The US Federal Aviation Administration is investigating how Southwest Airlines tracks the weight of checked bags on its flights.
The FAA began its probe in February 2018, the FAA said told CNN Business on Monday.
"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 performance data," the FAA said.
The Wall Street Journal first reported the story. It cited FAA officials and agency documents that indicated airline employees made mistakes that caused pilots to compute the wrong weight of planes at takeoffs.
The errors were described as "systemic and significant," occasionally causing the reported takeoff weights to be 1,000 pounds lower than the plane's actual weight.
According to the FAA's weight & balance handbook , a safety guideline for airplane operators, excess weight on an aircraft can cause a variety of problems, including the need for a higher takeoff speed, reduced cruising speeds, decreased maneuverability and higher stress imposed on landing gear.
Southwest said in a statement to CNN Business that there's an "ongoing effort to track and voluntarily report operational data to the FAA so that we can mitigate and eliminate any operational risks."
But the airline added that it has already put in place controls and procedures to address weight and balance issues. Southwest said it has shared those measures with the FAA.
"Southwest believes the controls and procedures we implemented throughout 2018 have enhanced our weight and balance program and resolved the issues that we originally reported to the FAA," the airline added.
The FAA has not linked any accidents to the weight discrepancies. But the agency said in its statement to CNN Business that it "will not close its investigation until it is satisfied that Southwest's corrective actions are consistent and sustained."
The news of the FAA probe also comes as Southwest is grappling with an "operational emergency" due to a labor dispute with mechanics.
Southwest said in a statement to CNN Business last Friday that the percentage of "out-of-service aircraft" with maintenance issues in its fleet had more than doubled in the past week.
"We are requiring all hands on deck to address maintenance items so that we may promptly return aircraft to service," a spokesperson for Southwest said in an email at the time.