WASHINGTON >> The Federal Aviation Administration said Monday it is proposing a $12 million civil fine against Southwest Airlines for failing to comply with safety regulations related to repairs on Boeing 737 jetliners.

It is the second-largest fine the agency has proposed against an airline. The largest proposed fine was against American Airlines for $24.2 million in August 2010. That one was ultimately settled for $24.9 million as part of American’s bankruptcy proceedings, although the final settlement included other safety violations not part of the original proposal.

The FAA said that beginning in 2006 Southwest made "extreme makeover" alterations to eliminate potential cracking of the aluminum skin on 44 jetliners. An FAA investigation determined that Southwest’s contractor, Aviation Technical Services Inc. of Everett, Washington, failed to follow proper procedures for replacing the fuselage as well as other work on the planes, the agency said. All of the work was done under the supervision of Southwest, which was responsible for seeing that it was done properly, the FAA said.

Southwest, which is based in Dallas, then returned the planes to service in 2009 and began flying them even after the FAA "put the airline on notice that these aircraft were not in compliance" with safety regulations, the agency said.

During its investigation, the FAA also found that Aviation Technical Services’ workers applied sealant beneath the new skin panels but did not install fasteners in all of the rivet holes fast enough for the sealant to be effective.

"This could have resulted in gaps between the skin and the surface to which it was being mounted. Such gaps could allow moisture to penetrate the skin and lead to corrosion," FAA said.

The contractor also failed to follow requirements to properly place the planes on jacks and shore them up while the work was being performed, the FAA said. If a plane is shored improperly during skin replacement, the airframe could shift and lead to subsequent problems with the new skin.

The FAA also said that Southwest failed to properly install a ground wire on water drain masts on two of its Boeing 737s in response to a safety order aimed at preventing lightning strikes. The planes were each operated on more than 20 passenger flights after Southwest Airlines became aware of the discrepancies but before the airline corrected the problem, the agency said.

"The FAA views maintenance very seriously, and it will not hesitate to take action against companies that fail to follow regulations," said FAA Administrator Michael Huerta.

Southwest Airlines has 30 days to respond to the proposed fine. Usually FAA officials negotiate extensively with an airline in cases of large fines before settling upon an amount. However, regulators and airline officials sometimes are unable to reach an agreement and the airline contests the fine.

Brandi King, a spokeswoman for Southwest, said the airline will "respond to the FAA allegations" in accordance with the agency’s procedures.

"Having fully resolved the repair issues some time ago, none of the items raised in the FAA letter affect aircraft currently being operated by Southwest Airlines," she said. "As always, Southwest is committed to continuously making enhancements to our internal procedures, as well as improvements related to oversight of our repair vendors."