Tuesday's sale appeared designed to fill seats on planes after the peak summer travel season ends and families return home for the start of the school year.
Dallas-based Southwest and AirTran said they were charging $49 each way on flights of 500 miles or less, $99 each way for 501 to 1,300 miles, and $139 each way for trips longer than 1,300 miles.
Southwest and other airlines have raised their base fares more than a dozen times since the start of 2011 to cope with rising prices for jet fuel -- their top expense. But government figures show spot prices for jet fuel have dropped by nearly one-fifth since April 1.
