Southwest Airlines has been under investigation by the Federal Aviation Administration for a year, with the probe focused on its aircraft weight and balance calculations, a critical step before takeoff.

The investigation, disclosed for the first time Monday by the Wall Street Journal, began in February 2018, FAA spokesman Gregory Martin confirmed to USA TODAY. There have been no fines nor enforcement action from the investigation to date.

"Since that time, the FAA has directed the development of a comprehensive solution to the methods and processes used by Southwest Airlines to determine this (weight and balance) performance data,'' he said in a statement. "The FAA will not close its investigation until it is satisfied that Southwest’s corrective actions are consistent and sustained.''

Southwest spokeswoman Brandy King said the airline's approach to calculating weight and balance — a detailed process that factors in the weight of bags, cargo and passengers and where they are positioned on the plane to help determine things like fuel needed and takeoff and landing speeds — has served it well for its 47-year history.

She said Southwest switched to a new system, approved by the FAA, in August 2017 to improve reliability, efficiencies and accuracy, and made more changes in 2018. This year, following a test at three airports in 2018, Southwest plans to start scanning each bag as it's loaded onto the plane, a process in place at other airlines. The airline's baggage handlers have been manually counting bags because Southwest's technology priority the past few years was its new reservations system.

"While no system can ever be perfect, our weight and balance program is a proven, reliable system with checks and balances installed to ensure safe operations by any measure or standard,'' King said in a statement.

The Journal's report, based on internal FAA documents and interviews with unnamed FAA officials, says as many as one third of Southwest's 4,000 daily flights could have operated with inaccurate weight data during certain periods due to the manual counting of bags by baggage handlers instead of computerized scanning. Southwest said there is no evidence to support that number. King also notes that safety margins are built into its system "to allow for the possibility of discrepancies and to assure performance limitations are never compromised.''

Why are aircraft weight and balance so important?

Weight and balance calculations on any type of plane are critical. Weight is the total weight of passengers, bags and cargo, while balance is how it's all proportioned on the plane.

There are significant risks to having an overweight or improperly balanced plane, especially in case of an emergency, said Brent Bowen, professor of aeronautical science at Embry-Riddle Aeronautical University in Prescott, Arizona.

He uses US Airways Flight 1549, the plane that successfully landed on the Hudson River 10 years ago after a bird strike knocked out both engines, as an example.

"If that plane had been overweight, it could have stalled and spun in (to the Hudson River),'' he said.

Similarly, a plane that is too heavy can cause problems if it skids toward the end of the runway during a snow or ice storm, Bowen said.

"If the plane is overweight it will have difficulty stopping,'' he said.

Weight and balance is also critical during extreme heat, such as the triple-digit temperatures in Phoenix in the summer.

"It makes the air feel like it's at a higher altitude,'' he said. "I don't think I would be comfortable using a (baggage weight) estimate in extreme conditions.''

The Journal said the investigation was initiated after allegations were raised by a whistle blower and on an FAA hotline for safety complaints.

Southwest, the nation's largest domestic carrier, says it voluntarily alerted the FAA to baggage weight discrepancies.

In early 2018, when the investigation was launched, the Journal said, "the FAA told the carrier 'there have been numerous reports of ground operations personnel and/or flight operations personnel not following Southwest Airlines procedures for entering correct and complete weight information' before takeoff. One early document called it a “high-risk concern.”

"The inaccuracies prompted swift FAA responses, and were considered serious enough to require Southwest to physically audit the number of bags unloaded daily from 25 percent of its flights,'' the Journal said. "That sample was later reduced, at Southwest’s request, to 15 percent. The airline continues to provide daily status reports to the agency.''

In the wake of the Southwest investigation, Bowen, who also publishes the annual Airline Quality Rating, said he plans to incorporate the topic of weight and balance in a new airline safety report card due out later this year.

News of the FAA investigation comes on the heels of an unrelated operations "emergency'' Southwest executives declared last week due to a higher than usual number of planes out of service. The airline, which has been in prolonged contract negotiations with its mechanics, ordered all maintenance hands on deck until the situation is resolved and said mechanics "alleging sickness'' will be required to provide a doctor's note.

And it comes as Southwest is in the final stages of seeking FAA certification to begin flights to Hawaii.