This may be the summer to take the vacation trip abroad you have thought a lot about but put off in the past because of hign air fares.

Although regular economy air fares abroad have continued to rise, and probably will be higher this year than ever, travelers across both the Atlantic and Pacific also may be able to take advantage of a growing number of promotional fares that have never been lower to a growing number of destinations.

A number of fortuitous circumstances have combined to give the consumer with a lust for foreign travel a break this year on air fares:

Britain's Freddie Laker and his low-fare, no-reservation Skytrain air passenger service between New York and London.

The competitive responses of the other airlines on that route to his fare.

The rippling effect on other airlines flying other routes that were forced to consider lowering some fares or risking the loss of potential passengers who would choose to go to London.

A president and, for the first time, a Civil Aeronautics Board committed to lower international air fares and willing to back philosophy with actions.

Though it is giving airline executives and planners headaches, one of the best things going for travelers is the failure of International Air Transport Association members to agree on a package of fares that they all would charge this year.

Just as the economics textbooks say, the "open rate" situation that cartel members dread has enabled competition to creep in. Practically every day brings an announcement by a U.S. or foreign airline seeking approval for a new fare or new, sometimes less-restrictive and more-attractive, conditions on their offerings.

Except for summer fares to London, Virtually everything else is subject to government approval - ours and theirs - and radical change as airlines seek to match or exceed what their competitors propose.

The result is a hodgepodge of air fares with confusing and complicated rules constantly in a state of flux - but they are designed to attract passengers, and may make this the year to take that trip abroad.

There are, of course, problems - not the least of which the fact that the U.S. dollar does not go nearly as far abroad as it did at one time or even recently.

And there is a possible problem with the cut-rate fares too. "Add one warning," CAB Chairman Alfred Kahn cautions. "We don't know how many seats are being made available at these very attractive fares.

"These widely advertised attractive fares may cause widespread cancellation of charters, and people will find themselves relatively late in the season calling up and finding no seats left at those prices," he worries aloud. "Won't you pay economy?" the traveler will be asked.

"There's a real possibility of a bait-and-switch technique here," he says. "What do you do if you call three weeks in advance and no more seats are available and there are no charters either?"

It is of obvious concern to the charter industry, too. With big-bugdet advertising announcing the availability of low fares on the scheduled airlines, the charter airlines and tour organizers who until now have provided consumers with the only widely available low fares fear travelers simply never will think of going charter, and that the charter market will be seriously injured as a result.

Donald Farmer Jr., director of the CAB's bureau of international aviation, notes that some of the airlines seeking approval for low-fare proposals incorporated capacity controls in them. For instance, Pan American World Airways said it would sell a maximum of 15 percent of the available weekly seats provided on each route to London when it propsed to extend cut-rate "standby" and "budget" fares, previously available only from New York, to other cities, including Washington.

Other major carriers to London have limits on the number of seats they can sell at the bargain prices as well. But airlines on some other routes have no capacity-control provisions, and it will be up to them to decide how many seats to sell at the lower prices.

In addition, most of the airlines who proposed selling up to a certain number or percentage of their seats at the lower prices are not necessarily guaranteeing that that number or percentage will be available, especially during the peak travel season. The seats could be filled up ahead of time with passengers paying regular or various excursion fares.

"The important message is that nobody knows how many seats are going to be available at the new low fares," Farmer says. "That's why the organizers are so worried."

It is also why the CAB is considering whether to require the airlines to disclose the number of seats available when they advertise cut-rate fares.

With those caveats, and some advice to check charter and tour options as well, here are some of the options on the scheduled airlines:

From Washington, a $326 round-trip ticket to London during the summer using the "standby" or "budget" fares of Pan Am or British Airways. The fare is $186 to London and $140 back. The regular economy class fare during the summer will be $409 each way, according to BA.

Standby passengers must appear at the airport at least three hours before departure time to receive a boarding pass if seats are available. "Budget" passengers must purchase their tickets at least three weeks in advance of the week they wish to depart. The airline is to contact them at least 10 days in advance with confirmation of the exact flight and time of departure.

Pan Am and BA will be limiting the numbers of tickets sold at these fares to a total of about 350 eats a week in each direction; if there is a big demand for "budget" tickets, there could be few standby tickets available.

The next-lowest scheduled fare to London this summer is the advance-purchase excursion (APEX) fare of $462 round trip. A traveler must stay away between 14 and 45 days and book the ticket 30 days in advance (with BA; Pan Am still carries a 45-day advance purchase period but is bound to change it) and pay for it within seven days after the reservation is confirmed. A cancellation fee of 10 percent of the fare or $50, whichever is higher, is assessed unless the cancellation is due to death or illness. A maximum of 25 percent or 30 percent of the seats in the airplane can be made available at APEX fares, a percentage rarely reached, according to a Pan Am spokesman.

Although no other standby and budget fares are yet available from Washington to other European cities, APEX fares have been the vehicle used by airlines to cities other than London for their low, or at least lower, fare offerings. For instance, if approved, an APEX ticket from Washington to Paris on TWA purchased 30 days in advance will cost $423 this summer, compared to a regular economy roundtrip ticket in the range of $800. If you think you might like to go to a certain country or countries, be sure to check each of the domestic and foreign airlines that fly there for their fare offerings, or have a travel agent do it. Unlike the days of the recent past when the rates were agreed upon, it is very possible that one carrier doesn't offer.

Leaving from other cities, espceially New York, should be considered. The prevalance of Super Saver domestic discount fares makes it possible to combine a low domestic round-trip fare with a low-fare charter or scheduled flight to make a trip from an other city feasible, even attractive.

For example, Air France is seeking permission to offer a "midweeker special" fare of $360 round trip between New York and Paris this summer for travelers taking their flights on Wednesday or Thurdsay evenings for stays of 14 to 45 days. If approved, combining that fare with a Super Saver discount of 30 percent or 40 per cent from the regular $80 round-trip Washington-New York fare might be a very attractive way to get to France this summer.

Also, a much greater number of standby and budget fares will be available to London from New York BA, Pan Am and TWA this summer - up to 2,450 a week in each direction for each carrier. The fare will ber $299 round trip this summer - $169 eastbound and $130 westbound - compared with the current $256. Standby and budget fares also are available for this summer from New York to Rome, Brussels, Belgrade and Frankfurt, at least at this time.

And don't forget about Freddie Laker, who is to a great degree responsible for the sweet chaos that is benefitting the air traveler. Laker Airways' Skytrain air pasenger service operates only between New York and London (although it is now competing with British Caledonian Airways for authority to operate between Los Angeles and London as well) and still offers the most economical fare (about $245 round trip, depending on the exchange rates.) The fare is $135 eastbound and 59 pounds westbound and is not expected to change this summer.

Tickets are sold on a first-come, first-served basis after 4 a.m. on the day of the flight at Laker Center in Queens, New York, easily accessible from the airports and Manhattan by public transportation. In London, return tickets or boarding passes are available at Gatwick Airport or a downtown train station. In the summer, Laker will have at least two airplanes, carrying 345 passengers each, flying in each direction each day. Passengers pay extra for the meals that are included in the ticket prices of the other airlines.

Telephone recorded messages tell callers the directions to the Laker Center and the number of seats still available each day.

Why not go around the world this summer? A $999 "Round-the-World in 80 Days" ticket from Pan Am entitles a traveler to go around the world on its flights in economy class on a standby basis. The normal economy fare is $1,849. A first-class standby fare of $1,599, almost half the $2,950 regular first-class fare, is also available.

Passengers who want to fix their itinerary at least 30 days prior to their planned departure from the United States can buy a reserved-seat economy ticket for $1,199 or a first-class seat for $1,899.

Passengers must stay overseas a minimum of 22 days and a maximum of 80 days. Many different routings are available. A traveler could begin on the West Coast, stopping in Honolulu and Tokyo, continuing to Hong Kong, Tehran, Istanbul, Frankfurt, London and back. Or someone could go to Europe first, then on to Delhi, Bangkok, Hong Kong, Tokyo and back.

The fares are available only on Pan Am flights, but travelers could leave a flight in one city and pick up a Pan Am flight in another so long as the journey continues in the same direction as it began, with no backtracking Standby passengers can make a maximum of 11 stopovers, while reservedseat passengers get unlimited stopovers.

A number of transpacific discount fares are also available from New York and several West Coast cities to New Zealand, Thailand, Guam, Hong Kong, Indonesia, the Philippines, Singapore, Australia, Fiji, American Samoa and Taipei. For instance, pan Am offers a budget fare of $429 one way to Hong Kong from New York; from California, a visit to Fiji is $289 one way.