The appearance of ads for charter flights is a harbinger of spring.

In winter, especially this one, there are plenty of trans-Atlantic scheduled-airline seats available, but when the weather begins to warm, ads for charter airlines and operators pop up like crocuses.The charter prices are normally less than those of the scheduled airlines but in their current fare wars, some prices are appearing for spring that beat the charter companies'.

The bottom fares on the scheduled lines are nonrefundable and sometimes require purchase 30 days ahead, and the number of seats available is tightly controlled.

Most public charter airlines these days make at least weekly trips to and from their destinations, and some have more frequent service - a far cry from the days when everyone had to become a member of some semimythical group to enable an "affinity charter" flight to make a single trip out and back.

In spring these companies expand their programs to accommodate added trans-Atlantic demand in the high season and serving places where sales will not sustain daily service. Last winter charters represented about 3 percent of overseas flights; in summer that number grows to more than 5 percent.

Some consumers shy away from charters because they consider them less reliable than regularly scheduled flights.

"In America a stigma attaches to the word charter," said Janice Holden of Martinair, a big Dutch charter company.

"Europeans always fly scheduled airlines for business and go charter for their vacations."

Indeed, a lawyer who specializes in travel-consumer litigation, Thomas A. Dickerson of Manhattan, says charter flights, in a time of uncertain economic health among airlines, may be financially superior to regularly scheduled air transportation.

He says the safest is the public charter, the most widely offered, which requires that both the tour operator and the air carrier set up separate accounts for each flight in which consumer money is held in escrow until the trip is completed. Public charters also require the tour operator to have a surety bond.

Every contract for a public charter must state clearly that it is a public charter. When passengers sign the contract, they may make their check payable to the escrow account, which is safer than making it out to the travel agent.

Charter flights to Ireland, Northern Ireland and Scotland operated by Sceptre Charters can be $200 cheaper in the high season than the advance purchase tickets sold by scheduled airlines for the same routes, according to Angela Pelosi, manager of the company. Scepter sells seats only through travel agents.

The flights are mostly aboard L-1011s.

The widebody charter L-1011s, according to Mrs. Pelosi, carry 362 passengers; a scheduled carrier using this plane for a flight with one class may carry 315 passengers, or 226 in three classes.

Mrs. Pelosi said tickets for this summer were being sold at $20 to $40 off until March 31. Here are a few of them, not including customs and immigration fees and departure taxes:

On May 17, Friday evening flights to Shannon from Kennedy Airport will start for $429 round trip May 17 to June 14. For the high season, until Aug. 24, the Shannon fare is $449. As of March 18, the best scheduled airline fare to Shannon for May is $499.

More flights will be added to the schedule in June, both to Belfast, Knock and Dublin and Glasgow. For tickets sold this week, the Glasgow fare is $499 and the Knock fare is $459. Return dates are not restricted.

Sceptre does not list its telephone number in the directory, but travel agents are able to reach it.

Two big charter companies, Martinair, which flies to Amsterdam, and Balair, which flies to Switzerland, offer weekly trans-Atlantic service all winter from Florida, but when spring comes they start service from the West Coast and later the Northeast.

These two lines have also added a further wrinkle: they have begun selling a class above economy with more space per passenger, eliminating the customary cramped seating of charters.

Balair calls it Relax class and Martinair calls it Star class, but the service is comparable to business class on scheduled airlines.

On Balair's DC-10s, according to Kathy Schindelheim, a spokeswoman, there are 63 of these better seats and 264 economy seats. This makes a total of 327 passengers; a scheduled airline operating a DC-10 with three classes might accommodate 215 to 258.

Martinair has 24 of the better seats on its 747s and 767s, said Ms. Holden, assistant vice president.

The 747 carries 513 people, contrasted with the scheduled airlines' total of 387 or 428 in three classes.

The Martinair 767 carries 266, as against 163 to 209 in a scheduled airliner in three classes.

Balair's Thursday flights out of Kennedy to Geneva and Zurich resume May 2; Saturday fights from Kennedy to Basel and Zurich resume June 15. Tuesday flights from San Francisco to Switzerland resume April 30.

Friday flights from Miami are offered year round, with warm-weather fares in effect May 3.

Thursday economy class Balair round-trip tickets from New York will range from $510 to $660, depending on the dates of departure and return; if you go over and back in May, for instance, it would be $510; a later return would raise the price. Business class will cost $810 to $960.

As of March 18, Pan Am's best fare to Switzerland in May is $498, which requires advance purchase.

Ms. Schindelheim pointed out that as the fare wars go on through the summer, Balair would be offering its same fares, even the day before, if the seat is available.

Martinair resumes New York to Amsterdam service May 17, from Newark. This is a move from Kennedy, where Martinair operated for five years.

At Martinair, seasonal West Coast flights also resume earlier, in this case service from Los Angeles, Seattle and Oakland on March 26. Toronto flights resume May 1 and Detroit flights May 10.

Not counting departure taxes and other fees, a round-trip economy ticket for a Wednesday flight from Newark between May 17 and May 31 would cost $548, and in business class it would cost $848.

An economy Martinair ticket bought 21 days in advance would cost $448 for travel in this period. As of March 18, the best round-trip fare to Amsterdam in May on Pan Am would be $318.

Ms. Holden said her company considered its fares competitive because demand was good and there seemed to be few seats available on the scheduled carriers at their bottom prices.

Balair and Martinair do sell directly to the public, as well as through travel agents and tour operators. Balair's number is (212) 581-3411, or, in California, Maine and Florida, (800) 322-5247.

Martinair's number is (516) 365-6146. Tickets on both are sold by Travac, a tour and charter company: (800) 872-8800.