Southwest Airlines is accused of conducting repairs to the planes' aluminum skin, with the help of Southwest contractor, Everett, that were not in line with federal rules. The repairs were made by Everett, but, under U.S. law, airlines are responsible for repairs made by contractors.
"Safety is our top priority, and that means holding airlines responsible for the repairs their contractors undertake," U.S. Transportation Secretary Anthony Foxx said in a statement.
"Southwest is committed to continuously making enhancements to our internal procedures, as well as improvements related to oversight of our repair vendors," said a Southwest spokeswoman.
The fine relates to three separate incidents involving Boeing 737 jetliners. In one of the cases, the airline conducted "extreme makeover" alterations to the outer aluminum skin to eliminate potential cracking on 44 planes. The FAA claims that Everett failed to stabilize the planes while placing them on the jacks.
If the plane is not stable it could shift during repairs and compromise the integrity of the new skin, according to the FAA.
"Southwest returned the jetliners to service and operated them when they were not in compliance with federal aviation regulations," the FAA said.
In another instance, the contractor failed to install fasteners in all rivet holes while applying sealant. The error could have resulted in gaps between the skin and the surface to which it was being mounted.
Southwest has 30 days from receipt of the suit to respond to the FAA.