IT HAS TAKEN a generation to land the big ones, but now the word should boom through the loudspeakers from one end of the terminal to the other and all the mobile lounges in between: "Greater Washington announces the arrival of Dulles International Airport." At the age of 23, Dulles is set to become a full-fledged, important world center of airline traffic. The turning point is the decision of United Airlines, the country's largest, to establish a huge "hub" at Dulles with more than 50 daily departures to 24 destinations and plans for more than 100 daily takeoffs within three years.

It's the big break on several counts. First, the move should inject hundreds of millions of dollars into the local economy through spending for hotels, restaurants, fuel, rental cars and other traveler services. Also, the enormous increase in flights and connections should make Dulles far more attractive to air travelers from this region. And if good management sense prevails, the "New Dulles" should cry out for physical improvements that a regional authority could work with airlines to put in place far more efficiently and economically than could the airport's present owner, Uncle Sam.

The rise of Dulles as a hub-quarters began in earnest earlier this year. New York Air started service from Dulles in February and then Pan Am made the going great in May with plans for its hub. Presidential Airways is in the competition there, too, all of which sets up some lively competition for customers.

When life comes to Dulles in this big way, new facilities will have to come as well. However smoothly the terminal has functioned through its lean years, it can't begin to handle an evening rush-hour with scores of new arriving and departing transcontintal, international and domestic flights unless new services are added. The airlines are prepared to do their part, but a midfield terminal at Dulles will be essential: that is something that can be done best through the proposal pending in Congress to get the federal government out of the airport-ownership business at Dulles and National. A regional authority could sell bonds to finance improvements -- and could respond quickly to airline and traveler needs.

There's the not-so-small matter of ground connections to Dulles too. The idea of a light-rail line -- as proposed by a group of private investors -- between the West Falls Church subway station and Dulles -- has its attractions. It may well prove less expensive to run buses constantly, but you can forget any hopes of quick federal aid for direct Metrorail service all the way.

Will National wither and die? Will Baltimore- Washington International lose out if a regional authority takes over the other two airports? Nothing points to either eventuality. What is likely -- and worth a first-class welcome -- is a healthy, balanced air transportation system that could contribute mightily to the economy of the entire Baltimore- Washington "common market."