Other times the installation manuals lists the ONLY equipment that can be used
In what terms would such a requirement be stated?
Here is typical wording from the GNS530W install manual. Most other sections have similar wording to permit other equivalent devices if they meet the specified criteria.
Appendix G APPROVED EQUIPMENT
The following tables list equipment that can be interfaced with the Garmin 500W Series units. This is not a complete listing however; it only represents those units that are approved under 400W/500W Series AML STC SA01933LA, with any exceptions noted by the appropriate table. When installing equipment on the aircraft always refer to the manufacturers’ documentation for complete pinout and interconnect information.
G.1 Audio Panels
Audio panels not listed below can still be approved under 400W/500W Series AML STC SA01933LA if all of the following conditions are met: The installation of the audio panel was previously was previously FAA-approved; The VHF COM audio and VHF NAV audio (if applicable) must be verified as described in sections 5.5.4 and 5.5.5.
For installations using 500W Series TAWS units, The audio panel must have an unswitched audio input that is used for the TAWS audio. The TAWS audio must be verified as described in sections 5.5.6 and 5.6.6.
OK – it is obvious that one should not be connecting equipment that is going to be a problem functionally. I would not imagine the certification of any product to remain valid if one then connects something to it that cripples the way it works.
However what I am looking for is some legal rule (or wording in the IM or the STC) which specifically prohibits connecting items which are not shown in the IM.
I would have thought that such a rule would amount to a restrictive practice, at least.
Garmin doesn’t specify what may or may not be interfaced to in its IM. It merely states what is covered in the STC. In some cases it is specific by naming the equipment and in others it is both specific and general by specifing an acceptable criteria for equipment that is not listed, but if met, the STC may be used. If a piece of equipment does not meet the IM requirements, it in no way means that it can’t be interfaced to, it merely means that the STC can’t be the approved data for that part of the installation. An example of this is the Aspen with its connected panel. It interfaces to the crossfill interface and is not approved by the Garmin STC IM. I would still document the installation using the Garmin STC and then add the STC from Aspen to justify the interconnection. In some cases, I have seen two 337’s used, one with the STC install as the approved data and one with a field approval for the attachment to equipment not mentioned in the STC IM.
Here is the statement from the IM for 500W series:
7.2.3 Equipment Interfaced to the 500W Series Unit
500W Series unit interfaces to aircraft systems other than those identified in this installation manual are outside the scope of this manual and may require further evaluation for certification and/or other FAA airworthiness approval.
IOW, it’s not my job. If Garmin doesn’t cover it, get your own approval. It doesn’t prohibit anything.
If a piece of equipment does not meet the IM requirements, it in no way means that it can’t be interfaced to, it merely means that the STC can’t be the approved data for that part of the installation
That is clear, and very interesting. Thank you @NCYankee.
Are any additional wires a Major Alteration? I can’t see the basis for that.
Such an installation would then be done as “IAW the STC” and a further logbook entry done to cover the extra wires.
I would think that any additional wires would be evaluated on their own merits as to whether or not it constituted a major or minor modification and the installer would make that determination.