I spoke to Peter Heffly yesterday about his installation of a Daynon D-100 in Don Kuehl's Ercoupe. Peter runs the avionics shop at Granbury Regional (KGDJ), and Don's airplane was based there until he sold it recently.
Peter is reading the applicable regulations and coming to the same conclusion as many of us-- there is no requirement for TSO equipment in the Ercoupe. Here are the facts: 1. The FAA is clear, TSO is NOT a stand-alone requirement for Part 91, Small Aircraft. It may be required for aircraft under part 135 or for certain types of operations, or for specific equipment (transponders, ELTs, etc.) TSO is a manufacturing standard. 2. The Ercoupe line was certified under CAR-3 and CAR-4a. The airplane was equipped with basic instruments (altimeter, airspeed, VSI, heading) that were NOT TSOed. The standard and requirement for certified aircraft did not exist at the time. 3. FAR Part 23 does not supercende the certification in TCDS A-718 or A-787 4. FAR 91.205 does not require any of the instruments to be TSOed. In fact, many Ercoupes continue to fly with just the basic (Non-TSO) equipment listed. 5. FAA approval is still required to install equipment in a certified production aircraft. A 337 submitted by a licensed mechanic that is not returned as disapproved by the FAA constitutes approval. Therefore, Peter concludes that installing a Dynon in an Ercoupe is authorized as long as it does not replace any of the equipment that the Ercoupe was certified with. YMMV-- as in Prof. Ed's case, some avionics shops and A&P's simply aren't willing to assume the liability for a non-TSO installation. And some simply fall back on the generalization that TSO equipment is required in a production aircraft. But I have yet to find anything that specifically prohibits installing a non-TSO part in an aircraft operated under Part 91 (with the exceptions noted) as long as you are not replacing a TSO part that was originally part of the TCDS. Of course, you still have to get FAA approval. P.S.-- We had the same kind of "word of mouth" regulations in the Air Force that were wrongly interpreted. They actually taught us at CCTS that you could not do a 360-degree turn in cell formation, when the actual restriction was no 360-degree turns during stream operations. Two entirely different things-- basically you could not do a 360-degree turn when there was another 3-ship cell behind you (streams of 3-ship bomber formations flying into combat or during an ORI), but an individual cell 360 was allowed.
