ARCHITEC EERO SAARINEN'S magnificent terminal building for Dulles International Airport - dedicated almost to the day 15 years ago - is one of the few generally admired works of modern architecture. In a poll taken by the Journal of the American Institute of Architects last year, Dulles ranked third (after the University of Virginia and Rockefeller Center) among all important structures built in this nation's first 200 years.

Architecture buffs are therefore upset to learn that the Federal Aviation Administration, which owns and operates Dulles, is about than those who worked with Saarinen have been selected for the job, and that the $7 million alteration plan is still under wraps, although bids will be invited early in December. There is a growing demand that Saarinen's masterpiece be put on the National Register of Historic Places. The Secretary of the Interior now has authority to confer landmark status on exceptional contemporary buildings and thereby give them some measure of protection against aesthetic tampering.

In our view and experience, the FAA makes a convincing case for the need to expand the Dulles concourse. It gets uncomfortably crowded during morning and evening rush hours, which affect international flights as much as domestic commuter traffic. Saarinen, to be sure, designed the building so that the structure can be elongated eastward and westward - section by section, in a way that would simply repeat the present design - to accommodate more mobile-lounge gates and more airplanes. But that, aside form the cost of such addition, would not solve the problem. The problem is that new and larger aircraft load and unload more people and more baggage than the airplanes of Saarinen's time and that the new security procedures also demand more space than Saarinen anticipated.

The FAA says it plans to provide this space by extending the podium on which the concourse rests by some 55 feet toward the field. In other words, the wall through which passenger enter and leave the mobile lounges will be pushed out. The great hall itself and the ingenious and convenient mobile-lounge system would not be changed. Trust us, FAA officials say in effect; we love Saarinen's masterpiece as much as anyone.

We'll buy that argument.We are even willing to trust the present FAA officials and their architects to make the alterations without impeding the integrity of the building's architecture. But we would trust them even more if they would allow Dulles to be put on the landmark list. They adamantly refuse to nominate the building on the grounds that this requires alterations to be approved by the Advisory Council on Historic Preservation. FAA considers this a bureaucratic nuisance. Trust us!

Well, we also trust the advisory council to be reasonable and to use good sense in accommodating needed function to aesthetic form. We think Dulles should be officially recognized as a great landmark building of our time as a safeguard against the whims of possibly less-sensitibe aviation administrations in the future.