The Federal Aviation Administration (FAA) is proposing a $12 million fine against American airline Southwest over negligence in complying with FAA-mandated repairs to Southwest's aircraft, the FAA stated in a recent press release.

The fine comes after an investigation by the FAA into Southwest's 'makeover' of its planes that included its engineering contractor, Aviation Technical Services, Inc. (ATS) Federal investigators determined that engineers installing new fuselage skins into the aircraft followed improper procedures, along with failing to properly place the airplanes into stabilizers during their upgrades.

Southwest was supervising the repair work, and thus had the responsibility of ensuring that proper procedures were followed.

The FAA alleges that Southwest then used these airlines in service when they were in violation of FAA Airworthiness Directives. When Southwest provided proper documentation as it pertained to the repairs in question, the FAA later approved the repairs.

The investigation turned up some worrying results: ATS engineers applied sealant beneath the new skin panels but did not install fasteners into all of the rivet holes, meaning that the sealant would not have been completely effective. This would allow moisture to enter into the fuselage, which could corrode parts and damage the aircraft mid-flight.

In addition, because the airplanes were not placed properly on jacks, the frame of the vehicle could shift and lead to problems with the new skin.

A third case alleges that Southwest failed to properly install a ground wire on the water drain masts of two of its Boeing 737 airplanes. The ground wire is necessary for when lightning strikes an airplane, which allows for the electrical current to pass along the skin of the aircraft without interfering with the sensitive computers and electronic devices on which the aircraft runs.

Southwest used the planes in violation of FAA safety codes on more than 20 passenger flights after the airline became aware of the issues but before the aircraft in question were grounded and repaired properly.

The airline has 30 days to respond to the allegations.