AIRLINE INDUSTRY : Competitors Match Northwest’s 45% Cut in Discounted Fares : Travel: In another price war, Alaska launches sale in response to promotion by Reno Air.

Major airlines Tuesday followed the lead of Northwest Airlines and cut discount fares by up to 45% for travel through much of the United States, Canada and the Caribbean.

Steep fare sales are common in the winter, when travel drops off and airlines try to fill empty seats.

“It’s January--there must be an air fare war,” said Thomas Nulty, president of Santa Ana-based Associated Travel Management. “The airlines can’t get by without having one.”

The biggest discounts apply to travel between Jan. 18 and Feb. 16 and for flights Monday through Thursday. The discounts are smaller on other days and for trips between Feb. 16 and April 15.

The discount tickets, which must be purchased by Jan. 11, require a Saturday night stay and must be bought at least two weeks in advance. The supply of discounted seats is limited.

“We wanted to get (travelers) thinking about the sun and ski destinations they have been wanting to see,” said Northwest spokeswoman Susan Halstead.

American, America West, Delta, United and USAir matched the Northwest sale to varying degrees. American, for example, applied the cuts to all its destinations in the continental United States but only to selective routes to Canada, Hawaii and the Caribbean where it competes with Northwest.

Under the sale, the price of a discounted, off-peak Los Angeles-New York round trip falls from $628 to $338 for travel by Feb. 16 and to $377 for trips between Feb. 17 and April 15. A Los Angeles-Dallas round trip--normally discounted at $491--will cost as little as $282 through Feb. 16, then rise to $306.

In another air fare war, Alaska Airlines cut discount fares on many of its major routes in response to a promotion by Reno Air. The non-refundable tickets, which must be purchased by Jan. 18, include round trips between several Southern California airports and Seattle and Portland for between $159 and $179. The sale applies to a limited number of seats and requires passengers to purchase tickets two weeks in advance.

Some travel agents said customers have grown accustomed to waiting for the post-New Year’s fare wars before buying a ticket.

“It’s almost gotten to be predictable, and that’s a problem,” said Ada Brown, owner of Seaside Travel in Long Beach. “When it’s predictable, people wait.”