Although airline service on the Concorde from Dulles International Airport will celebrate its fourth birthday next month, the sight of the sleek supersonic plane still obviously excites onlookers.

Passengers aboard other airplanes crane their necks to get a glimplse of the long-nosed plane landing or even sitting on the ground. And at the Dallas/Fort Worth Regional Airport, a recipient of Concorde service at subsonic speeds for the past 15 months, some passengers actually have missed flights watching the plane, according to officials of Braniff International, the only U.S. airline to fly the plane.

Despite the public's interest in the Concorde, however, transatlantic travel aboard it must inevitably remain a luxury for the rich and those on generous expense accounts.

A trip to Paris or London on the Concorde is just plain expensive. A round-trip ticket aboard Air France's Concorde to Paris from Washington costs a whopping $2,840, about $500 more than a first-class ticket on their conventional planes and more than three times the cost of a regular coach ticket. A trip to and from London on the Concordes operated by British Airways costs $2,522, also $500 more than a first-class ticket on a Boeing 747 and about three times the cost of a coach seat.

Despite the high cost of tickets, Concorde service generally has been a losing operation for both airlines operating them. sThe plane, a product of a joint French-Anglo research, development and production team, was expensive to build and gets increasingly expensive to operate as the cost of fuel continues to escalate. The plane consumes prodigious amounts of fuel, one airline official says in an obvious understatement.

At Mach II or twice the speed of sound -- which is 1,350 miles per hour, the speed at which the Concordes cross the Atlantic -- the plane consumers the same amount of fuel as a Boeing 747 although the Concorde, when full, carries about a quarter as many passengers.

Those who have flown on the Concorde -- and its boasts a lot of repeaters -- say it is air service at its most luxurious. Although the plane is narrowed-bodied -- seating 100 passengers in pairs along a center aisle -- Concorde travelers say there is plenty of leg room and distance in front seats for larger-than-usual tables on which are served the most sumptuous meals the airlines offer.

The high technology that went into building the drooping, needle-nosed aircraft allows it to travel to Europe in half the time it takes conventional jets, flying at altitudes almost twice the usual.

In its fiscal year ended March 31, 1980, British Airways made what it says was a slight profit on its overall Concorde services only because its service from New York has been operating with 82 percent of its seats filled, making a "rather substantial profit," according to a BA official. BA operates two Concordes a day between New York and London (except on Wednesdays, when it operates one).

In contrast, its three-times weekly service out of Dulles is operating at a 56 percent load factor, just below the 60 percent figure that BA says is its break-even point. Its London-Bahrain-Singapore service does even less well.

Air Frances's Concorde service out of New York is popular than British Airways'. -- London is considered a better business point than Paris -- and runs with even fewer seats filled out of Washington, where it operates the Concorde service four times a week.

An Air France spokesman said its New York Concorde service ran at a 71 percent load factor last year, while its Washington service was 50 percent filled.

The idea that either BA or AF can even entertain the idea of breaking even, not to mention making a profit, on operations of the supersonic plane is only possible because both British and French governments decided last year to allow their government-owned airlines to write off the monumental costs of development. The airlines need not take into account the costs of buying the plane, only operating it, when reporting profits and losses.