Almost overnight, Baltimore/Washington International Airport has become a popular takeoff point for bargain-priced charter flights, principally to Europe and the Caribbean.

BWI averages 30 charter flights a week during the summer tourist season, and some charters fly weekly throughout the year. By comparison, only one charter operator flies regularly out of Washington/Dulles. No charters regularly depart Washington/National, says an airport spokesman.

Frequent charter flights will fly from BWI this summer to London, Paris, Amsterdam, Frankfurt, Shannon, Bermuda, Cancu'n (Mexico), Nassau and Freeport (Bahamas), San Juan (Puerto Rico), St. Thomas and St. Croix (U.S. Virgin Islands), Tortola (British Virgin Islands), St. Maarten and Aruba.

The Dulles charter goes to St. Thomas every Sunday.

Some of these flights provide air transportation only; others offer hotel packages as an option and others are strictly air and hotel packages. While the European flights are seasonal, many of the island charters operate year-round.

On many charters -- but not all -- travelers can stay abroad as long as they choose, from one to several weeks, although a ticket surcharge may be imposed for longer stays.

An unusual charter offering in April is a chance to fly the supersonic Concorde one-way from London to BWI for a low fare of $800, which includes a one-way ticket from BWI to London aboard regular British Airways service. The trip has been organized by the Travel Committee, a long-established charter travel firm in Owings Mills, Md.

The first leg of this special trip departs April 18, and the Concorde return is a week later on April 25. A one-way ticket alone on a scheduled Concorde flight currently costs $2,399.

In 1983, the number of charter passengers flying out of BWI increased by 80 percent over the previous year, says airport spokeswoman Linda Greene, and in 1984 the increase was 21.5 percent. This year, with Europe again beckoning bargain-hunters, the growth is expected to continue.

A total of 171,886 travelers boarded charter flights at BWI in 1984, or about 35 percent of the 477,599 passengers taking international flights from the airport last year.

The emergence of BWI as a charter center has coincided with an improved U.S. economy and the beginning, three years ago, of very favorable dollar exchange rates that have sent record numbers of American vacationers across the Atlantic.

Airport administrator Ted Mathison cites Piedmont Airlines' decision to make BWI a hub as another growth factor. Piedmont is bringing additional international passengers on short hops from southern Virginia and North Carolina. The airport also has the advantage of drawing from both Baltimore and Washington.

Meanwhile at Dulles, charters are "an area we're trying to cultivate," says Cookie Ellis, public relations director for the Washington/Dulles Task Force, an organization promoting new air service at the facility. The St. Thomas charter is one result of a promotion effort directed at charter organizers.

"We know we have a lot of passengers who have to go to BWI or New York to take charters," she says.

The advantage of frequent charter service out of local airports is that passengers do not have to pay the extra air fare to get from Washington to other major charter gateways such as New York, Philadelphia and Boston.

Additionally, there is the convenience factor. Travelers don't have to change planes en route. And if there is a lengthy delay in departure, which sometimes happens, passengers may be able to wait it out at home instead of at an airport hotel.

Charter flights can be a good value, usually providing the cheapest way to fly to certain destinations abroad, but they aren't for everyone. At least one cautious Washington travel agency requires its customers to sign a statement that they have read a notice detailing the pros and cons of charter travel before the agency will sell them a ticket. It is also important to read the small print on the charter contract before signing.

The big plus, of course, is that charter flights tend to be less expensive than those of scheduled airlines.

For example, Pan Am's cheapest round-trip fare between Frankfurt, Germany and Washington/Dulles during the summer high season (June 1 to Sept. 14) will be $776 midweek and $825 from Friday through Sunday (based on a 21-day advance purchase). During the same period, DER, a German-based charter operator, will fly round trip from BWI for $539 to $599 (depending on the departure date).

Eastern's lowest round-trip air fare to St. Thomas currently is $653. Wainwright, a Bethlehem, Pa., charter operator, offers weekly round-trip departures from BWI (Saturday) and Washington/Dulles (Sunday) for $329.

One scheduled carrier that does challenge the charters is People Express. But on the transatlantic route, it flies only to London.

People Express' cheapest Washington-to-London round trip (with a plane change in Newark) is $398. The Travel Committee flies Friday to London with a round-trip air fare of $479 to $499. World Airways is charging $649 for a BWI-London round trip from June 1 to Sept. 14.

At these prices, there are differences in the services each airline provides. World Airways is a full-service airline. People Express, as a discount airline, provides fewer services; for example, passengers must pay extra for meals. Charter flights operate under different federal regulations. For example, a ticket is good only on a specific flight; you can't alter your itinerary at the last minute or use the ticket on another airline.

Special discounted fares on scheduled airlines, frequently offered through travel agencies, may sometimes be competitive with charter prices. (Discounted fares are often available to destinations not regularly serviced by charters -- such as Australia, Africa and the Far East.)

Weighed against the charter industry's low prices are several factors that travelers should consider:

Once a charter ticket is purchased, passengers generally have less flexibility in making changes. One charter operator, for example, offers no refund for a cancellation within 45 days of departure. Scheduled airlines also have cancellation penalties, but they are not nearly as stiff.

Taking a charter "is like buying a theater ticket," says Ray Greenly, consumer affairs director for the American Society of Travel Agents. "If you don't use it, you lose it." A traffic jam encountered en route to the airport that causes you to miss the plane could cost you the entire vacation. The same holds true if a connecting flight is delayed, and the charter departs without you.

Up to 10 days before departure, charter operators themselves can cancel a flight (with full refund), usually when they haven't sold enough seats. This could make it difficult for vacationers to make new arrangements, especially at the same low cost.

International flight departures can be delayed up to 48 hours -- at either end -- without penalty to the charter operator. Travelers might have to pay for meals and an extra night in a hotel if, for example, the plane has mechanical problems. A charter carrier may not have a backup plane available.

And though federal regulations require that ticket payments be placed in an escrow account to protect passengers, things still go wrong. For one reason or another, every summer brings a story of American tourists temporarily stranded abroad when a charter operation shuts down.

Nevertheless, thousands of travelers annually fly charters without difficulty, and at substantial savings.

Most charter organizers are wholesalers whose flights are sold to the public through travel agents. A good agent can offer advice on a charter firm's performance. Similarly, ASTA's consumer affairs office at 965-7520 can advise on a charter operator's complaint record. Many charter firms have been operating reliably for years and have a history of responding satisfactorily to complaints.

One good protection is to buy trip cancellation insurance, sold by travel agents. The cost is about $5 per $100 of coverage, or $25 to buy insurance for a $500 ticket. It should include coverage that provides full refund if the traveler must cancel for illness or death in the family or if the charter operator goes out of business.

Listed below are many of the charters that will be flying from Baltimore/Washington during the upcoming vacation season. In some instances, base prices are quoted for charter packages, with a note that taxes and service charges are extra. However, since March 19 charter operators are required by federal regulation to include the entire price a passenger must pay in all advertising. That means no more extras in small print beneath the big, bold number that catches your eye.

* Amsterdam: DC10 (wide-body) departures every Sunday at 7 p.m. between June 23 and Sept. 29, offered by Martinair-Holland, a longtime Dutch charter operation partially owned by KLM Royal Dutch airlines. There's a stopover in Boston en route to Amsterdam; nonstop returns on Sunday. From $398 to $478 round trip, depending on date of flight. Martinair is unusual in that it owns its own aircraft. Most other charter organizers have contractual arrangements with a variety of airlines to provide planes.

* Frankfurt: For the second year, DER Charters, a major German travel firm, is offering round-trip flights from BWI every Sunday at 1:50 p.m. from May 5 to Oct. 20, from $449 to $599. There's a stopover in Newark en route to Frankfurt; nonstop Sunday returns. Additionally, there are departures every Friday in April for $449 (and Friday returns). Service is aboard a DC10 of Condor Airlines, a subsidiary of Lufthansa German Airlines.

* London: Nonstop Friday departures from May 24 through Oct. 25 from $479 to $499, offered by The Travel Committee. Return flights on Saturday. Surcharge of $30 for stays of three weeks or longer. Service is aboard a wide-body L1011 of American Trans Air of Indianapolis.

* Paris: Another Travel Committee program. Nonstop Thursday departures from May 26 to Oct. 24 for $519, with Friday return. (Travelers can fly into London and out of Paris, or vice versa, but a surcharge is added.)

* Shannon: The charter trips to Ireland, offered by Emerald Tours of Alexandria, are seven-night packages. They include air fare and a choice of a guided motorcoach tour and hotel lodging or a do-it-yourself car rental program, with stays in bed-and-breakfast inns. Frequent departures from June through October. The per-person price ranges from $549 (Oct. 25) to $639 (July 21). Service aboard a DC10 of Jet 24/Jet Charter Service of Miami.

* Bermuda: This is the fourth season for the package-only trips to Bermuda, offered from June 2 through October by Apple Tours of Media, Pa. Departures Sunday at 7:30 a.m. for the 95-minute flight, returning the following Sunday at 7 p.m. The seven-night price depends on the quality of accommodations chosen, ranging from $529 per person at the Hamiltonian Hotel (without meals) to $1,149 at the luxurious Southampton Princess (two meals daily). Included in all packages are air fare, lodging, transfers, tips and taxes. Service aboard a B727 of Gulf Air of New Iberia, La.

* Cancun: Another Apple Tours program, now in its third year to the popular Mexican beach resort. Year-round Saturday departures, providing a choice of seven-night hotel packages. The per-person price ranges from $349 (Caribe International) to $749 (Hotel Camino Real), including air fare, lodging, transfers, tips and taxes. Air fare only is $279.

Three charter operators -- The Travel Committee, Wainwright and GWV Travel of Needham Heights, Mass. -- schedule frequent flights year-round to the Bahamas and a number of Caribbean islands, offering both hotel packages and air transportation only.

Travel Committee's destinations include Nassau and Freeport in the Bahamas, San Juan, St. Thomas, St. Croix, Tortola and St. Maarten. Wainwright flies to Freeport, St. Thomas (from both BWI and Washington/Dulles), St. Maarten and Aruba. GWV flies to Nassau and St. Maarten, and it also has flights to Cancu'n.

A wide range of prices and packages (including three-, four- and seven-night stays at selected destinations) are available. Among the options:

* St. Maarten: Travel Committee flies every Friday to this part-Dutch, part-French island, offering seven-night round-trip air/hotel packages in April and May that range from $399 (Summit Hotel) to $619 (Le Grand St. Martin Hotel). The rate is per person, and there is a 15 percent addition for "tax and service charges." The air-only round-trip fare is $329 to $339.

* Aruba: Wainwright's flights depart Sunday for the Netherlands Antilles. A seven-night air/hotel package goes for $399 (Talk of the Town) to $559 (Aruba Concorde). The rate is per person, plus a $40 tax and service charge. Air-only round-trip fare is $329.

The most-convenient way to find out about upcoming charter flights is to check with a travel agent and keep an eye on the travel ads.

Another primary source, used by agents, is "Jax Fax Travel Marketing Magazine," a thick monthly that details charter and discount flights and tour packages available from many U.S. cities, including Washington and Baltimore. Subscriptions are $12 a year from Jax Fax, 280 Tokeneke Rd., Darien, Conn. 06820.