51 cents, or 4.1%, to $11.89 in afternoon trading today. That is near the lower end of their 52-week range of between $11.02 and $16.96.