EVEN before autumn is officially here, airlines are touting their ''fall sales.'' This week, Continental, Trans World Airlines (TWA), and Northwest started fare discounts that should give bargain-hunting travelers a lift through December. The other major airlines are expected to join in shortly, since this is the unofficial start of the airline-discounting season. But even with the discounts, air fares are higher than last year, and travelers who can't comply with the sales' small print will be shelling out more for regular fares than in the past. ''It's a good deal for consumers if you can plan that far in advance,'' says Lee Howard, president of Airline Economics International Inc. in Arlington, Va. The three airlines initiating the season's sales are offering flyers up to 50 percent off on tickets purchased no later than tomorrow. Travel can take place through Dec. 15. There are some restrictions on which days bargain flyers can head for the runways. Says Tom Parsons, editor of Best Fares Discount Travel Magazine in Arlington, Texas, ''Some of the best travel bargains of the year are now available for fall getaways.'' For example, a New York to Honolulu airfare has suddenly dropped from $980 round trip to $570 round trip. For consumers who have not firmed up their plans yet or miss the airline promotions, there are likely to be still more fare sales. ''It happens every year. When the passenger traffic falls off, the kids go back to school, and the airlines are in the doldrums,'' Mr. Howard says. But, he points out, future discounts may not be as steep. Despite the discounts, consumers should expect to pay more money for their tickets than they paid for last year's bargains. For example, a trip from New York to New Orleans will cost $250 round trip on Continental, compared with $218 during last year's fall sales. And consumers who don't buy the discounted tickets can expect to pay higher prices this fall and winter. ''The airlines are attempting to take advantage of the demand for unrestricted transportation,'' says Howard, ''so people who don't want to stay over on weekends get it socked to them.'' For example, an unrestricted ticket from Dallas to Boston costs $1,214. But if an individual buys a discounted ticket, requiring a Friday night stay, the cost will only be $322. ''I don't know of any hotel room in Boston that sells for $900,'' Mr. Parsons says. The discounts come at a time when the airlines are flying high because of a relatively strong economy with plenty of leisure time travel. ''It's a banner year - the best in five years,'' says Kevin Murphy, an airline analyst with Morgan Stanley & Co., a New York investment banking firm. In some parts of the country, Parsons points out, there are even better deals because of real fare wars between upstart airlines and the major carriers. For example, Southwest Airlines, which has its own 50 percent off sale, now flies between Baltimore and Chicago for $94 round trip. An unrestricted fare on American Airlines is $386. Cut in half, it's $193. Another new airline, Air South, is offering the cheapest fare in the country - $44 round trip between Columbia, S.C., and Atlanta. This is almost cheaper than driving the 192 miles between the two cities. There can be similar savings by using less popular airports. For example, an unrestricted airfare from LaGuardia to Dallas is $572 round trip; from Dallas to Newark, round trip is only $199. ''It pays to look for the upstarts and alternate cities,'' Parsons advises. For travelers who want to take advantage of the latest round of discounts, it's worth reading the fine print. Tickets must be purchased within 24 hours of making a reservation, at least seven days prior to departure, and as much as 14 days on some destinations. There are restrictions, such as departures no earlier than noon Monday or noon Thursday or all day Saturday. Most of the airlines require an overnight stay on Friday or Saturday. Tickets are usually nonrefundable. Thanksgiving holiday travelers will also find the tickets are not good for vacations between November 22 to 26. To get the best deal for travel around the holiday, Parsons recommends leaving on Sunday through Tuesday before the holiday and returning the following Thursday, Friday, or Saturday. He also suggests consumers snap up the cheap seats while they last, since airlines typically reserve only a portion of their planes for lower-fare customers.