IT LOOKS AS IF a new era in international air travel is getting under way. President Carter has approved a set of low fares to London and told the Civil Aeronautics Board that he wants "low-fare, competitive international air service." On the same day, the long-awaited Laker Airways "Skytrain" began operations with its even lower fares. So travel to and from London will be a bargain - at least for a while.

How long it lasts, we suppose, depends not only on how many people decide they can afford vacations - or weekends - in London, but also upon how insistent the President is on putting his view into effect. The CAB had turned down the fares he approved on the ground that they were "predatory" - designed by the scheduled airlines to drive off competition from Mr. Laker and the charter airlines. The theory of predatory pricing, if we remember it, is that once the opposition disappears, so do the low prices. If the CAB was correct and the new fares do drive out the competition, the President must be prepared to force the big airlines to keep the lower fares in effect.

In truth, however, we don't expect any of that to happen. We think there are plenty of people around this country who would love to travel abroad except for the price of getting there and back. Now, suddenly, they have an enormously broadened choice. You can, of course, pay as much $1,312 to fly first class from New York to London and back (even more if you want the super speed of the Concorde). But you can also pay as little as $236 if you have the time and are willing to put up with the inconvenience of Mr. Laker's service. In between are fares of $256, $290, $467 and $626.For people who aren't tied to a rigid schedule, those lower prices have to be attractive. And a person could confect a pretty delectable brown-bag lunch for some part of the $390 difference between a seat on the Skytrain (no food) and one at regular economy fare (food that is nothing to write home about).

Of course, the cartel that has been making the rules for international air travel never has understood that, for most Americans, the cost of a ticket has something to do with the decision to fly. Many of the international airlines seem to prefer half-empty planes at high prices to full planes at low prices. That is the concept against which the CAB and the State Department will have to do battle if the President's goal is to be achieved. It will not be easy, because the idea of competition based on price is so alien to the airline industry. But it is a valid idea, and we look forward to its being proven in the coming year as people discover it is possible to see a part of the world cheap.