FROM THE hearings that have been going on the last two days at the Federal Aviation Administration, you would think the future of National Airport were about to be decided in a reasoned way. Not likely. A critical vote on the fate of National will probably be taken either today or tomorrow in, of all places, the House of Representatives.

Congressional advocates of unlimited airline service at National plan to ram through legislation blocking -- once again -- any rational airport policy. They are being aided by some airlines that are spreading scare stories around Capitol Hill concerning the early demise of the particular flights congressmen frequently use. Rep. Gene Gnyder, for instance, has sent out a "Dear Colleague" letter that says in large type: "Do you fly out of National? Forget it! Take the . . . to Dulles" The . . . is a drawing of a horse-drawn covered wagon.

Facts, unfortunately, have rarely entered into the congressional reaction to plans for National. Rep. Snyder's broadside, for instance, says, "There are no air carriers now going into National that meet the 1986 noise limits." That's true. But by 1986, almost all air carriers will have replaced the engines on the planes now flying into National or the planes themselves. Those replacements, if the airlines are pushed just a little, will meet the noise standards. Similiarly, an argument is being made in the House that the proposed National policy should be suspended for a year so it can be studied. No one mentions that Congress and the FAA have been studying National since 1965.

The proposals Secretary of Transportation Drew Lewis had made were not perfect. The local civic groups are right in contending the airport should be completely closed at night and that permitted flights should be curtailed more sharply. The complaints of some airlines that the proposals discriminate against them and in favor of their competitors are valid, too. But these provide no justification for congressional intervention at this time. The period for public comment on the proposals doesn't end for another month, and the earliest date on which the proposals could go into effect is late October.

Fortunately, 134 members of the House and 29 members of the Senate have signed letters supporting the general thrust of Secretary Lewis' plan. If their colleagues only take time to examine the issues, they will be able to beat back this latest bold effort to make congressional perquisites the basis of costly and wrongheaded policy.