“Kit” doesn’t tell you which kind of airplane you’re building

You are comparing two kit airplanes that look almost identical on paper. Both ship in crates, both come with a build manual, and both brochures say “kit.” Before you write a deposit check, ask the question that actually matters: what kind of airplane will this kit make when it is finished?

“Kit” only describes how the parts arrive. It does not decide how the finished airplane is certificated. The FAA treats an amateur-built kit and a light-sport kit as separate experimental routes, each with its own requirements, so working out which one you are on comes first.

The amateur-built route

Section 21.191(g) covers an aircraft “the major portion of which has been fabricated and assembled by persons who undertook the construction project solely for their own education or recreation.” FAA guidance gives “major portion” a number: more than 50 percent of the fabrication and assembly tasks, often called the “51-percent rule.” The FAA compares the builder’s work with everything needed to finish the airplane, leaving out standard purchased items such as engines, propellers, wheel and brake assemblies, instruments and standard hardware.

Kits get evaluated against that test. The FAA keeps a list of amateur-built kits it has evaluated, and its checklists credit each task to the kit manufacturer, to paid commercial help, or to the builder. The FAA does not certify or approve the kits themselves, though. It evaluates whether a builder who follows the manual can meet the major portion requirement. Hired help counts: in the FAA’s own example, a kit with 40 percent of the work already done by the manufacturer leaves room to contract out less than 10 percent.

The FAA also recommends talking with your local FAA office before you start. Bring a three-view sketch, drawing or photo of the project and a rough finish date, and say so if you plan to use paid help.

The light-sport kit route

Section 21.191(k) is a different purpose: operating light-sport category kit-built aircraft. It covers an aircraft of a type already certificated under § 21.190 and assembled from a kit following the manufacturer’s assembly instructions, which have to meet an applicable FAA-accepted consensus standard. Here the manufacturer has documentation duties: the FAA says light-sport manufacturers must provide a statement of compliance, on FAA Form 8130-15, for each new aircraft kit they deliver. The rule does not say how to show that a kit’s type has been certificated under § 21.190, so ask the supplier to show you in writing.

The older provisions for kit-assembled and previously certificated light-sport aircraft, §§ 21.191(i)(2) and (i)(3), no longer permit new experimental certificates after October 22, 2025. The current text separately provides (k) for kit-built aircraft, as above, and § 21.191(l) for former light-sport category aircraft. That cutoff concerns new certificates, not existing ones. The FAA’s FAQ says a recurrent certificate for an aircraft originally certificated under (i)(2) or (i)(3) is issued under (k) or (l).

A hypothetical example

Imagine you are comparing two kits. Kit A is sold as an amateur-built project. Kit B is sold as a light-sport kit with a manufacturer’s statement of compliance.

For Kit A, you would ask whether the kit leaves you able to do more than half of the fabrication and assembly yourself, and whether the FAA has evaluated it. For Kit B, you would ask for the statement of compliance, how the supplier shows the type is certificated under § 21.190, and which consensus standard the assembly instructions follow. Dates matter on Kit B: the FAA’s FAQ says its light-sport certification changes take effect July 24, 2026, and if a kit’s statement of compliance was signed before that date but you finish assembly afterward, the standards in place when the statement was issued apply and you may apply under § 21.191(k).

An example like this cannot tell you whether any real kit qualifies. That takes the supplier’s documents and the FAA.

The takeaway

Identify the certification route before you buy, and get the supplier’s answer in writing. Whichever route you take, keep a project log with photographs; the FAA’s inspector guidance calls them the most essential information for working out who did what. Then check pilot eligibility separately. How an airplane is certificated and what a sport pilot may fly are decided in different places, so a change in sport pilot privileges does not change the certification basis of a finished homebuilt.