Certified vs experimental
I had read/ heard in a posters you tube channel, that should an owner make an unapproved modification to his or her certified aircraft that it “ reverts” to experimental.

My immediate thought was this then.....if one had a plane with a great, solid airframe, but a timed out engine, and didn’t want to pay the ridiculous prices that lycoming et al charge, why not just drop a new Aeromomentum or something in it?
I recognize the loss of resale to some, but for the sake of argument, is there anything other than pursed lips that prevents you?
Further, if you are then in the “non certified” class, does that now mean your Avionics upgrades are now at the cheaper rate?    This cannot surely be the case or everyone would do it, but this fellow has quite the following, so thought I,d check.  

Are there repercussions He failed to mention? ...surely so
2 Replies
What you heard is mostly incorrect.

Should an owner make an unapproved modification to his or her certified aircraft, it simply becomes unairworthy, and cannot be flown legally unless/until either it is returned to its approved configuration or FAA approval is obtained for operation in its altered state.  While it's true that one can put an engine not certified for that airplane in that airplane under an Experimental airworthiness certificate, that certificate will be in the "Experimental-Showing compliance with regulations" class, not "Experimental-Amateur Built."  That is covered in 14 CFR 91.191:
(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.

Such a certificate will be issued only for a limited time (typically six months), after which:
  1. the airplane must be returned to its approved configuration, or
  2. FAA approval must be obtained for the alteration, or
  3. one (and only) one 6-month extension is obtained to complete the testing.
During that test period, strict limitations are established, including things like no passengers, a limited area for the flights to be done, no flights to/from any airports other than those few listed in the operating limitations, Day VFR only, etc.  And the engine will have to undergo the certification process for engines before the alteration is approved.  To take a certified airplane from Standard to Experimental-Amateur Built, more than half the resulting airplane will have to be the handiwork of the builder, and just dropping in a different engine won't get you there.  For an example of such a project, look at the Breezy, which is built from the wing of a certified airplane and a lot of pieces manufactured by the builder.

So, while it is true you can put a non-certified engine in a certified airplane, it can't be flown until the FAA issues an EXP-Showing Compliance airworthiness certificate (which is by no means guaranteed), and then only for a limited time under very limited conditions.  And there is definitely no "automatic" reversion to the Experimental category.
thank you very much for such a detailed and educational reply. I’m just getting started in all this and appreciate the time folks here take with us newbies without condescension. 
have a great week!
Greg Haley