The Federal Aviation Administration on Monday proposed a $12 million fine against Southwest Airlines for failing to comply with safety regulations related to repairs on its fleet. Beginning in 2006, the Dallas-based airline conducted “extreme makeover” alterations to eliminate potential cracking of the aluminum skin on 44 Boeing 737s, the FAA said. An FAA investigation found that both the airline and its contractor, Aviation Technical Services, Inc, of Everett, Washington, failed to follow procedures for replacing the fuselage as well as other work on the planes.