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  Last Update: 12 Mar 2010

- Engine Conversions on Certified Aircraft-

A Great Way to Spend a TON of Money

If you are contemplating swapping a liquid-cooled V8 into a certified aircraft, you better have a long, heart-to-heart talk with a highly-placed person in the FSDO responsible for your geographic area before committing any significant resources to the project.

Although the flexibility and willingness to work with engine conversions varies from district to district, the bottom line is this:

We are not aware of ANY LEGITIMATE WAY to obtain long-term authorization to fly a certificated aircraft which has been modified by the installation of an uncertified powerplant.

HERE are the reasons for that statement.

The Federal Aviation Regulations are quite specific regarding aircraft powerplants. FAR-Part 33 (14-CFR-33) defines the requirements for certified powerplants. FAR-33.1 states:

  • (a) This part prescribes airworthiness standards for the issuance of type certificates and changes to those certificates for aircraft engines.
  • (b) Each person who applies under Part 21 for such a certificate or change must show compliance with the applicable requirements of this part and the applicable requirements of Part 34 of this chapter.

In short, that means you can't get an STC for the installation of a non-certified powerplant in an aircraft which was originally certified in the Normal, Utility, Aerobatic or Transport categories. That leaves the Experimental category and the Restricted category.

The following list, excerpted from 14-CFR-21.191, explains the different types of Experimental certificates that are available and how they apply to the installation of a non-certified powerplant onto a certified airframe.

  • (a) Research and Development: Testing new aircraft design concepts, new aircraft equipment, new aircraft equipment, new aircraft installations.

          NOTE: An R&D certificate must be renewed annually. Your FSDO contact will usually issue the first one fairly easily. You will have to prove some substantial reasons to justify the issuance for a second year. In my region, there has never been one issued for the third year.
          ALSO, after the expiration of your R&D certificates, you will find it extremely difficult to convert the aircraft back into its original form (ie, restore it to being a standard Cessna-185 or whatever) because now the burden is on you, the modifier, to prove that the aircraft complies with the original type certificate .

  • (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 certificates and supplemental type certificates, flights to substantiate major design changes, and flights to show compliance with the function and reliability requirements of the regulations.

          This classification is what you would use for the flight test verification to obtain an STC for the installation of a different certified powerplant onto a certified airframe. The number of times this certificate can be renewed is also limited.

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

          This requires substantiation, periodic inspections, and ongoing renewals. The allowable operations under this classification are limited.

  • (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.

          The allowable aircraft operations under this classification are very limited.

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

          The allowable aircraft operations under this classification are very limited.

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

          FAR 21.195 is very specific about the restrictions that apply here.

  • (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.

          This certificate is clearly not applicable. (See NOTE below.)

  • (h) Operating Kit-built Aircraft: Operating a primary category aircraft that meets the criteria of paragraph 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 paragraph 21.184(a).

          This certificate is clearly not applicable.

The Restricted category offers at least the remote possibility, but we have not fully explored that avenue. BUT notice the name: RESTRICTED.

Another approach we frequently hear suggested is an approval by means of a One-Time STC. Again, STC's apply to certified engines in certified airframes.

Yet another often-suggested approach is an authorization by means of a Form-337. Realistically, I think it would be quite a challenge to find an A&P-IA who would be willing to risk losing his tickets by signing and submitting to the FAA a field approval which flies in the face of existing spirit, intent and practice. More important, since 337's must be approved by FSDO, it is unlikely that the field approval would be granted.

Bottom line: unless you are ready to obtain BOTH a type certificate and a production certificate for the powerplant, AND an STC for the installation into an existing certified airframe, you might as well forget it.

NOTE: We do know of a path which a few energetic builders have taken, with varying degrees of success. The steps are:

  1. disassemble the certified aircraft down to bare pieces (de-riveting the skins in the case of monocoque airframes),
  2. make a few new duplicates of existing parts along with the new pieces required for the desired modifications,
  3. reassemble the aircraft, using the duplicated original parts and incorporating the desired modifications,
  4. Generate copious photos and documentation of the rebuild process,
  5. Apply for Experimental-Homebuilt { FAR 21.191(g) } certification of your new "from scratch" aircraft as the "Hermann Umschlagplatz Firebreather Which Looks A Lot Like A Bonanza With A V8 Engine".

The success of that strategy involves a fair bit of luck in convincing the FAA Inspector or Designee that your project really is a "homebuilt" under the 14-CFR-21.191(g) definitions, and it entails certain risks with respect to deception of Government Officials, but it has been done.

The best plan for this strategy includes (1) energetic invocation of the "...for the education and recreation of the builder..." clause, (2) having pounds of documentation, including detailed engineering drawings of most parts, which you are alleging to have "handmade", and (3) excellent salesmanship skills.

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