What is the difference between experimental amateur and amateur built aircraft in the US, or are they the same?
If not, is there a difference re specifications, inspections, engines, passengers, etc.?
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The FAA provides pretty clear definitions and/or explanations:
Title 14, Code of Federal Regulations (14 CFR), part 21, section 21.191(g), defines an amateur-built aircraft as an aircraft "the major portion of which has been fabricated and assembled by person(s) who undertook the construction project solely for their own education or recreation."
They define Experimental as a category, since it has to do with how the airworthiness cert is issued:
A special airworthiness certificate in the experimental category is issued to operate an aircraft that does not have a type certificate or does not conform to its type certificate and is in a condition for safe operation. Additionally, this certificate is issued to operate a primary category kit-built aircraft that was assembled without the supervision and quality control of the production certificate holder.
Special airworthiness certificates may be issued in the experimental category for:
Operating amateur-built aircraft: to operate an amateur-built aircraft in which the major portion has been fabricated and assembled by persons for their own recreation or education.
Operating kit-built aircraft: to operate a primary category aircraft that was assembled by a person from a kit manufactured by the holder of a production certificate for that kit, without the supervision and quality control of the production certificate holder.
So... All amateur built aircraft are experimental (flown under an experimental airworthy type certificate) but not all experimental's are amateur built, for example a pre-production aircraft may need to be registered as an experimental for test flights before a type certificate is received.
"Experimental" was originally applied to development test aircraft only. The application of the term to amateur builts is an artifact of the early days of homebuilding in the early 50s, when the Civil Aviation Authority (FAA's predecessor) was persuaded to create a formal licensing structure for amateur built airplanes created for personal use, and they decided to piggyback on the existing Experimental category normally used for test aircraft, as the path of least bureaucratic effort.
So you have a somewhat odd categorization, done nowhere else, where airplanes NOT created for development and testing, but just built for people to have fun and fly, still have EXPERIMENTAL plastered on the side.
In Canada the homebuilt category was originally called "Ultralight", but then in the 70s actual ultralights appeared on the scene and the name wasn't really suited to 180 mph airplanes like Thorp T-18s, so a new Amateur Built catagory was created to cover traditional homebuilts, with ultralights just ultralights.
As I understand it (I am not a pilot, nor do I live in the US) in the US:
"amateur built" is a subset of "experimental".
"experimental" aircraft are assesed and issued with airworthyness certificates on an individual basis under requirements less rigorous than the normal type-certification process. However the FAA doesn't want manufacturers using experimental certification as a way to do an end-run round the normal certification process, so they put limits on what aircraft can be registered as "experimental".
"amateur built" is a subset of experimental for aircraft that are more than 51% built by amateurs and is one of the least restrictive experimental categories.
Another subcategory of experimental that the aviation hobbyist may run into is "experimental exhibition", this category is often used for ex-warbirds and is also sometimes used as a means to heavily customize aircraft. The downside as I understand it is you have to convince the FAA that you will genuinely be using the aircraft for exhibition purposes.
Regarding passengers as I understand it no experimental aircraft is allowed to carry passengers for hire and some are not allowed to carry passengers at all. As I understand it amateur builts are allowed to carry passengers not for hire after passing some additional testing/proving. I don't know if the rules on carrying passengers are different for other types of experimental.