Airlines Slash Discount Fares on Fall Flights : Travel: The cuts of 25% to 45%, initiated by Northwest and expiring Friday, are among the season’s largest.

The nation’s airlines Monday cut discount air fares 25% to 45% to perk up what is traditionally a slow travel season.

Although carriers have already held several promotions for fall travel, the current sale, which expires Friday, offers travelers some of the largest discounts of the season.

“It will stimulate demand because anytime you get prices this low you get people interested,” said Thomas Nulty, president of Santa Ana-based Associated Travel Management.

Northwest Airlines launched this fare war, which includes destinations throughout the United States, Mexico and the Caribbean. American, United and other major carriers matched Northwest on competing routes.

The non-refundable tickets must be purchased at least two weeks before departure and require a Saturday night stay. Travel must be completed by Dec. 16. However, the sale does not apply to flights on Nov. 24 and 28--two of the most heavily traveled days because of Thanksgiving.

Under the sale, the deepest discounts apply to travel on Tuesdays, Wednesdays and Saturdays. A sample of the lowest round-trip prices from Los Angeles include $335 to Boston, $318 to Minneapolis and $335 to Honolulu.

“Historically, the fall season brings with it airline fare sales,” Michael E. Levine, executive vice president of marketing at Northwest, said in a statement. “These fares provide value to travelers who are unlikely to see fall fares at these levels again.”

Northwest’s sale foiled--for now--the airline industry’s $20 increase on discounted round-trip tickets purchased two weeks in advance. Trans World Airlines, which led the pack in raising fares over the weekend, said it plans to hike prices Saturday after the current sale expires.

While TWA is trying to raise prices, the airline is also reportedly planning to increase the frequent-flier miles passengers earn while traveling this fall.

Harold Sirkin, a transportation specialist at the Boston Consulting Group, said the current crop of airline promotions will attract only a small number of passengers. Many travelers have bought tickets during previous sales, he said.

What the sale will do, he said, “is continue to cause the consumer to recognize that there will always be a sale. They will wait until the sale comes.”

At Seaside Travel in Long Beach, travel agents were busier than usual as the fall sale began, said owner Ada Brown.

“Everybody is waiting for the next fare war,” Brown said. “I don’t know how the airlines are ever going to turn that around.”