It was an ugly crash. But at least it was in a beautiful place.

Don Vetter's pennyweight aircraft had been in the air two minutes and was still making a perfect spiral climb when it took a direct hit from a World War II-vintage plane at least an ounce heavier. The crowd gasped. Both aircraft ended up on the huge floral carpet, but only one would fly again that day.

"It's not any stronger than it has to be to do its job" -- which, of course, doesn't include midair collisions -- Vetter said as he studied the wreckage. It resembled a translucent dragonfly from an unexplored evolutionary path. He removed its wounded tail, delicately setting it aside.

"When you fly planes of this class with all these others," he said with sporting resignation, "you might anticipate this would happen."

And indeed you might, for yesterday was a rare opportunity for the brotherhood of balsa wood and tissue paper. The National Building Museum hosted an indoor air show in its Great Hall.

For more than four hours, one of Washington's great spaces was the scene of the sort of activity that's almost not legal any more -- people having a great deal of fun with unpredictably moving mechanical objects.

There are a lot of builders of little airplanes around. The Academy of Model Aeronautics, the pastime's biggest organization, has 160,000 members. But it's a group constantly in search of a playground.

Fliers seeking to test super-light, indoors-only aircraft are usually forced to conduct guerrilla operations in vacant gymnasiums and field houses. And these are, in the main, elderly men and fathers with adolescent children.

"This is the only place in the metropolitan area that lets us fly," Norman Davison, 74, a retired nuclear engineer, said ruefully between many flights of numerous craft.

"There's never been an injury. I don't know of even a nick in a column or paint," said Paul Spreiregen, 71, a Washington architect and model flier who arranged for use of the Great Hall about eight years ago. Now, it's open as an air park about three Sundays a year.

The secret? These airplanes are each powered by an elastic band twisted about a thousand times -- "the motor," the fliers all call it without a hint of irony.

Despite the delicateness of the craft -- or perhaps because of it -- there were moments of knuckle-whitening suspense yesterday. There were moments of exultation. And there were moments that didn't seem quite as absurd to the people they happened to as one might have expected.

Dave Mitchell, 42, a furniture maker from Takoma Park, took a blue airplane in his right cheek, plucked it off and held it out, spinning-prop down, for its owner to reclaim -- all without removing his eyes from his own soaring craft.

"The reason I didn't miss a beat is because my plane was in the midst of an impossible feat," he said. The plane flew around one of the huge, faux-marble columns three times without hitting the column or a huge banner hanging next to it. "When something miraculous happens, you don't mind."

One of the afternoon's events was a last-in-the-air contest for "Phantom Flash" aircraft. They have paper-covered wings about 14 inches long, which are set in a shallow V and fastened to a balsa-stick fuselage. On the upper surface of each wing tip is a skull-and-bones decal. These are no-nonsense, boys-only aircraft designed in the 1930s, which is when some of yesterday's contestants built their first ones.

Thirteen took off simultaneously. For 30 seconds they made tight, nearly identical counter-clockwise circles, until each plane's aeronautical personality emerged. One landed on a cornice, and two on the second-floor balcony. But, unaccountably, none collided. They spent their four minutes in the air like a silent and nearly sentient swarm of insects.

Although there were six competitions, most of the day was given over to dozens of people running hundreds of test flights.

The planes are hard enough to make, but there's no guarantee that even a picture-perfect one will stay in the air. Dozens of steps of tuning are needed for that -- setting the rudder to create the proper turning radius, adjusting the wings to prevent stall, trimming the propeller to accommodate the rubber band's torque.

Some adjustments are so fine that they are made by imperceptibly bending the wood after moistening it with one's breath. At least that's the way Pete Zseleczky, 13, an eighth-grader at Severn Middle School in Annapolis, did it.

Pete took up model-airplaning after football season. He has made two.

"I'm really thrilled he is interested in this," said John Zseleczky, a naval architect, as his son followed, mother hen-like, a plane flying about head high in the middle of the atrium. "I'm an engineer, so of course I love it."

One reason for the popularity of models designed a half-century ago -- aside from their simple beauty -- is that people were returning to the field of long-ago disappointment and frustration.

"They are models we built as kids and never flew. No one knew enough to make them fly. But now we can," said Stew Meyers, 66, who is an aeronautical engineer at Goddard Space Flight Center.

Meyers grew up in Northern Virginia. He built his first model when he was 7 and World War II was underway. One of the highlights of his childhood was a flight in a Ford Tri-Motor from Washington to Cleveland.

"I was ga-ga about airplanes," he said.

A "masterful flier" is how Mike Moskow, 77, of Severna Park (who flew four of his own planes yesterday) described Meyers after watching a scale model of a prewar Curtiss Robin flawlessly go through its paces. He was right.

The planes get their energy from a tiny "motor" -- an elastic band twisted about a thousand times.Ed Zapolski, of Arlington, launches his plane in the Great Hall of the National Building Museum during an event hosted by the D.C. Maxecuters, a model airplane club. Club members say it is difficult to find a large indoor area in which to fly their planes.