Here they come again.

Playing another variation of the bargain fares tune, the nation's major airlines are trumpeting another group of those bargain-basement fares that last year filled millions of empty airline seats and sent the carriers running to the bank.

It all sounds like a white sale at Woodies, the Hecht Co. or J.C. Penney. And what the airlines are doing is similar to a white sale: They're selling goods -- is this case empty seats -- that few people usually want.

Trans World Airways was the first carrier to introduce the newest bargain-basement concept in which travelers can get extra discounts, and take a companion for 50 percent off or a child under 12 for free during February and March, traditionally the airline industry's worst two sales months.

"The reason for this is really simple." said American spokesman Al Becker. "We're trying to increase traffic during a time which is historically a slow period for us. Our business is very seasonal, there are a lot of peaks and valleys.... We think the Super Saver fares are the answer because it has been those fares that have had the capacity to generate new business to fill empty seats."

Although prospective travelers should consult with the airlines or a travel agent to find out what the benefits and restrictions are on specific flights, most of the new fares work something like this:

The Super Saver fares now offer larger discounts and fewer restrictions. Narmally, super saver rates give travelers a 30 to 50 percent discount, but the February-March discounts will be 35 to 55 percent, or an average of 5 percent more. Passengers had been required to make reservations at least 30 days in advance but now can book flights as little as 7 days before traveling. Also, the minimum-stay requirement has been lowered from 7 days to staying until the next Sunday, making fares available for weekend trips.

Travelers paying full fare can take an adult companion for 50 percent of the regular coach fare. The companion must stay until the following Sunday but not more than 7 days.

Some carriers are allowing children under 12 years old to fly for free when accompanied by an adult who has purcased a regularly priced coach seat.

The savings can be impressive.

On a TWA round-trip flight between New York and Los Angeles, a family of four with two children under age 12 can fly for as little as $450, which is $1,000 off the normal coach fare. Similarly, the same family can fly round-trip to Dallas from Washington on Braniff International for $538, a 37 percent savings over the $854 regular coach fare.

Passengers not traveling with family members also can benefit. American's round-trip sale fare from Washington to Los Angeles on a week day is $235 compared with a regular super saver fare of $257 and a standard coach fare of $428.

Other major airlines offering similar packages or meeting their competition in certain markets include Continental, Eastern, Delta and North-west. National Airlines and Pan American World Airways are not having any comparable air fare sale.

Airline officials say they hope to attract a wide range of travelers, including business customers who may choose to take their spouse or children on a company trip or families that may decide to go on vacation early.

Harsh winter weather in the North and Midwest may make the fares even more attractive to those wishing to take an early vacation in a warmer part of the country, airline officials say.

The airlines say they do not know if the "white sale" will be successful, but because February and March are normally such slow months for the carriers, they have plenty of excess seating with which to experiment.

"This whole thing is an attempt to experiment with low fares during an off part of the year," said Ron Miller, director of pricing and market research at Chicago-based United Airlines. "February and March are really bad times for the airlines. When you give away seats for free to kids and offer deeper discount fares, you don't stand as much to lose."

For instance, on its Dallas to Washington route, Braniff reported that its planes were on the average 53 percent full during February and March. However, during July and August, the peak of the summer traveling season, load factors on the same route average 60 percent.

Airline executives explained that although the discounts are greater and the restrictions lighter, they hope additional seats can be filled without pulling too many regular-paying customers to discount seats.

"These are marketing judgments," American's Becker said of the restrictions. "What you are trying to do is fill empty seats without diverting too much of the traffic that would be flying with you anyway. The best way to do this is to create some restrictions on certain fares."

For this reason, most of the carriers have limited discounts on their domestic routes, and United Airlines has excluded its flights to Hawaii and Florida, routes that normally draw heavy passenger loads during the winter months.

The airlines have been promoting the new fares with newsparper and magazine advertising and television commercials. Most of the carriers advertise most heavily in the cities where they have the most flights.

Whether the airlines will use the "white sale" approach during other slow periods remains to be seen.Most airline officials say they will have to wait and see if this sale is successful before deciding whether the technique would work at other times of the yaer.

"Let's see if this works," TWA spokesman Angus McCoure said when asked if his airline would repeat the technique. "I don't think it will be extended past March, as this really is our low period. The other 10 months of the year aren't that tough on us, but in the current cli