Am I right that
From the old AC 20-138A:
(iii) Minor alteration: Under the original AC 20-138, installation of GPS equipment required the use of approved data (under an STC or major alteration) because GPS was a new and unique technology. However, since GPS technology is now common and considerable experience has been obtained in the installation of GPS, approved data for every installation is no longer appropriate. Instead, installations that do not qualify as major alterations above should be accomplished as minor alterations. These installations should be based on acceptable data including the following:
(A) Data previously approved as applicable to the aircraft (e.g., STC, Approved Model List).
(B) Data previously approved as applicable to a different make/model aircraft (e.g., an initial STC obtained by the equipment manufacturer), provided the installation is installed in accordance with the manufacturer’s instructions, any equipment interfaces are adequately addressed in the installation instructions for the GNSS equipment and the equipment with which it is interfacing, and the equipment is installed in accordance with the guidelines in this AC. For example, installation of GNSS navigation equipment that only interfaces with an antenna, power, ground, an external HSI/CDI with a single source selector switch, and a left/right (deviation-based) autopilot would typically be considered a minor alteration.
Thank you for that, NCYankee. It never occurred to me to look in AC20-138*… should have known after this project which you will remember 
So, you can install a first or a second GPS, with an autopilot connection to the first or both GPSs, and it is a Minor Alteration so long as there is no LPV capability?
Today, it would be stupid to not have LPV connected up, however (even in Europe). Am I right in that LPV then makes it a Major Alteration and thus needs either an STC or a Field Approval?
Yes, but all the WAAS GPS have an STC with an AML, so this is rarely an issue. The AFMS has a few boxes that need to be checked, particularly for the KFC225.
