“I wonder what percentage of the fleet could / would fit a mode C transponder if they were required to?”
If the regulatory structure remained the same regarding max empty weight etc, maybe an additional 10 – 15%. The rest would be unable to comply for a variety of reasons. If you start looking at individual cases it gets very complicated indeed! It was this that changed the CAA’s mind on compulsory carriage, it is just not possible for many 1000’s of aircraft using current tec and current rules.
Mode S has advantages in high traffic density, according to the CAA this is important, but the US experience is different.
Rod1