In reality all RVSM mods are mostly avionics. Some more so than others. All aircraft being setup for RVSM have the aircraft skin mapped to check on the accuracy of the pitot/static system. The the system is upgraded to meet the requirements of RVSM.
In the Lear 35/36 mod it depends on the Autopilot installed. The 530 a/p requires no changes for RVSM. The FC200 requires that the aircraft get the same pitot/static system as the FC530 a/p. Rosemont pitot tubes. The fuselege static ports and the shoulder ports are removed. The altimiters are changed to a electronic altimiters with a standby altimiter being added. The ADC is changed from a analog to a digital unit, plus another unit is added with switching to allow the each ADC uit to drive both sides of the instrument panel. Since the FC200 is an analog autopilot, a converter is added to the system to alow it to accept digital information.
Flew a LR36 with the RVSM STC installed. Both altimiters were within 10 feet of each other.
It is my understanding that the LR20 STC is basicly bootstrapped off of the LR35 STC. Goodrich is supporting the FC110 autopilot and claims that they can bring it into the performance requirements of RVSM. But there are STCs for Lears to put newer autopilots in the aircraft, that will accept digitial information. I have been told the biggest problem with the FC110 is that the static system variances due to attitude and speed changes. And my experience with the Lear agrees with that.
