If there is an STC that applies to a certain engine, and you modify the engine to conform. Would you need another stc to put this modified engine back in the airplane?

Never heard of a non-airframe STC. The whole idea of an stc is an addition to an airframe type certificate. What STC are you talkiing about?

Engines do have Type Certificates. Pponk is a good example of an engine STC, they even provide a new data tag. A separate STC is required to install the modified engine on a given airframe.

A good example is mogas use: one STC for the engine and another for the airframe.
I imagine that Pponk's 470-50 STC is similar--
one STC to modify the engine, and another approving that modified engine in the airplane.

Never really thought of it that way. An STC'ed engine mod without an airframe stc is pretty useless though. As per the mogas stc. theoretically there is one for the engine, but you can't get it without the airframe one also. I would imagine the pponk is really the same. You can't get one without the other.

Mark Y. wrote:Never heard of a non-airframe STC. The whole idea of an stc is an addition to an airframe type certificate. What STC are you talkiing about?

The Vitatoe STC to convert the 520 and 550 to crossflow cylinders needs 2 STCs as well. One for the engine, one for going onto your airframe. Similar to PPonk.

Mark Y. wrote:Never really thought of it that way. An STC'ed engine mod without an airframe stc is pretty useless though. As per the mogas stc. theoretically there is one for the engine, but you can't get it without the airframe one also. I would imagine the pponk is really the same. You can't get one without the other.

My old C150/150TD had a mogas STC when I bought it, but it was for the original O200 engine.
I contacted (EAA or Petersen?) and they sold me a new mogas STC for the new bigger engine.
They charged me the difference between the original STC cost ($100) and the current cost for the bigger engine ($225),
which certainly seemed fair to me.

I imagine that if you had an engine which was STC'd for mogas and were installing it in an airframe which didn't have one,
they would sell you an STC for the airframe only.
Not sure what they'd do re pricing on that one.
Lots of people wouldn't bother, but without an STC covering the airframe as well as the engine, mogas use wouldn't be legal IMHO.