She wants to go from cover girl to undercover agent, from spike heels to flatfoot.

Top model Aria Riccardo, 21, whose face is well-known from cosmetic ads and magazine covers, has just passed the written exam to be a New York City police officer.

It's not that she wants to turn in the designer clothes she models for the standard blue uniform, or reject her $2,000-a-day modeling rate for $51.23 per diem as a rookie. It's possible to do both, she says. "Both jobs give you a lot of freedom," she said yesterday. "I can model for a full day and then work for the police at night."

Her father, a retired New York City police officer who helped coach her for the police exam, has tried to discourage her from a job she thinks will be "exciting." She pooh-poohs her boyfriend's warning that police work is dangerous. "Anyone can walk out on the street here [in New York] and something can happen to them," Riccardo says.

But why does she want to do this?

"I just want to help people," she says. And she wants to help change a view that all police officers are "mean," she adds. "I want to show that policemen are human."

Her start in modeling also came as a surprise. At age 16 a friend who lived near Riccardo in Queens wanted a summer modeling job and asked Riccardo, who was more streetwise about Manhattan, to help her find the modeling agency where she had an appointment. It turned out to be Riccardo that the Zoli agency wanted, not her friend.

Modeling assignments came quickly. A few months after she signed with Zoli she was sent to Rome to do pictures for Italian Bazaar. The next year she was off to Japan for Issey Miyake and Hanae Mori and to Milan and Paris for the ready-to-wear shows. She had to quit the High School of Fashion Industries for a more flexible schedule at the Professional Children's School.

She was a "nervous wreck" when she got her first runway job from designer Stephen Burrows. "I had never worn high heels. I was a tall, skinny, flat-chested kid that no one looked at. I certainly didn't want to be taller so I always wore flats," she said. "Now I'm used to being in and out of shoes of all kinds."

She met boyfriend Michael Holder, also a model, on her first assignment. They lived together until recently. "I needed my own space. He's a lot older--he'll be 35 in June and he worries about me a lot," she says. She comes to Washington frequently to see friends, including an old beau who was once at medical student at at Howard University.

Next on her schedule are more tests before she becomes a probationary officer, the lowest rank on the police force. The written part was easy. In addition to her father's help, she took a special class on Friday nights for two months. She was a good student but her instructor was skeptical. "Where'd you get the fancy car? I don't have a fancy car," he would say. "What else do you do, Riccardo?" he wanted to know. She never told him. "I said, 'It doesn't matter.' "

She expects no difficulty with the next hurdle, the physical test, which includes climbing a six-foot-high wall. "Then I have to pull this 150-pound dummy they call Mr. Bill. I weigh less than he does." She weighs 115 pounds.

Recently she modeled the clothes of Tunisian designer Azzedine Alaie. "They are fitted and sexy, they feel womanly and sleek. They get you lots of attention," she says.

But now she wants to work toward a job as an undercover detective, wearing her own clothes. For starters, though, she would wear the uniform blue shirt, pants and tie. "Maybe we could get Halston to design some new ones," she says.