AT FIRST GLANCE, the proposal of Texas International Airlines to fly people from Washington to New Orleans for $54 is alluring. That's half the normal tourist fare and makes the idea of a long winter weekend on Bourbon Street seem less of a dream. But this proposal, like so many that are enticing, has a catch in it. And that should lead the Civil Aeronautics Board to say thanks, but no thanks.

The catch is that Texas International wants to provide this fare on direct flights in and out of National Airpport. If the CAN permits that, it will be opening a door that should stay shut. A mileage limit was placed on direct flights out of National a decade ago to reduce the air traffic there and bolster operations at Dulles. New Orleans is outside the limit and that, as far as we are concerned, should settle the matter. If TIA gets a direct route to New Orleans, it will be hard to justify withholding similar routes from other airlines. That would destroy one of the most painfully negotiated restrictions in air travel history.

TIA should remember that two other airports serve this metropolitan area. If it wants to be alluring, it can be so just as well from Dulles or Baltimore as from National.