THE WASHINGTON traveler was grumbling. She and her husband had recently retuturned to Washington from a vacation in Europe on a budget charter flight from Paris, and for them it had not been a happy experience.

For one thing, the flight's departure was delayed for four hours, and during the tedious wait in the airport they could find no one to explain what was going on. And then, though they understood the flight was nonstop back to Washington, it landed first in Boston where, after clearing customs, they encountered another long delay. Again, they had difficulty getting an explanation.

"It ruined our vacation," she says. "After 10 days of relaxing, we were so miserable coming back."

Frequent travelers probably won't consider a four-hour delay, even on a scheduled airline flight, extraordinary. But what really agitated the couple was not having anyone to turn to for information when the complications arose. The charter company, Travel Committee Inc. of Baltimore, appeared to have no counter or staff at either Paris or Boston airports.

"There was no charter operator you could complain to," says the woman. "You got no feeling of being a customer." In response, Norma Guard, a spokesman for Travel Committee, says the charter was flown by Pan American, and that airline's staff was responsible for flight information.

The couple was among the tens of thousands of Americans annually who sign up for public charter flights and tours, where the idea is to pack the plane as full as possible to keep the cost low. One estimate is that 25 percent of last summer's vacation travelers to Europe booked on a nonscheduled airline, a substantial chunk of the transatlantic business.

And, based on reports from travel agents and the Civil Aeronautics Board, most of them were at least satisfied enough not to make any complaints.

"Off the top of my head," says Dean Witt of CAB's office of congressional, community and consumer affairs, "I don't think that the percentage of complaints on charter operations is any greater than that for scheduled service."

The attraction of a charter flight, of course, is bargain rates, which may save travelers up to hundreds of dollars on airfare and tour packages. For example, Travel Committee is offering a limited number of $451 roundtrip charters weekly to London from Baltimore/Washington airport, compared to $549 advertised by Trans World Airlines. However, any ticket-buyer is well-advised to comparison shop; in the face of competition, scheduled airlines have been cutting rates.

"The fact is," says Witt, charter flights are "very reasonable transportation. For the thousands who have been transported without any problems, it's a real beneficial segment of the industry."

But complaints do get filed, sometimes because passengers misunderstand the nature of the charter business. They expect the same kind of service from charters they get from a scheduled airline, and that isn't necessarily so.

"There are those who think they're going first class, who weren't pampered, and they beef," says H. William Cordes, vice president of consumer affairs for the America Society of Travel Agents. "If they were properly prepared by a travel agent for the possible inconvenience, then they would be able to absorb it, and they don't complain."

Cordes' office heard complaints last summer about a half-dozen charter operators who were "often simply inaccessible" to passengers or travel agents who had made reservations. Some ticket-holders, he says, could not get through by phone to cancel until it was too late to avoid a cancellation penalty fee. "It was frustrating."

As one Washington travel agent, who estimates 60 percent of his vacation clientele books on charters, advises customers: "You get what you pay for."

Should you consider a charter? And how do you find out who is flying where for what price?

The answer to the first question is really up to you--once you know what you get and don't get from a charter.

In years past, some charter firms operated on fairly shaky financial ground, and there were reports of travelers abroad whose pre-paid flights home were suddenly canceled because the firm went out of business. In the late '70s, however, the CAB introduced regulations aimed at eliminating many of the problems.

Now, explains Witt, any firm entering the public charter business must obtain some form of surety bond in the amount of the total fare collected for each flight. Or, if the charter is part of a regular series, the bond is $10,000 per flight but passenger payments are to be placed in a bank escrow account. Passengers write out their check either to a travel agent or the escrow account but not to the charter firm. The charter firm does not get the money until the trip is completed.

This seems to be working, but occasionally, says Witt, some charter firms "do get off the track. Some you can't trust to the extent you would like to."

Public charters are different from "affinity" charters, in which an organization arranges a flight for its members and they share the cost. Affinity charters are not included under the consumer protection provisions of public charters. Seats are restricted to members and immediate families, and the flight can only be advertised among the membership, unless the organization files a prospectus with the CAB as a public charter.

The prospectus for each public charter must include a participant contract spelling out all the conditions for the flight, including refund policies, which can be fairly stringent. The participant contract usually is the small print on the travel brochure, and it is important that passengers read it because this is where one discovers how charters differ from scheduled flights.

Some charter operators will make no refunds on airfare within 60 days of the flight. Scheduled airlines also often have refund limitations on their special fares, but the penalty rarely is so stiff. Passengers canceling out on a TWA advance-purchase (APEX) ticket within 21 days, for example, lose only $50.

Taking this a bit farther, charter passengers must show up at the airport on time (or hope their connecting flight is prompt), says the CAB, because if the charter takes off without them, "you're probably out of luck." The only hope is if the charter operator takes pity and can fit a tardy passenger on a later trip.

To avoid loss of money because of a last-minute cancellaton due to an illness or a death in the family, charter passengers should consider purchasing trip cancellation insurance. Some charter operators recommend it strongly.

Charter passengers should also be aware that the operator can cancel a trip (with full refund) up to 10 days before departure, usually because enough seats have not been sold to make the flight profitable. That can be a blow to people who have made a lot of plans--hotels abroad, a house-sitter at home--or who have no flexibility in their vacation schedule.

"Like any other flight," says the CAB, "a charter is subject to delays," and certain regulations apply. On an international flight, the delay can be up to 48 hours before the charter operator must come up with substitute transportation or a refund. On a domestic flight, the delay need only be six hours.

As a practical matter, passengers probably are better protected from extensive delays when their charter is flown by a large scheduled carrier (Pan American, United, American). If an aircraft has mechanical problems, the big carriers have more flexibility in providing a substitute, especially at peak travel times when a small charter airline company may have all of its planes heavily committed.

Other points the CAB says charter passengers should consider:

* Seating may be more crowded than you are used to.

* Baggage won't be checked from a connecting flight to a charter or vice versa. You must allow time to retrieve your suitcase from the connecting flight and check it with the charter.

* As the departure date approaches, the charter operator may reduce the price of a ticket to fill the rest of the plane. Passengers who have already paid are not entitled to the cheaper rate.

* All charter flights and tours are subject to changes. You may cancel with a refund only in the case of four "major changes," specifically: a change in the departure or return date, "unless the date change results from a flight delay"; a change in the departure or return city; a change in the hotels in a package tour; or an increase in price, if it is more than 10 percent of what you paid.

The simplest way to find out what public charters are available is to check with a reputable travel agent. Most subscribe to Jax Fax Travel Marketing Magazine, a monthly listing of thousands of flights. (The magazine is aimed at travel industry professionals, but a year's introductory subcription is $12 from Jet Airtransport Exchange Inc., 280 Tokeneke Rd., Darien, Conn. 06820.) Also, flights frequently are advertised in newspapers and magazines.

Jax Fax includes listings from a number of U.S. cities to about 80 U.S. and foreign destinations. The most heavily traveled route is to Europe. There are considerably fewer listings for South America and the Pacific.

To protect yourself on a charter offering:

* Ask your travel agent about the firm's performance record or phone CAB's consumer affairs office at 673-6047 for the complaint record.

* Read the operator/participant contract closely before paying any money. It spells out what you are buying.

"Charters can be a wise travel investment when you find one within your price range, and if you can be flexible in your travel plans," says the CAB. "Just be sure you know the conditions for the trip you're buying before you pay for it."