I'm specifically thinking about the Arrow III. There are hundreds or possibly thousands of turbo Arrow III's, but almost no non-turbos.

Could I buy an Arrow III Turbo (PA-28R-201T) that is at TBO and just put in a non turbo engine? Would it then become a PA-28-201? Can you do that to any aircraft that has a non turbo version as well? For instance, could you convert an Cirrus SR22T to an SR22?

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    $\begingroup$ It has been known to happen. The C320, for example, may be converted to a normally aspirated aircraft by replacing the TSIO-470s with a pair of IO-520s. $\endgroup$ – acpilot Dec 1 '17 at 4:47
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    $\begingroup$ In order to do that you would need to have an STC since the aircraft was certified with a turbo. I’ve read a lot about Arrows and don’t recall seeing anything about an STC. A cursory search for the Arrow doesn’t bring up any either. There are certainly none for the Cirrus. However, what many people have done with their Arrow, including the one I fly, is to replace the factory fixed-wastegate turbo setup with one from Merlyn. The variable wastegate fixes the problems of overboosting with the original and provides about a 5% increase in speed and a higher top altitude. $\endgroup$ – JScarry Dec 1 '17 at 15:14
  • $\begingroup$ I think you are wrong on your estimation of the number of non-turbo Arrow IIIs. Checking on Trade-a-Plane I found four turbos and three non-turbos for sale. That matches my recollection of the ratio when we were looking. $\endgroup$ – JScarry Dec 1 '17 at 15:19
  • $\begingroup$ You could also draft up some engineering docs and ask for a 337 from your local FSDO. Why you would do this instead of simply buying an NA Arrow is something I'm curious to know. Seems like a lot of expensive and time consuming work only to reduce the performance capabilities of a light SE airplane. $\endgroup$ – acpilot Dec 1 '17 at 16:27
  • $\begingroup$ @JScarry if you actually look up the N numbers, you'll see that a lot of the Arrow IIIs on the market are actually turbos, just not advertised as such. There are three non-turbo arrows on the market right now that I know about, one in Switzerland, and two in the US that are about 20k above our flying clubs current budget. $\endgroup$ – Arel Dec 1 '17 at 19:26

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