The FAA today said it planned to levy a $10.2 million civil penalty against Southwest Airlines for operating 46 airplanes that hadn’t undergone mandatory inspections for fuselage fatigue cracking. Subsequently, the airline found that six of the 46 airplanes had developed fatigue cracks.

“The FAA is taking action against Southwest Airlines for a failing to follow rules that are designed to protect passengers and crew,” said FAA Associate Administrator for Aviation Safety Nicholas Sabatini. “We expect the airline industry to fully comply with all FAA directives and take corrective action.”

The FAA alleges that from June 18, 2006, to March 14, 2007, Southwest operated 59,791 flights without performing repetitive inspections of certain fuselage areas on its Boeing 737s to detect fatigue cracking as required by a Sept. 8, 2004 Airworthiness Directive. The agency also alleges that after the airline discovered that it had failed to perform inspections, it continued to operate the same 46 aircraft on another 1,451 flights between March 15, 2007 and March 23, 2007. The FAA said in a statement that the amount of the civil penalty reflects the serious nature of those “deliberate” violations.

The AD in question mandated repetitive external detailed and eddy-current inspections at intervals of no more than 4,500 flight cycles to detect fatigue cracking in areas of the fuselage skin on some Boeing 737 models.

Southwest Airlines has 30 days from receipt of the FAA’s civil penalty letter to respond to the agency.