Dulles International Airport is both a circus and a tomb. The graceful, wing-shaped terminal, which on a foggy morning seems to float in the center of 10,000 acres of Northern Virginia flatland, is the 41st busiest airport in the United States. Yet, between 3 and 7 p.m., Dulles is as busy or busier than any airport in the country.

It is called the crunch, the peak or the spike. There are long lines, baggage delays and foreign travelers from places like Guatemala and Saudi Arabia walking about sharing an emotion one interpreter calls "absolute panic."

During the four hours, Dulles copes with about 40 percent of the 153 airline flights and the 7,800 passengers who move through the airport every day.

Before the crunch hits and after it is over, Dulles seems to snooze. The starkly modern air-travel terminal assumes the somewhat somnolent Southern mood of Virginia towns like Flint Hill, Rixeyville and Boyce where many of the airport's 7,000 employes lives.

What follows in this story is an account of a peace-to-panic day typical of Dulles:

At noon on a recent Friday, David Bennett, a 22 year-old student at San Diego State, sits waiting in the near-empty main concourse for a 4:45 flight back to school. He's reading "The Black Death," a tale about the bubonic plague in New York City, and is soaking up the Musak that slides down the 18-foot-high panes of glass which are the airport's walls.

Bennett flew into Dulles two days ago to visit his father, an Army engineer who flew in from Saudi Arabia to visit his son. They spent two days together, a day for each year they'd been apart.

At 12:50, three air traffic controllers in the 170-foot high control tower feel the floor beneath their feet tremble as a London-bound Bristish Airways Concorde rifles down the 11,500-foot-long west runway. In good weather people still come out to Dulles to watch the Concorde, which first took off from the airport May 24, 1976. Among air traffic controllers, however, the trill is clearly gone. The three in the tower give the swept-wing, needle-nosed jet a passive stare.

At 1:30, Ky Taylor, a 20-year-old who lives in the Chantily area of Fairfax Country, is asked by an FBI agent if she or any member of her family ever plotted to overthrow the governmen. This is part of an interview taking place in the FBI office in Dulles, which is squirreled away in a hall way where few passengers venture.

The FBI, according to the agents at Dulles, has an office at the airport "because it is a convenient place." The agents say they occasionally check for fugitives at the airport and regularly conduct recruiting interviews.

The downtown FBI office, the agents say, needs clerk-typists. Ky Taylor says she is willing to work downtown, but wonders if is isn't "strange" coming to an international airport for and FBI job interview.

At 1:55, Capt. Hans Herdejhergen of the German Air Force browses in the main concourse for a paperback to read on the way to EI Paso. It is his 14th trip through Dulles from Cologne. Herdejhergen's opinion of Dulles: "For a short time, I like it. But it is too far from town. You can't go somewhere."

At 2:03, air traffic controller Patrick Thawley, 31 strolls across the main concourse, taking a break from the glowing green radar screen and eating a butter-pecan ice cream cone. Only on Fridays, Thawley says, does he permit himself such a diversion.

At 2:36, a loud voice echoes through the room where the mobile lounge drivers lounge: "Mel, hey Mel!" Melvin Belt, who got up at 4:30 a.m. fixed his lunch and drove 64 miles to work, slowly moves to his feet from his supine position on a sofa and heads for a flight from Denver.

Belt, who for eight years has been driving the 70,000-Pound mobile lounges that haul passengers between the terminal and their planes, rolls off at 15 miles an hour on the one-half mile drive to United Flight 166.

Belt has seen, he says, "a lot of celebrities," people like Gregory Peck, Andy Devine and the like. Belt says he doesn't count members of Congress as celebrities.

At 2:55, the passengers from Denver crowd into the mobil lounge. Robert and Mercedes Eicholz, suntanned, bundled in fur coats and carring a dog box, take a seat at the front of the lounge.

The Eicholzes live sometimes in Aspen, sometimes in Washington. When they move they take their dog, a rare shih tzu. Their shih tzu is named after Tu Fu, the great Chinese poet born in 712. The dog, they say, flies well.

Robert Eicholz, a retired Washington attorney, says Dulles is the "nicest" airport in the United States because "you don't have to walk miles and miles like in Denver."

At 3:15, the Dulles crunch starts. Passengers in the main concourse drown out the Muzak. In the control tower's radar room, the number of blips on the screens is growing as minutes pass. Chuck Moyer, an air traffic controller with 21 years experience, is bringing in seven jets on one of the two radar screens that minitor incoming traffic. In another hour, Moyers will handle between 10 and 15 planes at once, making sure they are at least five miles apart.

Dulles has three runways and the capacity to handle 130 planes an hour, which rarely happens, according to Jim McCafferty, assistant chief at the tower.

At 4:10 flights from London and Paris are arriving as departing passengers headed for the West Coast queue up in lines 30 or 40 deep behind the metal detectors.

Erika Fahrney, who just arrived from Switzerland after a 15 hour flight that connected in London, turns away from a customs officer and runs. Her daughter, Mary Ann, 1, is running toward a small door that opens to the outside. The mother, looking exhausted, wins the race and carries Mary Ann back to customs.

In a waiting room in the interntional passenger arrival area, a bald Iranian man smiles elatedly and speaks machine-gun Arbic to Donald Morton, airport manager for British Airways. Before the Iranian, who does not speak English met Morton, who speaks Arabic, his only contact in Washington was a piece of paper with his son's phone number written on it. The number is dialed, the son is home and he will pick up his father immediatley.

At 4:30, Grete Korsgaard, who speaks Danish, Norwegian, Swedish, German and, of course, English, tells a woman, in English, to take a bus into Washington and avoid dishing out $25 for a cab.

Korsgaard, the one paid member of the International Visitors Information Service staff, which has been providing assistance to foreign travelers at Dulles since 1976, said a Frenchman was one referred to her after he tried to take a taxi to Orlando, Fla.

At 4:45 the bar is full. In the baggage area, designed to handle the luggage of the more than 300 who ride on wide-bodied jets, a baggage train wait for a half hour for room to unload.

Outside, the loop of the Dulles access road is clogged with traffic. Dulles Airport police estimate 89 percent of the traffic is commuters from the Reston-Herndon area using the road intended by the Federal Aviation Administration for use only by airport traffic.

At 5:40, Thomas Edwards, a cargo agent at Pan American's international freight warehouse, tosses fresh greenery from Guatemala used for funeral floral arrangements into a Baltimore-bound truck. "A lot of people must have died this week in Baltimore," Edwards remarked to no one in particular.

At 6:45 Dulles is getting sleepy again. It is dark outside and the terminal glows like a transluscent wing. Janet, who works at the Hartz rent a car counter has no one to wait on. An hour earlier, she had 40 customers.

At 9:55 out on the Dulles Access Road two airport police cars are parked, looking for teen-agers with heavy feet and hot cars.

At 9:57 in the Marriott motel near the terminal, a round-faced woman with dark hair and a concerned smile greets a visitor from the airport: "Hi, Are you here for marriage encounter?" The hotel, the visitor learns, is the site for a seminar that was part of a marriage enrichment program.

At 11:35 a noisy, ski pole-dragging crowd assembles in the near-deserted airport in front of the United ticket window. They are here for a $400 which includes six days of lift tickets and hotel accommodations.

Ed Randel, a chemical engineer from Baltimore who is putting his skis in a plastic bag, has the flu.

Randal says he plans to go to bed when he gets to Sun Valley. "I got a lot of money invested in this trip. I go or I lose it," he explains with a wan face. Behind him, Patsy Hottinger, who lives in Loudoun County and works the 9:30 to 6 a.m. shift, is vacuuming rugs.

At 1:10 a.m. Elmer Lloyd eases the accelerator on his electric floor scrubber. No one besides Lloyd can be seen in the east baggage claim area. Lloyd says he walks 12 miles a night, wears out three pairs of work shoes a year and finds the work safer than driving a truck.

Lloyd, who works the midnight to 6 a.m. shift, greets a visitor warmly and says: "There are night I don't see anybody."