DALLAS (CBSDFW.COM/AP) — The Federal Aviation Administration is seeking to fine Southwest Airlines $3.9 million for failing to properly calculate the weight of planes and make sure that loads were balanced before more than 21,500 flights in 2018.

The FAA alleges between May 1, 2018, and August 9, 2018, Southwest operated 44 aircraft on a total of 21,505 flights with incorrect operational empty weights, and center of gravity or moment data.

Weight is used to determine the maximum number of passengers on a plane and how much fuel it should carry, and cargo must be balanced to avoid have too much weight in the front or back, which can affect the way a plane flies.

A Southwest Boeing 737 lands at Los Angeles International Airport on May 24, 2018. (credit: Daniel Slim/AFP/Getty Images)

The FAA alleges that Southwest’s operation of these aircraft was contrary to the airline’s approved weight-and-balance program and FAA-issued operations specifications.

Southwest has 30 days to respond to the FAA’s proposed civil penalty.

Airlines often try to negotiate down the size of FAA fines.

(© Copyright 2020 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)