Rules & Regulations of Airplane Building
By Ron Alexander

We hear the word “experimental” used within the sport aviation industry on a regular basis. The most common use of experimental applies to a classification of an airworthiness certificate used for a custom built airplane. This is different from the airworthiness category assigned to an airplane that is mass produced by a manufacturer which is then sold to the general public. I will explore the exact meaning of the word experimental later in this article. Suffice to say that FAR’s (Federal Aviation Regulations) pertaining to the operation of experimental airplanes can be confusing. I will attempt to clarify the confusion that exists and to simplify the regulations as they apply to building an airplane. Each phase of building and operating an amateur-built airplane will be discussed along with the applicable regulations.

In general, we are very privileged to have only a minimum number of regulations that actually pertain to building and flying our amateur-built airplane. When a manufacturer plans to mass produce an airplane, they are required by FAR’s to comply with design standards that are detailed in FAR Part 23. This regulation is very restrictive as to design, weight, speed, etc.. Amateur builders are not restricted by Part 23 or any other certification regulations. Basically, our only restriction is that we must construct and assemble the majority of the airplane. (Most airplane kit manufacturers actually voluntarily comply with the guidelines of Part 23.) Part 23 is titled “Airworthiness Standards: Normal, utility, acrobatic, and commuter category airplanes.” As the builder of our own airplane, which will not be mass produced, we are limited only by our imagination and ingenuity. Of course, when we build our own airplane we are going to impose strict limitations and restrictions concerning quality of construction, materials used, etc.. We certainly want a safe, reliable airplane to fly and in which to carry our passengers.

Lets define the “experimental” category and see how it applies to our amateur- built airplane. To legally fly within the United States, we must have 4 documents on board; an airworthiness certificate, a registration certificate, a copy of the operating limitations, and the weight and balance for our airplane. Airworthiness certificates are classified under 2 categories according to FAR 21.175 – standard and special. Standard airworthiness certificates are issued for most production airplanes and they are usually classed under the normal category. We are interested in special airworthiness certificates that are further broken down into several additional categories of which one is “experimental.” Experimental airworthiness certificates are issued for different purposes. These purposes are: (1) research and development, (2) to conduct flight tests to show compliance with airworthiness regulations, (3) for crew training, (4) for exhibition, (5) for air racing, (6) to conduct market surveys and sales demonstrations, (7) to operate an amateur-built airplane, and (8) to operate a kit-built aircraft that was assembled by a person from a kit manufactured by the holder of a production certificate for that kit.

We will primarily concern ourselves with purpose number 7, to operate an amateur-built airplane. Fully 95% of all airplanes that we build from a set of plans or from a kit will be certificated under the amateur-built classification. Purpose number 8, the kit-built classification, only applies to kit manufacturers who have certified their airplane under a type certificate termed a “primary category” aircraft. To date, only one kitplane manufacturer falls in this category to my knowledge. All other kitplane manufacturers sell their kits to be classed under the experimental certificate for the purpose of operating an amateur-built aircraft. FAR 21.191(g) is the heart of all regulations for the builder of an airplane. This regulation states the following: “Operating amateur-built aircraft. Operating an aircraft the major portion of which has been fabricated and assembled by persons who undertook the project solely for their own education or recreation.” This regulation is the essence of custom aircraft building. The intent of the classification is very clear. Notice that one or more persons may build the airplane but they must build it only for their own enjoyment or education.

Ultralight airplanes fall under a different set of rules. If your completed airplane meets the requirements of FAR 103.1, it is classed as an ultralight vehicle and as such does not require an airworthiness certificate. Briefly, these requirements are: single pilot, used for recreation only, weighing less than 254 pounds empty weight, fuel capacity not to exceed 5 U.S. gallons, not capable of more than 55 knots in level flight, and a power-off stall speed not exceeding 24 knots. As you can readily observe, the majority of custom built airplanes exceed one or more of these criteria. Often, the owner of an ultralight airplane will choose to certificate their aircraft under the experimental category. This is usually done to comply with the regulations regarding weight, passengers, etc.. Note that the operator of an ultralight does not have to be a certificated pilot contrasted to the operator of an amateur-built airplane who, of course, must be a licensed pilot and the holder of a current medical certificate.

To continue our discussion of FAR 21.191(g), it is clear that to certificate an airplane under the experimental category for amateur-built operation, we must assemble and construct at least 51% of the airplane. The FAA emphasizes this restriction in at least two publications. The first is FAA Order 8130.2C, which is the airworthiness certification manual used by FAA Inspectors as a guide to inspect an airplane and to issue an airworthiness certificate. On page 116 of that guide, the following guidelines appear under the eligibility section. (1) “Amateur-built aircraft may be eligible for an experimental airworthiness certificate when the applicant presents satisfactory evidence that the aircraft was fabricated and assembled by an individual or group of individuals.” This section goes on to state that the project must be undertaken for educational or recreational purposes and the FAA must find that the airplane complies with acceptable standards. Aircraft that are manufactured and assembled as a business for sale are not considered to be amateur-built. This statement appears within the Order: “NOTE: Amateur-built kit owner(s) will jeopardize eligibility for certification under FAR 21.191(g) if someone else builds the airplane.” The applicant for amateur-built certification must sign a notarized form (FAA Form 8130-12), certifying the major portion was fabricated and assembled for educational or recreational purposes, and that evidence is available to support the statement. The second place the 51% rule is emphasized is in Advisory Circular 20-27D on page 5 under 7(b). This section simply emphasizes the major portion rule.

When you purchase an airplane kit from a manufacturer, the kit should be listed on the FAA listing of kits that have been evaluated to ensure that 51% of the building will be completed by the purchaser (this is commonly known as the “major portion” rule). I want to emphasize that the FAA in no way endorses any of these kits nor do they approve kit manufacturers. They simply evaluate the kits solely for the purpose of determining if an aircraft built from the kit will meet the major portion criteria. A listing of these kits is available from your local FAA office. I do not recommend purchasing a kit that is not on this listing unless you are prepared to prove to the FAA Inspector that the kit meets the proper criteria.

The FAA does not expect the builder to personally fabricate every part of the airplane. A number of items can be purchased and several tasks can be contracted commercially. FAA Advisory Circular 20-139 titled “Commercial Assistance During Construction of Amateur-Built Aircraft”, provides a very detailed guide concerning what can be purchased complete and what can be contracted commercially. Engines, propellers, wheel and brake assemblies, and standard aircraft hardware are examples of items that may be purchased. Installation of avionics, painting an airplane, and upholstery items are examples of tasks that may be contracted. The bottom line of the entire discussion is that you must prove to the FAA Inspector who issues your airworthiness certificate that you have complied with FAR 21.191(g). Next month we will discuss the necessary documentation to present to the inspector to assure your compliance.

If you decide to allow someone else to build your airplane to be certificated as amateur-built, you will be required to license it under the experimental category for the purpose of exhibition. This category is much more restrictive than amateur-built. The purpose of this category is to allow the holder to exhibit their airplane at air shows, motion pictures, television filming, etc., and of course to fly to and from these productions. I will not spend time discussing this category since it is rarely used.

Now that I have discussed the general regulations concerning building your airplane, I will detail specific regulations as they apply to each phase of building, flying, and maintaining an amateur-built airplane. I would recommend that you obtain a copy of the regulations for your own reference. Several books are available that contain the FAR’s along with computer discs containing all of the FAA regulations. The FAA also maintains a web site with all regulations. This site can be found at

Initial Building Phase

The first phase of construction is, of course, the building phase. I would highly recommend that before you begin your project you ask your local FAA office for their information packet that is available relating to amateur-built airplanes. Part of this packet is Advisory Circular 20-27D, that you will refer to regularly. Regarding regulations governing the first phase, we have discussed in detail FAR21.191(g). Another regulation, FAR 21.173, presents the eligibility for an airworthiness certificate. FAR 21.191 defines all purposes that are allowed for licensing under the experimental category including, of course, amateur-built. FAR 21.175 defines the classifications of airworthiness certificates. FAR 21.193 contains the information that must be submitted for an experimental certificate. Advisory Circular 20-27D presents this information much more completely. FAA Part 45 details the markings that are necessary for your aircraft with respect to what is required, size, location, etc.. FAR 45.23 is where we are told that we will display the word “experimental” in letters not less than 2 inches high nor more than 6 inches high near the entrance to the cabin or cockpit. FAR 45.29 provides us with the size of registration marks and specifically allows us, as owners of experimental aircraft, to use 3 inch high numbers and letters providing our maximum cruising speed is less than 180 knots. If our cruising speed is higher than 180 knots, then we are required to use 12 inch letters and numbers. An additional regulation applies if our airplane had an experimental certificate issued more than 30 years ago. This regulation allows us to use numbers and letters only 2 inches high. FAR45.22 specifies the rules as they apply to the older airplanes. Details of spacing, width, and other factors are discussed in this section.

Continuing the building stage, FAR 47.15 informs us about registration numbers. You may select an “N” number of your choice providing the number is currently not in use on another airplane. FAR 47.33 lists the information that must be submitted with your application for the “N” number. If you intend to fly your airplane at night or under Instrument Flight Rules, you are required to have specific equipment. The necessary equipment, including instruments, radios, etc., is outlined in FAR 91.205. This regulation also tells you what is needed for VFR flight during the day. FAR 91.207 outlines the requirements for emergency locator transmitters (ELT). The requirements for an ELT are basically the same for all airplanes, including amateur-built. It should be noted that if you remain within 50 nautical miles of your home airport and you are engaged in flight training, you are not required to have an ELT. Also, if you have a single place airplane you are not required to install an ELT.

Obviously, there are a number of other issues involved in the building phase.

In next month’s article, I will present a detailed checklist of the items you will need to certificate your aircraft under amateur-built. At that time I will also talk about the FAA inspection process and the required documentation and papers. After the FAA inspects your airplane, the inspector will issue a set of Operating Limitations. Those limitations then become regulations for operation of your aircraft and they are actually part of the special airworthiness certificate. The airworthiness certificate will be issued at the time of the inspection and will contain two phases. Phase 1 is the initial flight testing phase of the aircraft, and Phase 2 lists the operating limitations that go into effect upon completion of the flight testing. Phase 2 applies for the duration of the certificate.

Flight Testing

FAR 91.305 defines a flight test area. Basically, it states that you must conduct your flight testing over sparsely populated areas having light air traffic. FAR 91.319 provides a listing of operating limitations. As I mentioned, when your aircraft is inspected you will be given a copy of operating limitations. Usually, the inspector will issue Phase 1 and Phase 2 at the time of inspection providing you with 2 sets of operating limitations; flight testing and subsequent operation. The flight test area is defined within the Phase 1 limitations along with the required number of hours you must fly the aircraft. The primary restrictions regarding flight testing are: (1) no passengers, (2) day, VFR only, (3) no operation over congested areas, (4) you must advise ATC that you are experimental, and (5) the pilot must have the appropriate ratings. Of course, the general operating rules under FAR Part 91 are applicable. Phase 1 operating limitations have an expiration time of 12 months from date of issue. All flight testing must be completed within that time period or the aircraft must be reinspected. One of the restrictions, in FAR 91.319, that is interesting is that in order to have the Phase 1 restrictions lifted you must prove that the aircraft has no hazardous operating characteristics and that it is controllable throughout its normal range of speeds and maneuvers. The FAA has an Advisory Circular that is very helpful in providing guidelines for flight testing. This circular, Advisory Circular 90-89, is necessary to read prior to your first flight. Also, the EAA Flight Advisor program is highly recommended. The flight testing phase should be an enjoyable conclusion to your building experience and it will be if planned and executed properly.

Normal Operation of Your Amateur-Built

Once again, all of the general operating rules under FAR Part 91 apply to daily operations of your aircraft. In addition, the operating limitations presented under FAR 91.319 and as issued by the FAA Inspector at the time of inspection govern. After completion of Phase 1, you are then allowed to carry passengers and fly at night or IFR if so equipped. Phase 2 limitations do add some restrictions that merit discussion. First of all, you may not carry passengers or property for hire. Secondly, any major changes that are made to the airplane as defined by FAR 21.93 require inspection by the FAA prior to further flight. A minor change is defined as one that has no appreciable effect on the weight, balance, structure, or anything affecting the airworthiness. Examples of a major change would be a different horsepower engine, a different pitch propeller, a change in basic design, etc.. If a major change is made notify the FAA in writing providing the details of the change to ascertain whether or not an inspection will be required. Thirdly, you may not operate your airplane unless it has received a condition inspection (annual inspection). This will be discussed in the next section.

Maintaining Your Airplane

As I mentioned in the previous section, a condition inspection is required every 12 calendar months on amateur-built aircraft. This check is similar to an annual inspection required by FAR Part 43 on production airplanes. The Phase 2 Operating Limitations specifically refer to FAR Part 43, Appendix D, as the guide to performing this inspection. The inspection can be performed by any licensed A & P mechanic, an FAA Approved Repair Station, or by the builder of the airplane provided the builder obtains a “Repairman’s Certificate” from the FAA. FAA Advisory Circular 65-23A is available for information concerning application and privileges of this certificate. In short, the primary builder of the airplane is eligible to apply for this certificate which then permits inspection of the airplane and a logbook endorsement of the condition check. It is noteworthy that the primary builder must be one person. If a group of people builds an airplane, only one can be designated as the primary builder. In addition, the issuance of the repairman’s certificate only applies to the one airplane that has been built by the primary builder and no other airplane regardless of same type, etc..

Normal maintenance on an experimental airplane can be performed virtually by anyone regardless of credentials. Once again, this does not apply to the condition check previously discussed. You can perform maintenance items on the engine whether or not it is “certified”. Once a certified engine is placed on an amateur-built aircraft and is operated, it no longer conforms to its type design. This means that the engine can no longer be placed on any aircraft other than an amateur-built until it has been inspected and found to meet its type design. It also must be found to be in a condition for safe operation “airworthy”. Once again, common sense should rule. We do not want to overhaul an engine on our airplane unless we are equipped to do so with tools and proper knowledge.

I will point out that FAR Part 43 specifically states that the rules of that part do not apply to amateur-built airplanes. With that in mind, anyone can maintain the airplane. However, remember in our earlier discussion that Part 43, Appendix D was referenced in Phase 2 operating limitations presented to the builder at the time of inspection. It is referenced as a guide to be used in conducting condition inspections. That means Part 43, Appendix D does apply to the condition inspection because of this reference. The FAA has further clarified AD (Airworthiness Directives) as they apply to amateur-built airplanes. Airworthiness Directives cannot apply to any part on an amateur-built airplane unless that specific airplane is cited along with who should do the work and to what standards. The reason for this is because once an approved part is placed on an experimental airplane it is no longer considered an approved part. Again, let me emphasis that just because a regulation does not require an action it still may be prudent and within our best interest to conform to an AD note. We are striving to improve the safety record of this industry and in all cases we must act on the side of common sense and good practice.

Regulations Involving The Sale of Your Amateur-Built

There are few regulations governing a sale of your airplane. The airworthiness certificate is transferable with the airplane even though it is experimental. (FAR 21.179) The proper bill of sale and registration documents must be completed when you sell the airplane. Of particular interest is the fact that the new owner may maintain the newly purchased airplane, but may not perform the condition check. The repairman’s certificate is not transferred with the airplane. It remains with the original primary builder. That person legally may still perform the condition check if you can persuade them to do so. If you are purchasing a partially completed kit you need to obtain the proper documentation to ensure you will meet the major portion rule. FAA Advisory Circular 20-27D has the following warning: “CAUTION: Purchasers of partially completed kits should obtain all fabrication and assembly records from the previous owner(s). This may enable the builder who completes the aircraft to be eligible for amateur-built certification.” Once again, a call to your FAA Inspector will prevent future problems. The time spent by the original builder is usually applied toward the total time required to build the airplane. Documentation is necessary.


The following table will provide a concise summary of Federal Aviation Regulations as they apply to amateur-built airplanes. Next month I will detail forms, documentation, etc., necessary to certificate your amateur-built aircraft along with an easy to use checklist.

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This article and others to follow are written by EAA/SportAir Workshop personnel. A series of “hands-on” workshops on building airplanes is presented throughout the country for your education. A schedule of locations and dates of upcoming workshops follows. For information on available workshops, prices, curriculum, etc., call 800-967-5746 or web site