The 51% rule decides whether your project earns an amateur-built airworthiness certificate or gets denied. Many first-time builders assume it is about hours, weight, or money. It is not. This article explains what the rule actually measures, how the FAA checks it, and the concrete steps that keep your eligibility intact from day one.
What the 51% rule actually requires
The legal basis is FAR 21.191(g), which grants an experimental amateur-built certificate when the major portion of the aircraft was fabricated and assembled by people who did the work for their own education or recreation. “Major portion” means more than half of the required fabrication and assembly tasks. That is where the informal name “51% rule” comes from.
The key point: the FAA counts tasks, not the things builders usually track. It does not matter that the engine, propeller, avionics, and instruments were bought complete. Those are standard commercial parts and are excluded from the tally on both sides. What matters is who performed the airframe fabrication and assembly work.
Fabrication versus assembly
Fabrication is making a part from raw or rough material: cutting, bending, drilling, riveting a rib, laying up a composite skin. Assembly is joining finished parts: bolting a wing to a fuselage, connecting control cables. The rule requires that amateurs perform the major portion across both categories, not just one. A builder who fabricates almost nothing but assembles everything can still fall short.
How the FAA measures it: the checklist and the kit list
The FAA uses an Amateur-Built Fabrication and Assembly Checklist. It lists the tasks for a given aircraft and marks who is expected to do each one. Kit manufacturers can submit a design to the National Kit Evaluation Team (NKET), which evaluates the kit against that checklist. Kits that pass appear on the FAA’s listed-kits document, and building one of those kits per the manufacturer’s instructions creates a strong presumption of eligibility.
If your kit is not on that list, you are not automatically disqualified, but the burden shifts to you. Your DAR or FAA inspector will work through the checklist using your documentation to confirm amateurs did the major portion.
Commercial assistance: where builders lose eligibility
You are allowed to get help. You can hire a shop for a tricky composite layup, take a builder-assist week at the factory, or have a professional set up your panel. The trap is quantity. If paid professionals perform so many fabrication and assembly tasks that amateurs no longer did the major portion, the aircraft fails the rule, regardless of how much you personally learned.
This is a real risk with “quick-build” kits combined with heavy builder-assist programs. Each shortcut is fine alone. Stacked together, they can push the professional share past 50%.
A real-world scenario
Consider a builder who buys a quick-build metal kit, then pays a professional shop to install the entire wiring harness, mount the engine, and rig the controls, while doing mostly cosmetic finishing himself. On paper he spent hundreds of hours. But the checklist shows professionals performed most of the assembly tasks and the factory pre-completed much of the fabrication. At inspection, the numbers do not support amateur major portion. That aircraft may be certificated in a different experimental category, but not as amateur-built.
Common mistakes and how to fix them
- Assuming hours equal eligibility. Fix: track completed tasks against the checklist, not clock time.
- No photo record. Fix: photograph every major task with you or a fellow amateur doing the work; date each image.
- Confusing the builder’s log with the aircraft logbook. Fix: keep a separate construction log documenting who fabricated and assembled what.
- Over-buying builder assist without counting it. Fix: before booking paid help, ask the shop which checklist tasks they will complete, and keep the amateur share above half.
- Choosing a kit that is not FAA-listed without checking. Fix: confirm the kit’s status on the FAA listed-kits document before you buy.
Action steps to protect your eligibility
- Verify your kit appears on the FAA’s evaluated/listed kit document before purchase.
- Obtain the Fabrication and Assembly Checklist for your project early.
- Start a dated builder’s log with photos on the very first task.
- Log the name of the person performing each significant task.
- Before hiring any professional work, count it against the checklist.
- Contact an EAA Technical Counselor to review your documentation mid-build, not at the end.
Conclusion and next step
The 51% rule is about tasks and evidence, not effort. Treat your builder’s log as the proof it is. Your next concrete step: download the checklist for your aircraft and mark the first ten tasks you plan to complete, so eligibility is a running total rather than a surprise at inspection.
Frequently asked questions
Does buying a complete engine hurt my 51% eligibility?
No. Engines, propellers, wheels, instruments, and similar standard commercial parts are excluded from the count. Installing them counts as assembly work, but purchasing them finished does not reduce your amateur share.
Can I use a professional builder-assist center and still qualify?
Yes, within limits. Help is permitted as long as amateurs still perform the major portion of fabrication and assembly. The failure comes from stacking too much paid work, not from using assistance at all.
What happens if my kit is not on the FAA list?
You can still pursue an amateur-built certificate, but you must prove eligibility through documentation and the checklist rather than relying on the presumption a listed kit provides.
Is the builder’s log legally required?
It is your primary evidence. Without a dated, photographic record of who did the work, an inspector has little to verify the major portion against, which can delay or jeopardize certification.
References
- FAA Advisory Circular AC 20-27, Certification and Operation of Amateur-Built Aircraft.
- 14 CFR 21.191(g), Experimental certificates for amateur-built aircraft.
- Experimental Aircraft Association (EAA), builder resources and Technical Counselor program.