For students in Herndon High School's aviation program, the sky is literally the limit.

Under the direction of Jay Blair, 30 teen-agers are building, bolt by bolt, a two-passenger biplane called a Steen Skybolt that will eventually have a 180-horsepower engine. The upper wing spans 24 feet.

It is said to be the first airplane to be built by high school students in Fairfax County and one of only two planes being built by students in the Washington area. The other is a single-passenger airplane under construction at Laurel High School in Prince George's County.

When it is completed in early 1986, said Blair, the school will apply for certification of the plane by the Federal Aviation Administration.

Blair, 41, said he came up with the idea shortly after taking over the school's aviation program in December 1978. "Rather than have aviation be a dull physics or science class, I decided to give the kids hands-on experience," he said. "I wanted to do just as much as we could within the confines of the school."

A friend suggested building the Steen Skybolt, Blair said, because the planes generally have a good resale value, are relatively easy to fly and are built with standard aircraft components. It was icing on the cake, said Blair, that the plane is a two-seater. "Students need to feel as if they can ride in it someday. If it only had one seat, they wouldn't get that feeling," he said.

Blair, who holds a commercial pilot's license, said he had never been involved in building an airplane before he started the project. "I've learned as much as the kids," he said. Even now, he added, "I'm still learning as I go."

David Edney, who graduated in June after spending four years working on the plane, said it is better built than most do-it-yourself planes. "We've had people from the EAA Experimental Aircraft Association come in, and they've said that the general workmanship is better on this plane than on most home-builts they've seen." He said this was the result of Blair's high standards. "If we messed something up, we had to do it over and over and over until we got it right."

Edney recalled that while it was sometimes frustrating to be involved in such a massive undertaking, especially at the beginning, the effort was worth it. "You'd start something and say, 'Okay, this isn't going to work,' so you had to figure out some other way to do it. But once you figured it out, it felt pretty good," he said.

Despite the hard work entailed in building the plane, aviation is one of the most popular classes offered by the school, students said. "This class gets me through the day," said Trevor Brown, a sophomore now in his second year in the program.

Norm Viands, a senior taking the class for the third year, said, "The thing about this class is that you always have something to do. And when you finish, you can look at the plane and say, 'I built that.' "

Viands said the accomplishment of which he is most proud is building a test panel for the wing. Seniors Chris James and Jeff Peacock built the plane's 33-gallon fiberglass fuel tank. "It was one of the hardest things to make on the whole plane," Peacock said.

Blair, who also teaches a consumer mathematics class, said the aviation program is divided into two beginning and two advanced classes, with each class period lasting 45 minutes. First-year students are not allowed to work on the plane, he said. They concentrate, instead, on learning the basics of aviation. Grades, Blair said, are based on how a student approaches the class and not necessarily on ability. "If a kid is serious about what he is doing and doesn't get involved in 'bull sessions,' then he'll generally do okay," he noted.

Blair's use of "he" is not just a figure of speech, for in the six years he has conducted the class, there have been no female students in the advanced classes. "I think there were a few girls in Aviation I a while back," he said. He attributed the lack of female students to the fact that girls at the school, for whatever reason, do not seem as interested in engineering and mechanics as boys.

The class, said Blair, "is whatever the kids make of it, from a general introduction to aviation to serious training for a career in the field." A number of students, he noted, have begun aviation careers.

David Edney, for instance, is one of many former students in the program now at Embry-Riddle Aeronautical University in Daytona Beach, Fla., considered a top-notch aviation school. Edney noted that he has not met anyone who built a plane -- or even a part of it -- in high school. "It definitely put me ahead."

But, said Blair, the program's success has not shielded it from a familiar problem: lack of money. The plane will end up costing between $13,000 and $15,000. More immediately, said Blair, "We're looking at anywhere from $6,000 to $8,000 for an engine. Once we get that, we'll be rolling." Blair, who said the program receives about $600 a year from the county, adds that he hopes to secure a grant from industry for the engine.

The plane is "about 60 to 70 percent finished," he said. When they are done building the plane, students said, they will paint it red, black and white -- the school colors -- and number the plane N155HH. All planes in the United States are registered with the prefix N, said Blair, and the rest stands for Room 155 Herndon High, the classroom that serves as the plane's hangar.

Blair, who will probably serve as the plane's first pilot, said it should be "no big particular problem" to fly the craft. But he admits to some apprehension. "I've been away from it flying for years. Hell, I might even get scared."