CAREENING feet-first down a steep mountain at 80 miles per hour in a tiny sled is Bonny Warner's idea of a good time.A member of the United States Olympic luge team, she divides her life between the rigorous thrills of the luge and flying - as a flight instructor and as second officer for United Airlines. The qualities that make a good luger, she says, are those needed to make a good pilot - mental stability, self-confidence, precision, finesse, coordination, cool head under pressure, courage, and appropriate caution. "Some people think I must be a daredevil," says Ms. Warner in an interview between flights. "But the truth is, you can't be reckless in sliding or in flying. I'm not a risk-taker. No hang-gliding or sky-diving for me." Mental calm is essential, and after that, the exact anticipation of each move and the precision performance of the most subtle motions makes the best luger. Warner enjoys precision. It's a good thing, since both sliding and flying require it. The luge is steered primarily with the feet, but really with the whole body. The slightest movement of the shoulder can change the direction of the sled. Enormous upper-body strength is required just to hold the position - lying back on a short sled so that the neck and head extend over the rear of the sled, the head raised only enough to see the path. On the curves, a slider will pull three to four times his gravitational weight. Looking like a space cadet in skin-tight suit and close-fitting helmet, the luger launches the sled sitting up, pushing off by hand at the starting gat e. A fast launch and instant assumption of the prone position help ensure top speed down the winding track three-quarters of a mile long. Luge is the only Olympic sport timed to the 1000th of a second. "It looks like the slider is not doing anything," says Warner, "but you are controlling precisely the path of the sled. The shortest path is not the fastest, neither is the longest path the fastest." It's the middle ground she seeks, and she knows every track she races by heart. Warner trains five months a year, seven days a week, from 7 a.m. till 10 at night. The luge slide may look frightful with its high- banked walls at each turn, but Warner says it's perfectly safe if you know what you are doing - safer than almost any other advanced sport. Olympic luge injuries are few and very minor, despite the fact that "you may be 20 feet up a wall going 60 miles per hour, but you're held to the sled by centrifugal force." The slider must keep his or her body straight, turning the sled with leg pressure on the runner and an opposite shoulder pressure on the sled's bo dy. Men's tracks are longer and, because men have greater upper-body strength, they do not compete with women. Weight is an advantage, so lighter sliders wear lead weights. But because a smaller, weighted body is also faster than a larger body of the same weight, there is a formula for balancing the two. As a 14-year-old high school student, Warner heard an explorer describe how he had made a list of 100 things he wanted to do with his life. He'd done most of them. "I couldn't think of 100 things, but I could think of five I wanted to do: Go to college, participate in the Olympics, build my own log cabin, be a pilot, and work on TV." Bonny Warner has achieved nearly all those goals. At 17 she saw her first luge track at Lake Placid, N.Y. A fair skiier and an avid hockey player, she knew it would take a lot of money to train as a skier for the Olympics. But luge was a relatively new sport in the US at the time, and there still wasn't a great deal of competition. Warner took some lessons and decided luge was for her. She went to Germany where she naively asked the German Olympic coach if she could train with his team. He thought she must be very advanced to ask to train with the best team in the world, but soon discovered she was a raw beginner. He decided to train her anyway, so she wouldn't hurt herself. And when she tried out for the American team the following season, her German training had been so excellent she was immediately accepted. That was in 1980. Today, she says, it takes 10 years to become competitive at the sport. There is still only one luge track in the US, so luge is not a recreational sport there yet. One of the world's oldest sports, it is believed luge began in the Alpine countries, spreading north through Europe. From a southern French dialect, the word "luge" means sled. By 1480, luging was an established competitive sport.