US-based Southwest Airlines Co, the only major US carrier that doesn't assign seats, will test new boarding methods to reduce the time its jets sit idle between flights.

The tests follow the airline's recent experiment with assigned seating and will begin this week on selected flights from San Diego. Dallas-based Southwest has used a first-come, first-served boarding system since it started flying in 1971, Bloomberg said.

"They're trying to give customers the service they want to maintain market share," Jim Corridore, a Standard and Poor's analyst in New York, said in an interview on October 13. "At the same time, they don't want to jeopardize quick boarding, because it's been their bread and butter."

The new tests will be a variation of Southwest's practice of putting passengers into three boarding groups based on check-in times and letting them pick seats once they're on the plane, spokeswoman Beth Harbin said. The test groups' size and order will differ from what Southwest uses now, she said. She declined to be more specific.

"We are going to be taking a look at a couple of variations more along the lines of how you organize customers for boarding," Harbin said. "Can you enhance customer service and can you be more efficient and reduce boarding time?"

Southwest wants to avoid increasing the average 25 minutes its planes spend at gates between flights. The assigned-seating tests that began in July produced mixed results, the airline has said.

Southwest wanted to see whether the change would accelerate boarding and appeal to more customers. The lack of assigned seats is a chief complaint from some fliers, while others value being able to select seats as they board, Southwest has said.

A possible switch to assigned seats "is still on the table," Harbin said.

In addition to Southwest, at least three other airlines recently have changed boarding methods or tested new procedures in attempts to boost efficiency. Reducing time at airport gates can allow airlines to add flights without adding planes.

"Any time spent parked at the gate is less time they can spend in the air," Corridore said. "Added up over time, it can be an additional flight a day."