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I'm reading up on building an airplane, and it says the builder must accomplish 51 percent of the work to get an Experimental Amateur Built airworthiness certificate. But what happens if a certain type of plane doesn't meet the 51 percent rule, how is it certificated then?

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  • $\begingroup$ "Experimental" is a term for homebuilts only used in the US. $\endgroup$
    – John K
    Commented Aug 9, 2021 at 14:33
  • $\begingroup$ FWIW, AFAIK all kit planes sold in the USA meet the 51% rule. The only way you wouldn't is if you somehow got the kit vendor to do a significant assembly or set of assemblies for you. The one possible concern would be if you purchased a partially completed kit plane, and I read about that being done enough that there seems to be a way to deal with it. $\endgroup$
    – Zeiss Ikon
    Commented Aug 9, 2021 at 14:45
  • $\begingroup$ I think you're asking about FAA regulations? If so, please edit your question to clarify, including adding the faa-regulations tag. This question might be relevant. $\endgroup$
    – Pondlife
    Commented Aug 9, 2021 at 16:04
  • $\begingroup$ Seems as though the problem is with defining what goes into "the work". If you buy a complete engine, or avionics package, instead of building one from parts, you've saved yourself a lot of work. How would that be different from e.g. buying completed wing assemblies? $\endgroup$
    – jamesqf
    Commented Aug 9, 2021 at 18:10
  • $\begingroup$ @Bianfable of course. My point was the term experimental when used on that context makes it clear the OP is in the US. $\endgroup$
    – John K
    Commented Aug 10, 2021 at 1:08

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If you don't comply with the 51% rule as explained in AC 20-27G - Certification and Operation of Amateur-Built Aircraft the FAA will not issue an Experimental-Amateur Built airworthiness certificate.

What happens after that up to you and what the FAA determines is the specific reasons for non-compliance. If the reasons can be fixed and it brought into compliance, they can eventually grant the certificate. If not, you would have to pursue certification under another category, most likely Experimental-Exhibition. The downside is that will come with significant restrictions. That will have a significant negative impact the value of the aircraft. There's also no guarantee that it can be certificated at all.

As @ZeissIkon said, all US kits are 51% rule compliant, so the risk is when you get 'Builder Assistance' in the terms of the FAA. Follow the guidelines and you'll be fine.

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It is still certified "experimental", you just can't apply for or receive the repairman certificate, so you need to treat it just like a certificated aircraft when it comes to annual and repairs.

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    $\begingroup$ Without knowing what country's regulations apply, how do we know for certain? Or are you confident this applies in every jurisdiction on Earth? $\endgroup$
    – Zeiss Ikon
    Commented Aug 9, 2021 at 14:20
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    $\begingroup$ That experimental cert won't be the same as homebuilt 51% "amatuer built" experimental cert. It'll be under one of the other experimental categories and will come with a lot more restrictions. A lot of people questioned the CAA's (pre-FAA) use of that term "experimental" for amateur builts in the early 50s due to the confusion it created, but it was done as an expedient to speed up the regulatory process at the time, and it's stuck. $\endgroup$
    – John K
    Commented Aug 9, 2021 at 14:32
  • $\begingroup$ @ZeissIkon Given the OP's other questions are all US/FAA specific, I extrapolated that and the "51% rule". $\endgroup$
    – Ron Beyer
    Commented Aug 9, 2021 at 14:36
  • $\begingroup$ @JohnK That's a good point, it's "experimental" but not "amatuer built". There are a number of restrictions, such as flying near built-up areas, at night/VFR only, etc. $\endgroup$
    – Ron Beyer
    Commented Aug 9, 2021 at 14:40
  • $\begingroup$ Here in Canada they used to call homebuilts "Ultralights" until the 70s. Then ultralight ultralights came along and confusion reigned. It was changed to "amateur built" in the 80s. In some ways the rules are looser here than the US. Ex; there is no Repairment Cert requirement in Can for homebuilts. Whoever's name is on the C of R has authority to sign off on all maintenance, repairs, and annuals. Only ELTs and Xponders 2 yr encoder recerts need an approved shop sign off. $\endgroup$
    – John K
    Commented Aug 9, 2021 at 14:53

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