When two or three pilots get together on the ground, it's called hangar flying, a euphemism for internal yammering about the time this radio went out, or the time that landing gear didn't come down or when that controller at Dulles wouldn't let so-and-so do you know what . . .

When 400,000 flying enthusiasts get together as they have here since last weekend, call it certified aviation monomania. Oshkosh is usually known for the overalls it manufactures; during the first week of August it becomes pilot heaven.

Where else, for instance, would thousands of people sit under trees escaping the heat of the day by listening to communications with a control tower on their portable radios?

"Bonanza 38125, you're ninth for the approach, follow the blue and white home-built with the rear engine."

It's the stuff that separates the putters from the fliers. On an average day here up to 10,000 planes land and take off making Wittman Field three times busier than Chicago's O'Hare, the world's busiest airport. It's so busy here that you can develop New York subway paranoia, a.k.a. convention dread - the fear your're going to run into the one person in a million you don't want to see.

Consider Jim Geirg, who flew 800 miles here from Connecticut in his Citabria to get away from his hometown flying buddies.He's clear to land, pulls off the runway, hops out of his plane and discovers his tie-down neighbor from Danbury Airport is right behind him.

Year round, Oshkosh is the home of 53,000 residents, many of them metal workers and wood processors. The town lies in the center of Wisconsin's flat, lake-spotted summer vacation area, about 75 miles northwest of Milwaukee. Although it's visited by more fliers than any other group of people, the city is best known for its gray and white-stripped Oshkosh-by-Gosh overalls.

The Experimental Aircraft Association, whish sponsors the Fly-In, began coming here in 1970, after two previous sites had become too small. The first annual Fly-In, a quarter of a century ago, was held at Milwaukee's Timmerman Field. About 40 planes showed up. By 1960 crowds forced the event to Rockford, Ill. Ten years later industrialization around that airport forced the second move.

While virtually all of the people here are aviation nuts, they do represent a broad sample of America - a steelworkers from Birmingham; a psychiatrist from Boston; an aeronautical engineer up from cape Canaveral; a car mechanic from Montana; a United Airlines pilot; a city cop from Macon, Ga.

For 30 minutes a man from St. Paul and five other pilots fly their make-believe planes on a course to runway 36 at New Jersey's Peterboro - they're trying out a flight simulator in the gymnasium-sized exhibit. It's a circuitous path - though a standard instrument approach the commercial pilots fly every day - and at the end of it all, a man running the computer says a little nervously, "I wouldn't get into a plane with any of these guys" - all of whom are pilots.

"Well," says St. Paul, "that's why I do it for pleasure. Anybody who's flying to New Jersey is crazy."

Down the row a guy from Bozeman, Mont., is talking about how he and his brother got interested in flying during World War II.

"That way it," he said. "The ultimate way you could prove you were a man - shooting [someone] out of the sky. I was a failure. I couldn't pass the camouflage test. That's to say I couldn't read things printed in funny colors [he's color blind] so I couldn't keep up with my brother who went into the service. I had to learn on my own. He got out and got a job as an agpilot [spraying crops]. He did that for almost 30 years and only had one accident. Flew his plane into the trees. He got out and had about 10 feet to jump down and he slipped on the wing, fell and broke his collar bone. Damn, that wasn't the only time Roger wasn't careful. He said the reason he was so safe all those years was that he'd never fly under power lines, like most sprayers do."

There's a man here who invented what he calls the flying car, which he brought along. You land the thing and in 10 minutes the wings are off and the propeller shaft is connected to the wheels - presto - you've got a car on your hands. "You never have to have anyone come out to the airport to pick you up," he said.

And there's a man from Chicago who flies a plane called Breezy which has no cabin just a bunch of exposed struts.

"Sure it gets cold in Chicago," he said. "You just wear a snowmobile suit in the winter when you're flying."

You want checks with a photo of your airplane? What color paper? How about old generators from fighter bombers, miniature parachute packs, 50 kinds of headsets, a book containing photos of every airport in Kentucky, 1,100 home-built and military aircraft on display, a United Airlines 707 flying 50 feet above the deck, a special chemical that will stop your windshield from fogging? They're all here.

And then there is the man demonstrating the Convenience Bag, "for motion sickness and liquid disposal in flight." An automatic seal, he's saying, prevents leaking and spills but when somebody picks the thing up in his hands a stream of water erupts.

A guy from Texas is staring at Fift, the only B-29 Superfortress that's still operational - now owned by a group called the Confederate Air Force - and he's recalling his days in the Pacific and nights sleeping on the huge, tail surfaces of the plane and his main memory of the war - he's repeating this over and over - is that the Japanese used to try to come aboard to steal syringes of morphine from the first aid kits.

"You know," he says, five times in succession, "Those people were really addicted to drugs."

And some kid - who has obviously been dragged to the show - says that morphine addiction is nothing compare to aviation monomania.

And this guy says that's the most un-American thing he's ever heard.