MISSION PEAK, CALIF. -- Kevin Giffen was engrossed in the flight of his sail plane, keeping an eye on the cloud bank above him and the rocky ridge below, when he first saw them. "Body parts," he said. "Feet hanging out of the clouds."

Startled but not particularly concerned, Giffen finished his flight and, back on the ground, told and retold the story of his close encounter with the hang gliders just a few feet over his head.

"I was just amazed that they were up in the clouds," he said.

Pilots such as Giffen, 62, manager of a small private airport in nearby Fremont, have flown their motorless sail planes above 2,500-foot Mission Peak since the 1920s. The hang-glider pilots, harnessed into kite-like wings, are relative newcomers.

Both share the space, usually good-naturedly and occasionally with reference to an informal agreement that outlines rules for their coexistence in a stretch of sky just a few miles from the path of airliners bound for San Francisco International Airport, 25 miles to the northwest.

The peak's attraction, hang-glider pilot Patrick Denevan said, is the "texture of the air" that generally surrounds it, air that on a good day promises enough healthy updrafts to help keep them aloft for 30 minutes to an hour. It is rare to find not only such air but also a bit of open space on the eastern edge of the urban-suburban sprawl that stretches uninterrupted from San Francisco to San Jose, 50 miles down the coast.

At its highest point, the 1,875-acre Mission Peak Regional Preserve is more than open. It is deserted. Denevan, 39, owner of a hang-gliding school and equipment store, had passed only a handful of hikers as his four-wheel-drive van crept up the peak's twisting dirt road, and now he was alone at the top.

He had come to check out a new hang glider purchased by one of his customers, to take the machine on its first flight since factory test pilots approved it. He unrolled the two blue, green and white wings, which together spanned 35 feet, hitched them to their metal harness and awkwardly carried the contraption to the sloping edge of the peak before strapping himself in.

After analyzing the wind for a minute, he took four running steps downhill and lifted off for a 30-minute flight, gliding toward the valley, catching an updraft to rise 100 feet or more above the peak and repeating the process again and again.

The takeoff fit the image that he knows most people have of hang gliding.

"People think you learn to fly by leaping off a cliff," Denevan said later. But beginners do not. Instead, he said, they are kept just a few feet off the ground while they master the basics -- how to take off, turn and land -- and until they pass a written test on the rules of their sport, only loosely regulated by the Federal Aviation Administration (FAA). "People have to be really well-qualified to fly more than five or six or 10 feet off the ground," he said.

Deneven's school, opened in 1973, draws a large number of scientists and engineers, he said, probably in part because of the appeal that hang gliding holds for people who "are able to figure things out . . . and know it's not magic that keeps us up there."

Giffen's clientele at the sky-sailing glider port is similar -- physicists and computer technicians, bankers and retired airline pilots, all likely to express fascination with wind and aerodynamics. Their studies are more stringently outlined by the FAA, which issues them licenses as glider pilots when they can prove that they meet government standards. Their flights involve more equipment -- not only the two-place sail plane for the student and the instructor but also a tow plane and its pilot to pull them 3,000 feet or more into the air for lessons or sightseeing flights.

Only the more advanced sail-plane pilots soar at Mission Peak's ridge, and they ask tow planes to steer them there only when they think that the signs indicate good wind and a long flight. If their hunch is wrong and they compound that mistake in judgment by letting their sail planes drop too low in a narrow gap between two peaks, they risk smashing into the rocks, Giffen said.

He recalled a drawer full of photographs of between 30 and 40 sail planes that crashed on those rocks. The photos once illustrated instructions given to pilots before their first flights at the ridge.

The instructions still are given, without the photos but with tips on how to recognize what might be a good day on the ridge.

One of the good signs, according to Emil Kissel, 71, a 50-year veteran of soaring, is the sight of hang gliders on the mountainside, seeking out spots that have the strongest updrafts.

"I find them to be a big help," he said. "I watch them in thermals {rising warm air}, and they know what they're doing."

The ridge has changed since 1946 when Kissel first flew over the cattle ranch that spread across the hillside and the farmers' fields that filled flatlands to the west. Cattle still graze on the hills, but the fields have been replaced gradually by houses, apartments, shopping centers and light-industrial complexes stretching from the base of the ridge to the fence around the 21-acre glider port.

The future of the port itself is uncertain because of plans to build an automobile-sales park on the west side of the field. Those plans drove the previous operator away in the spring of 1989, closing the port for several weeks before Giffen and three other sail-plane pilots managed to revive it.

There has been no real battle

to save the glider port, which opened in 1959. Instead, the operators are considering moving, perhaps far enough away that another spot would replace Mission Peak as the destination for their best pilots.

Whatever the eventual alternative, things would not be quite the same anywhere else, Giffen said. "There's no ridge like this," he declared. "The pleasures would deteriorate."