Section 9(b) in this circular summarizes things pretty well:
http://www.soaridaho.com/Schreder/PDF_Documents/ac20-27E.pdf
Para #6
6. BACKGROUND. Part 21 provides for the issuance of FAA Form 8130-7, Special Airworthiness Certificate, in the Experimental Category, to permit the operation of an amateur-built aircraft.
Section 21.191(g) defines an amateur-built aircraft as an aircraft in which the major portion of the aircraft has been fabricated and assembled by persons who undertook the construction project solely for their own education or recreation. During the construction of an amateur-built aircraft, commercially produced components and parts, including engine and engine accessories, propellers, tires, landing gear, main and tail rotor blades, rotor hubs, wheel and brake assemblies, forgings, castings, and extrusions may be used. In addition, standard aircraft hardware such as pulleys, bell cranks, rod ends, bearings, bolts, rivets, etc., may be used.
Commercial assistance in the fabrication and assembly of an
amateur-built aircraft is allowed and is discussed in this AC. Specific details on the use of commercial assistance are available in AC 20-139.
PLus from the AC :
completion of certain tasks or processes involved in the construction of the aircraft. During all instructional activity, the amateur-builder must be present to accomplish the tasks and all subsequent fabrication and assembling of parts for which commercial instruction is being rendered.
Tasks completed by the amateur-builder would be identified on the FAA Form 8000-38 under AMATEUR or in the assembly manual. (snip)
b. Commercial assistance may be obtained for non-checklist items on a kit that has been evaluated by the FAA. A non-checklist item is a task or process that is not listed in the checklist. These items also include painting and the installation of interior upholstery or avionics beyond basic regulatory requirements. Such a task or process would not be required to be personally completed by the amateur-builder for.
Does the fact that I have an A&P-IA certificate make me a commercial builder?
If I bought an airframe kit, built it, and then sold it, would that be a used part, or builder assistance?
