Check Those Frequent-Flier Miles Before They Flit Away : Airlines: Expiration dates loom for many programs. You may want to earn miles quickly, or give extra miles to charity.

Frequent fliers who are earning mileage credits on several airlines should keep a close eye on their accounts, but especially on those programs--American, Northwest and United, among the major U.S. carriers--in which the miles can expire. If you aren’t watchful, you could suddenly be facing a deadline requiring you to use the miles immediately or lose them.

This is the dilemma confronting one busy business traveler who received her American Airlines mileage summary in the mail recently and was shocked to read that 7,554 miles in her account are due to expire Dec. 31. Sure, the airline had tried to keep her informed of the expiration date, she admits, but until two weeks ago she really hadn’t taken the time to read the warnings. Now she’s got just a few weeks to save her threatened miles.

What should she do?

Currently, her American account totals 24,700 miles, which is just 300 miles short of the 25,000 miles she needs to qualify for a free round-trip ticket anywhere in the United States. If she fails to act promptly, her account total will be slashed at the end of the year to just 17,146 miles--which puts her a long way off from a free ticket. In hard cash, her loss would amount to about $150, based on the generally accepted estimate that each mileage credit is worth two cents.

In truth, her case is not critical. Acquiring 300 more miles in the next three months should not be difficult--although it may cost her a few bucks. She would be in much worse shape, obviously, if she were several thousand mileage credits short of her goal. Then she would have to decide if the threatened miles really were worth the effort and expense to save them.

If the traveler does succeed in earning another 300 miles, qualifying for a free ticket, she must apply for an American AAdvantage certificate before Dec. 31. Then, under American’s frequent-flier rules, she has a year to exchange the certificate for a ticket. And the ticket is good for a year after she obtains it. In effect, the validity of her miles is extended for 24 more months.

Most of the major carriers have set a three-year expiration date for accrued mileage. Generally, any miles earned in 1995, whether it’s in January or December, will expire on Dec. 31, 1998--if they are not used before then. The airlines adopted this practice to better assess the number of outstanding miles that could be exchanged for free tickets. Mileage credits that are not used within three years are erased from the books.

Unfortunately, establishing a time limit works a hardship on infrequent travelers, who may not be able to earn 25,000 miles in three years. And it puts at a disadvantage anyone trying to accumulate enough miles for a pair of coach-class tickets to Europe or Asia, which require 60,000 and 80,000 miles respectively on Northwest. For many travelers, the miles can begin to expire before they reach these levels.

In much the same fix, but for exactly the opposite reason, are very frequent travelers. They sometimes earn so many miles so quickly that they don’t have the time--or the inclination--to take all the free trips to which they are entitled.

Rather than watch the mileage credits expire, they treat relatives and friends to free flights. A well-traveled businessman told me recently he regularly gave free tickets to his employees.

If someone really can’t use the miles, American maintains a Miles for Kids in Need Program to which its frequent-flier members can donate expiring credits. American matches the miles--three miles for each one donated--to provide free tickets to children requiring travel for medical care or other emergency needs.

So far, Continental, TWA and USAir allow mileage credits to accumulate without a time limit. Delta initiated a three-year expiration period on May 1 when it inaugurated SkyMiles, the latest version of its frequent-flier program. But Delta is somewhat more lenient. Any miles earned after May 1 will remain valid indefinitely as long as the SkyMiles member takes a qualifying Delta or Delta Connection flight at least once every 36 months.

At least one airline, Northwest, allows its program members to pay to extend the validity of expiring mileage credits. Until this year, Northwest issued a Fly-Write voucher for each 20,000 miles earned. These vouchers, which can be used to obtain a free ticket, have a validity of three years. To extend the validity an additional year, the airline charges $195, according to spokeswoman Marta Laughlin.

Nowadays, there are plenty of ways to pick up frequent-flier miles quickly when necessary:

A traveler who is shy 300 miles could book a short round-trip flight. To be sure, this is one of the more expensive options. A sympathetic phone clerk at a frequent-flier desk suggested the traveler book a room at a hotel that awards mileage credits to passengers--or rent a car for a day.

Many frequent fliers hold Visa cards that award one mileage credit on a specific airline for each dollar charged on the card.

Joe Hopkins, a spokesman for United, which offers such a card, suggests anyone needing miles in a hurry should start charging as many everyday purchases as possible until sufficient miles have been earned.

Beyond these basic mileage builders, a wide range of special promotions allow frequent fliers to earn miles--and without flying. These programs usually are announced in regular bulletins mailed to frequent-flier program members.