Sec. 91.413
"
ATC transponder tests and inspections.
(a) No persons may use an ATC transponder that is specified in 91.215(a), 121.345(c), or Sec. 135.143(c) of this chapterunless, within the preceding 24 calendar months, the ATC transponder has been tested and inspected and found to comply with appendix F of part 43 of this chapter; and " ...
If you have the equipment installed, it must be tested and inspected to use it.
I am a DAR. I expect the inspection and test to be done when I get there. If it is not, you cannot legally use the transponder till it is.
See
FAR 91.213 Inoperative Instruments and equipment.
FAR
91.215 ATC Transponder and altitude reporting equipment and use.
If you want me to do the initial airworthiness inspection of your aircraft and it has a transponder installed, I expect that it will have had
91.411 /
91.413 completed (depending on IFR or VFR Day Only limitations that are in the Operating Limitations I am issuing) before I see the airplane.
If the transponder is installed and it is not tested and inspected, I will tell you what you read above that you cannot legally fly the airplane and use the transponder till it has been tested and inspected.
Yes there are ways that you can operate the airplane without having the transponder tested and inspected. I have told you what you need to do if you do not have it tested.
If there is some way that you can fly using a transponder in your RV aircraft that has not been tested and inspected in the preceeding 24 calendar months, please show me how with a link to the data.