A rude awakening could be in store for thousands of frequent-flier club members nationwide who got certificates a year or so ago entitling them to a free domestic round-trip ticket in exchange for 20,000 miles.

Many traded in mileage for certificates because the airlines were about to raise the threshold for a free trip to 25,000 miles. Most big U.S. airlines adopted the new award level early this year.But for fliers who haven't taken their free flights yet, many of those one-year certificates will expire between now and February, and club members who try to book a quick trip with them may well find that there's a "sorry, full" sign on flights on the busiest routes. Each flight has only a limited number of seats allotted for award travel.

"For anybody who has procrastinated and wants to go to a popular destination on a popular day, it's going to be very tough," said Dan Brock, USAir's vice president in charge of the airline's Frequent Traveler program.

Brock said passengers should be flexible in trying to use their certificates - fly midweek and consider cities other than the hottest travel destinations.

Certificate-holders who can't get a flight that suits them have some options, but they must act before the certificate expiration date - or see their 20,000 miles vanish.

Some airlines will let you book a ticket good up to a year in advance with the certificate, effectively extending the certificate's value and also increasing the chance that you can book your preferred time and destination.

Another option with some airlines is to reinstate, for a fee - $40 in the case of USAir and American Airlines - the certificate mileage back into your program account, though there may be restrictions.

To get details of the frequent-flier programs contact:

- USAir Frequent Traveler, 800-872-4738

- American AAdvantage, 800-882-8880

- Continental OnePass, 800-621-7467

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- United Mileage Plus, 800-421-4655

- Delta Frequent Flyer, 800-323-2323

- Northwest WorldPerks, 800-447-3757

- TWA Frequent Flight Bonus, 800-325-4815.

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