This is really interesting and contravenes “advice” dished out to a lot of Socata TB owners about fitting parts approved for a TB GT:
In most cases, modifying any aircraft with parts or components from a later model aircraft is automatically a minor alteration. Why? Because the very definition of a major alteration is based on making a change that is not part of the type design of the aircraft.
As long as the factory didn’t have to get a new type certificate, the update is eligible for any aircraft covered by that FAA TC. Date of production is irrelevant. The only issue is that the entire scope of the update has to be installed, you can’t install half of an upgraded system.
Luscombe 8As are pretty good example: they came with both square and round tail surfaces and towards the end of production some aircraft came from the factory with a mixture of both, depending on what they had to hand! Within 1946, two different types of fabric wings/struts and one completely different kind of metal wings & struts were produced before metal wings became the sole option in 1947. Fuselage tanks and wings tanks were available, in two different configurations. All these are interchanged with only a mechanic’s logbook entry, with maybe 2/3 of the 6000 planes produced having today been reduced to parts stock.
You can’t however install just one wing tank without a field approval (or retain the fuselage tank with wing tanks installed), because that was not an approved configuration.
Presumably this concession is not available at all on EASA-regs, otherwise it would make a mockery of all the stuff like 2 x GNS = Major Mod.
You can install 2 x GNS on the TB GT models but not on the pre-GT, reportedly because the 2 x GNS appeared only after S/N 2000 (basically the GT run). Multiple pre-GT owners on EASA reg have been up that road, without success.
On N-reg, there is no issue anyway since 2 x GPS is not a Major alternation, but in this case it sounds like installing any GNS (or 2 x GNS) on any TB aircraft is a Minor mod due to the above concession even if there is an autopilot connection. This is potentially priceless!
