The grimace on Mayor Anthony A. Williams's face grew too pained for Senior Airman Sheron Mosely to ignore.

She paused in the midst of a long briefing yesterday morning, stopped describing all the things that could go wrong if the mayor had to bail out of the D.C. Air National Guard F-16 jet in which he was about to fly.

"Do you still want to continue with this?" Mosely asked the mayor. "You're starting to look a little nervous."

But Williams (D) waved her on. It was too late to turn back now.

Williams had come to Andrews Air Force Base, the D.C. Air Guard's headquarters, for what he described as "a once-in-a-lifetime experience." D.C. Guard officials invited the mayor to take the ride as part of an introduction to the military organization. As an added incentive, officials set it up to coincide with the mayor's 48th birthday.

By virtue of its location near the nation's capital, the D.C. Air Guard routinely flies dignitaries, from Pentagon officials to congressional aides. Earlier this summer, the Guard took the president of Ghana on a spin in an F-16. "He was in town, and the White House promised him a ride," a Guard official said.

Williams is the first D.C. mayor to go up, although council member Jack Evans (D-Ward 2) got a ride several years ago, officials said.

Williams "is the leader of the D.C. government, which we hope will be one of our biggest advocates and recruiters," explained Maj. Gen. Warren Freeman, commander of the D.C. National Guard.

Guard officials also said they wanted Williams to get an idea of the military resources the city could call on, both in times of emergency or for community improvement projects.

"It gets the mayor interested, so he thinks to use us," said Col. David Wherley, commander of the Guard's 113th Air Wing. "Now, obviously, [an F-16] is a capability he's not going to use in downtown Washington. But the resources and people who are part of this are all available."

Williams, for his part, saw the ride as just one more of his mayoral duties. "It's more glamorous, obviously, but to me it's like riding along on the snowplows or the garbage trucks," he said.

The VIP flights do not waste taxpayer money, officials said. "We'd be flying anyway," Freeman said. D.C. Guard F-16s fly from Andrews almost every day on training runs.

Williams spent nearly three years in the Air Force in the 1970s, but back then he was in public affairs, far from the cockpit of a fighter jet.

Yesterday's flight was close to a full-day affair for the mayor, who arrived at Andrews about 10 a.m. and was given a physical, a green flight suit and assorted training.

After the training, Williams nibbled nervously on some grapes while he awaited the order to board the plane.

"It's okay to eat this?" Williams asked Freeman. "I want to minimize any air sickness."

"You might not want to eat a full lunch," Freeman said tactfully.

As a last measure before walking out to the tarmac, Williams tucked an air sickness bag into a pocket.

William's pilot, Maj. Bill Shelton, flew straight up on a high-G-force takeoff, climbing 15,000 feet in 45 seconds--"a helluva climb," as later described by the mayor.

When the F-16 landed after an hour's flight and rolled to a stop in front of the Guard hangar, the canopy popped up and an ebullient mayor appeared. "He did great," Shelton said. "He never said stop the whole time."

Shelton even let the mayor take the plane through several loops.

"The plane," Williams noted, "didn't crash."