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Aircraft Maintenance: Explaining experimental

As we were driving to the airport the other day, my son Jake asked an interesting question: “If we can build an experimental aircraft and fly it anywhere, why can’t we just change our Bonanza any way we want and make that experimental too?”  Simple question, complicated answer.

To understand the concept of an experimental aircraft, it’s important to understand that it’s an aircraft's airworthiness certificate that ultimately makes the aircraft legal to fly in the eyes of the FAA (airworthiness matters too, of course). There are two different classifications of FAA airworthiness certificates: a standard airworthiness certificate and a special airworthiness certificate.

A standard airworthiness certificate allows for the operation of type certified aircraft in categories such as normal, utility, acrobatic, commuter, transport, and balloons. A special airworthiness certificate allows for the operation of type certified and non-type certified aircraft in special categories including primary, restricted, light sport, and experimental, among others.

Special airworthiness certificates allow for variations in the standards, testing, and regulations that an aircraft must meet in order to satisfy the special missions and cases that we all need in the world of aviation. This includes special-task aircraft in the restricted category, such as aircraft modified or created for use in agricultural work, aerial surveying and patrolling, aerial advertising, and other applications, but it also includes my favorite, the experimental category.

Under the experimental category, aircraft can be certified to fly for a wide variety of purposes. The different purposes are explained quite well in 14 CFR 21.91:

21.191 Experimental certificates.

Experimental certificates are issued for the following purposes:

(a) Research and development. Testing new aircraft design concepts, new aircraft equipment, new aircraft installations, new aircraft operating techniques, or new uses for aircraft.

(b) Showing compliance with regulations. Conducting flight tests and other operations to show compliance with the airworthiness regulations including flights to show compliance for issuance of type and supplemental type certificates, flights to substantiate major design changes, and flights to show compliance with the function and reliability requirements of the regulations.

(c) Crew training. Training of the applicant's flight crews.

(d) Exhibition. Exhibiting the aircraft's flight capabilities, performance, or unusual characteristics at air shows, motion picture, television, and similar productions, and the maintenance of exhibition flight proficiency, including (for persons exhibiting aircraft) flying to and from such air shows and productions.

(e) Air racing. Participating in air races, including (for such participants) practicing for such air races and flying to and from racing events.

(f) Market surveys. Use of aircraft for purposes of conducting market surveys, sales demonstrations, and customer crew training only as provided in §21.195.

(g) Operating amateur-built aircraft. Operating an aircraft the major portion of which has been fabricated and assembled by persons who undertook the construction project solely for their own education or recreation.

(h) Operating primary kit-built aircraft. Operating a primary category aircraft that meets the criteria of §21.24(a)(1) 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 under §21.184(a).

(i) Operating light-sport aircraft. Operating a light-sport aircraft that—

(1) Has not been issued a U.S. or foreign airworthiness certificate and does not meet the provisions of §103.1 of this chapter. An experimental certificate will not be issued under this paragraph for these aircraft after January 31, 2008;

(2) Has been assembled -

(i) From an aircraft kit for which the applicant can provide the information required by §21.193(e); and

(ii) In accordance with manufacturer's assembly instructions that meet an applicable consensus standard; or

(3) Has been previously issued a special airworthiness certificate in the light-sport category under §21.190.

The catch…

Example of an airworthiness certificate displayed in an experimental aircraft. Photo courtesy of Jeff Simon.

The devil is in the details and, in this case, the key is that the aircraft is only certified for the purposes of the certificate that was issued. Each aircraft issued an experimental airworthiness certificate has operating limitations attached to the certificate that regulate the testing period, maintenance, and operations. For example, it specifies that the aircraft cannot be used for compensation or hire. You can find an example of an experimental amateur-built airworthiness certificate here.

The most common reasons that previously certified, normal/utility category aircraft are moved to the experimental category is for product development. Anyone developing modifications to certified aircraft that affect flight has to develop and test these products. To do that, you need to take a deep breath and surrender the aircraft’s current airworthiness certificate and get a new, highly restricted experimental certificate to facilitate this work. However, that means you’ll only be flying for the purposes of the task at hand. You can’t move the aircraft into the experimental category, begin testing a new propeller, and then take the family on vacation to visit grandma. You’ll have to wait until the work is complete and the aircraft has been returned to its original airworthiness certificate.

By the way, that process of taking the aircraft out of the experimental category and obtaining an airworthiness certificate in its original category is no walk in the park. The FAA will want to evaluate the entire history and condition of the aircraft to verify that it meets the original type design. So, anyone looking to use their own aircraft for product development would be wise to go through the maintenance and logbooks very carefully before giving up their current airworthiness certificate. I’ve heard horror stories about the process of getting a standard airworthiness certificate back if old logs are missing or other issues are discovered in the process.

The answers to Jake’s questions are rarely brief ones. But, they’re always great opportunities for education (usually, my education). In the end, I had to tell him that we can’t just move our Bonanza into the experimental category so that we can go “hog-wild” on aircraft mods and still fly to the beach. Fortunately, we’re building the T–51D Mustang for exactly that! You can follow that story here. Until next time…Happy Flying!

Jeff Simon
Jeff Simon
Jeff Simon is an A&P mechanic, IA, pilot, and aircraft owner. He has spent the last 22 years promoting owner-assisted aircraft maintenance and created the first inspection tool for geared alternator couplings available at Jeff is also the creator of SocialFlight, the free mobile app and website that maps more than 20,000 aviation events, hundred-dollar hamburger destinations, and also offers educational aviation videos. Free apps are available for iOS and Android devices, and users can also visit
Topics: Aircraft Maintenance, Experimental
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