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.
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.
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.
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.
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.
Other times the installation manuals lists the ONLY equipment that can be used
In what terms would such a requirement be stated?
Most avionics list give some suggestions for connecting certain equipment, in my opinion it would that be possible to use other (equivalent) equipment.
Other times the installation manuals lists the ONLY equipment that can be used (without additional certification). If that is the case additional certification is required.
I spoke to Sandel in detail.
It turns out that the GTN750 puts up a prompt asking if you want to fly the (LPV) approach and you have to confirm that, and it then removes the discrete “GPS active” signal going to the KFC225 so the KFC225 thinks it is flying an ILS – which AIUI is basically what you were saying, NCYankee.
Interestingly, in roll steering mode, the autopilot is in the HDG mode the whole time – enroute and approach. Not NAV mode, like one does with older avionics.
But this was a bit of a digression… I still don’t get the rules on interconnecting equipment not shown in the IM. There cannot be a general prohibition on it, otherwise (as I say above) no new equipment could ever be introduced on the market. So it seems to me that some installers invent prohibitions as it suits them.
What would concern me is the conflict between GPS and fake ILS that the autopilot requires. This is handled by an additional discrete signal from the GPS. The Sandel would not generate it from the Arinc data. The FAA specifically held up the WAAS vertical guidance with the KFC225 and KAP140 over this issue. One can hook it up to a GNS or GTN and fake the signal without configuring the GPS for Prompt, but as soon as the autopilot detects the source change when the FAF becomes the active fix, the autopilot will disconnect APR mode. One can press the APR button a second time and the autopilot will follow the guidance from that point, but this is considered unacceptable by the FAA, hence the invention of the additional discrete signal and Prompt mode operation of the GPS. There is a separate check mark that applies to the GNS/GTN AFMS when the KFC225/KAP140 autopilot is used.
What would concern you, NCYankee?
This isn’t auto-slew (which I already have) – this is roll steering via ARINC429.
