Western Airlines today begins a new promotion that will allow passengers flying full-fare on a 400-mile trip to take another trip--to Hawaii and back for $100.

The new "Mahalo" fare plan is designed to increase passenger traffic on Western flights from now until Feb. 28, a time when airline passenger traffic traditionally is slow and is already suffering because of the recession. The Mahalo fare--the Hawaiian word means "thanks"--also will reward full-fare passengers for flying on Western, an official of the airline said.

Under the fare plan, a traveler must fly 400 miles or more round trip on Western using a full-fare coach or first-class ticket between now and Feb. 28. That entitles the traveler to take another trip--to Hawaii --for $100 round trip. The regular round trip coach fare to Hawaii on Western is currently $996, a Western official said. The trip to Hawaii must begin by May 27.

As an example of how the plan works, a Washington-area traveler could get the $100 trip to Hawaii by flying to Minneapolis and back on Western. The regular economy coach ticket to Minneapolis is $348 round trip on Western.

A Western official said the fare program, which will be marketed aggressively, is part of an ambitious comeback strategy for the Los Angeles-based airline, which is estimated to have lost $60 million in the fourth quarter of 1981.