Richard: You are sort of mixing your metaphors...
The C to D conversion is not an STC. In order for an aircraft to be airworthy it must meet the type specification or its properly modified specification. Without the stainless steel skin the plene does not meet the type specification for a model D. If it has a "D" airworthiness certificate, the certificate is not in force due to the non-conformance. As an example, the 1320# gross weight STC requires the 1 3/8" trim tab. If you remove the trim tab and replace it with the smaller one, the plane again does not meet its type specification as the STC becomes part of the type specification (properly modified) for that plane, so the airworthiness certificate is not in force, again. Say, instead, you remove the whole elevator and replace it with a split elevator. For the sake of simplicity let's assume there is a hypothetical STC to install the split elevator. You no longer have the required 1 3/8" trim tab referred to in the 1320# STC. Now, it is the responsibility of the IA inspecting the split elevator installation to determine that this modification does not conflict with any other prior modifications. In this case it's a no-brainer as the 1320# STC specifically allows a properly approved split elevator assembly. John Cooper Skyport Services www.skyportservices.net
