A mechanical replica of the giant prehistoric pterosaur went into a beak dive and crashed during a demonstration flight at Andrews Air Force Base yesterday. There were no injuries -- except to the model, which was decapitated on impact, disappointing thousands of spectators who had hoped to see the $700,000 flying reptile in action.

The lifelike, radio-controlled contraption played a leading role in the National Air and Space Museum's new movie about natural and mechanical flight. But people will have to see the film, "On the Wing," making its debut next month, if they want to catch the creature in some of its better moments.

"I'm very disappointed for the crowd," said Walter Boyne, the museum's director and one of several red-faced Smithsonian Institution officials and project engineers who watched the crash. "But that's the way it goes."

Or doesn't.

The pterosaur was making its first public appearance at the annual Department of Defense Joint Services Open House at Andrews when it ran into trouble just seconds into the first of two scheduled exhibition flights. With thousands watching from a distance, the 44-pound plastic replica toppled out of control as soon as it was released from a towline that pulled it to a height of 400 feet.

Engineers from the firm that designed and built the model said the tail boom, used to provide stability on takeoffs, dropped off several seconds too soon -- before the operator on the ground had activated the "bird's" automatic pilot.

The model made a brief recovery in the air after the auto-pilot was turned on, but by then the force of the wind against the pterosaur during its hapless dive had cracked the plate holding the creature's head to its neck. It plummeted, hitting the ground near a runway and snapping off its head.

The sight of the pterosaur crashing to earth was not a new one. The original beast, which disappeared from the skies about 65 million years ago, had an almost flight-defiant shape -- no tail, long neck and big head -- and these gawky-looking features make the replica difficult to keep in the air.

During months of testing in California, various models crashed 50 times before enough of the kinks were worked out to allow filming. But since then, Boyne said, the final model had flown "perfectly" 21 times in a row before yesterday's mishap.

"For some reason, the tail boom released itself prematurely," said Ray Morgan, who operated the radio controls. He speculated that competing radio signals may have caused some interference and said it was the first time in months of testing that the boom had fallen off too soon.

"This particular failure never happened before," he said.

The headless replica from the Age of Dinosaurs was recovered and trucked back to the main airfield, where it drew a crowd of sympathetic onlookers.

Others, however, dubbed it a "Dinoflop," and soon the fickle air show audience was agog at five Marines dangling from a helicopter. Nearby, the sleek aircraft of the Navy's famed Blue Angels aerobatic team sat on the runway awaiting an afternoon performance.

Air Force officials said the Andrews open house drew a crowd of more than 300,000, including thousands who never made it onto the base and chose to set up camp with their beer and binoculars along bordering roadways. Maryland State Police said traffic was backed up several miles in both directions on the Capital Beltway because so many motorists were trying to attend the air show.

Paul MacCready, best known for his prize-winning exploits with human and solar-powered aircraft, looked on glumly as engineers from his firm, AeroVironment Inc., set about repairing the model, which will be put on display at the museum this week.

Dressed in a floppy hat, patiently explaining to one and all what had gone wrong, MacCready said he still thought the two-year pterosaur project was a success because his team had created a wing-flapping duplicate good enough to star in the film.

Because of technical problems and a film deadline, the final model has a wingspan of 18 feet, half the size of the real thing.

The replica was built to resemble what is believed to be the largest pterosaur ever in existence. Its fossil remains were found in west Texas in 1972 and given the name Quetzalcoatlus northropi. Quetzalcoatlus is a Latinized form of the Aztec god that took the form of a winged serpent, while northropi is a reference to the Northrop Corp. and its experimental Delta Wing airplane, which like the pterosaur, had no tail. The Latin word pterosaurus means winged lizard.

The museum, AeroVironment and Johnson Wax shared the cost of designing and building the pterosaur and will share in the expected profits from the sale of T-shirts, models, books and other products. The model is called "QN -- The Time Traveler."

For yesterday's expected flight, more than 100 members of the media tromped over a weedy, bug-infested airfield to get a closer look at the creature. And though MacCready warned that the model might "do something different than what you expect," neither he nor the press seemed prepared for the QN's ignominious downfall.

Spectators "oohed" and "aahed" when the model took off and cried out in concern when the replica started to tumble. But later, when the creature had been scooped up off the ground and it was time for what Boyne called the post mortem, there were the inevitable jokes, even from the QN's most ardent backers.

Grumbled one official, a paleontologist: "Now you know why it's extinct."