Of course, to build, maintain, and fly a custom-built aircraft you’ll need to follow some rules. There always are rules. Though the Wright brothers were the first folks to build and fly their own plane, it wasn’t until 1947 that the FAA allowed “homebuilt” aircraft to be certified for flight. Not long after that, Paul Poberezny founded the Experimental Aircraft Association (EAA), the leading resource for people who build what are legally called “experimental” aircraft and popularly known as homebuilt or custom-built aircraft.
How do experimental aircraft get around the FAA certification process? They don’t. They fall under a special rule, CFR-14-21.191(g), that covers how experimental or amateur-built aircraft can be built and operated. In simple language, it says that such aircraft are built with the sole intent of enhancing the builder’s education and recreation, and that the builder must have done a majority (at least 51 percent) of the airframe construction, not including the engine, propeller, interior, an so on. You’ll hear this referred to as the 51-percent rule.
The FAA has since issued an advisory circular (AC-20-139) that reaffirmed and clarified the rules for building experimental aircraft, including what a manufacturer can and cannot do in building an aircraft kit for you to assemble. So any experimental/amateur-built/custom-built you build from scratch or a kit must comply with these FAA regulations if you want to get it legally certified as airworthy. Without FAA certification it’s a funny-looking car.
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Fortunately, those who offer aircraft kits work with the FAA to make sure their planes, if built according to directions, not only comply with the 51-percent rule but also are airworthy. In fact, the last stage of construction is having your aircraft checked out by an FAA designee and signed off as airworthy. Believe me, the designee isn’t going to sign you off until he or she is thoroughly convinced that you’ve built an airworthy aircraft.
"Man cannot discover new oceans unless he has the courage to lose sight of the shore."
-- Andre Gide