Southwest, which is based in Dallas, then returned the planes to service in 2009 and they were flown about 100,000 times. About a third of the flights occurred after the FAA "put the airline on notice that these aircraft were not in compliance" with safety regulations, the agency said. The FAA said it later approved repairs to the planes after the airline provided proper documentation that the repairs met safety standards.
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."
In 2009, Southwest settled on a fine of $7.5 million with the FAA for allegedly operating Boeing 737s on nearly 60,000 flights without performing mandatory inspections of the fuselage for cracking. The FAA had originally recommended more than $10 million in penalties.
In April 2011, a hole opened in the roof of a Southwest Boeing 737 with 118 people onboard, forcing an emergency landing Yuma, Arizona. The FAA said at the time that the plane wasn't among the 44 planes improperly repaired at Aviation Technical Services. The National Transportation Safety Board later blamed a manufacturing flaw involving the installation of rivets, saying it caused a section of fuselage skin in place to peel open.