The US Federal Aviation Administration (FAA) has proposed a $12 million fine against Southwest Airlines for violating regulations in three instances related to maintenance work on its Boeing 737s.

The airline had conducted repairs beginning in 2006 on 44 aircraft to prevent potential cracking of the aluminium fuselage skins. Everett, Washington-based MRO firm Aviation Technical Services (ATS) was contracted to perform the work.

"Investigators determined that ATS failed to follow proper procedures for replacing the fuselage skins on these aircraft. FAA investigators also determined that ATS failed to follow required procedures for placing the airplanes on jacks and stabilising them. All of the work was done under the supervision of Southwest Airlines, which was responsible for ensuring that procedures were properly followed," says the agency.

It adds that the airline had returned the aircraft to service while the aircraft were not in compliance with federal aviation regulations, and had operated "numerous flights" in 2009. The agency says it subsequently approved the repairs when the airline provided proper documentation.

The FAA alleges that ATS workers had applied sealant under the new fuselage skin panels but did not install fasteners in all of the rivet holes for the sealant to be effective. Such an omission could have resulted in gaps between the skin and the fuselage surface, which could have led to moisture penetrating the skin and to corrosion.

ATS workers also failed to properly place the aircraft on jacks while the work was being performed. "If a plane is shored improperly during skin replacement, the airframe could shift and lead to subsequent problems with the new skin," says the agency.

In addition, the FAA alleges that the airline failed to properly install a ground wire on water drain masts on two 737s in response to a FAA airworthiness directive concerning lightning strikes on these parts. The aircraft were each operated on more than 20 passenger flights after the airline became aware of the problem.

The airline has 30 days from the receipt of the FAA's civil penalty letter to respond, says the FAA.

In response to the FAA's proposed penalty, Southwest tells Flightglobal that the FAA letter "concerns repair issues that were addressed several years ago."

"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," it adds. "Safety is paramount and we always strive for full compliance with established and approved processes and procedures."

The airline will respond to the FAA's allegations in accordance with procedures, it says.

Source: Cirium Dashboard