"Danger, Will Robinson".
Now having disclosed my age bracket, may I offer a few comments ? A couple of posters, whose identities I haven't figured out (Progressive and halfmanhalfbiscuit) appear to have some background in the design discipline so my comments should be read as being additional to theirs.
(a) although I have been out of the design delegate side of things for a decade or so, previously I had around 30 years as a CAR 22/35/36 and, earlier, ANR 21/40/41 Authorised Person and, for my sins, produced the occasional STC. (Hopefully my memory is correct with the Reg numbers).
(b) when it comes to STCs, a basic and underpinning consideration is that the STC is applicable ONLY to the configurations implicit in the STC S/N applicability range. That is to say, the STC folk are saying that their mod is OK to install on a bog standard aircraft (as defined by the applicability so it may not be OEM bog standard) - but if it has had any OTHER mods then all bets are off.
(c) the hyperlinked documents are well worth reading - in detail rather than superficially. CAAP 35-7(0), in particular, is the sort of document I would have loved to have had access to in my previous lives.
(d) from the CAAP at §8 -
8.1 It is the responsibility of the installer, not the registered operator, to ensure that the modification or repair is approved and can be installed in accordance with the supplied instructions.
Holders of modification or repair approvals should provide their clients or customers with a copy of the approval document. The installer must take into consideration that the particular aircraft may have other modifications or repairs that impact upon the incorporation of the modification or repair.
8.2 If the modification or repair cannot be incorporated in accordance with the supplier’s instructions, or there is a compatibility problem with other modifications and repairs, then an assessment of the modification or repair or the aircraft or both by CASA or a CAR 35 IoA holder is required. The basis for compliance with the airworthiness requirements is not always obvious and it cannot be assumed that a small change is trivial.
(e) and the AWB
The STC is normally approved based on a specific aircraft configuration. As additional modifications are installed, the interface and interaction with those unexpected modifications can be significant. The person installing the modification is required to review the STC and the aircraft being modified and ensure there are no unexpected modifications in the aircraft being modified that will conflict with the STC. The interaction with any such modifications must be evaluated. For complex interactions, specialist technical advice should be sought to ensure there are no adverse consequences. Installation of significant avionic equipment into an aircraft with a FADEC, is one example. Any change to the STC to ensure necessary compatibility must be separately approved.
(f) Now, I have had many years' involvement with LAMEs of all disciplines and hold them in high regard. However, the Industry long ago evolved to a point where no one individual has all the smarts. In general, the LAME is not well placed to make a determination in respect of incompatibility ESPECIALLY with anything to do with electonics. Trust me, that is the fact of the matter. In reality aircraft work now is a multidisciplinary activity and needs input from a bunch of people to make things work properly.
(g) by all means, the LAME can make the call but then he/she carries the can. Not having any legal competence, I don't know where this leaves the CofR holder or, indeed, the PIC, if the LAME got it wrong and the smoking hole in the ground were able to be linked to that mistake.
well an avionics shop should be able to do the vast majority of the research in 15 minutes or less
That may well be the case for some matters. Consider though, that one of my (very highly qualified and experienced on Type) guys, in just the past week or so, spent four (4) days before he was able to track down an autopilot fault in a very electric system. The problem ended up being a widget inside another widget .. etc ... and he had access to the entire WDM data as well as the aircraft.
Food for thought, I hope ?