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Avionics installation/replacement without STC/change approval (based on a TC installation)

I recall having read here on EuroGA that you don’t need any kind of approval (or STC) to install equipment on an aircraft which is available as a factory option on that aircraft model.

E.g. if there is a factory option to have a particular make and model of HSI and your aircraft only has a DG, you may install such a HSI without approval/STC.

Is this correct or have I imagined reading it?

If it is correct, what documentation would be needed? The aircraft MM?

ESKC (Uppsala/Sundbro), Sweden

This is standard on e.g. TB20/21 where (for airframe s/n 2000+) a GNS430+530 was done on the Socata Type Certificate.

Re what documentation is needed, I don’t know but presumably the TCDS will list the equipment.

Shoreham EGKA, United Kingdom

For the particular aircraft model I had in mind (C172R), the TCDS doesn’t make any mention of equipment except that the required equipment can be found in the certification regulations. On the other hand, the POH has a detailed list of required, standard and optional equipment. But I don’t know what status that list has.

ESKC (Uppsala/Sundbro), Sweden

Exactly what is the equipment you have in mind ?

Generally speaking, if the equipment in question is listed in the Manufacturer’s’ Equipment list and there is a supplemental section for it in the POH then you should be good to go.

It’s rare to see avionics and such equipment in TCDS, excepting major integrated systems such as the G1000 or Avidyne EFIS.


I can’t see how the data in a POH, maintenance manual or IPC on its own can be used to perform and certify a modification. Any change requires classification as Minor or Major (leaving aside the different wordings used by EASA or FAA) so who takes responsibility for this?
Whilst the data may give you a list of part numbers that the OEM has previously installed, there are no installation instructions – for example location and sizes for drilling equipment rack mounting holes, or specification of the material and specific rivets, hole pattern and sizes for antenna doublers. What happens about addesssing the changes to the electrical load analysis? If in Peter’s example, a GNS430 was to be installed, how and by whom does the AFM supplement get issued? There may be

Normally where an OEM approves retrofits post-production, they issue a Service Bulletin, whereas a third party will issue some minor change data or an STC package.

Avionics geek.
Fairoaks. EGTF

Aren’t you over-complicating this, wigglyamp?

The equipment installation was certified under the factory TC. The new installation is done IAW the wiring diagrams in the MM, and trivia like

location and sizes for drilling equipment rack mounting holes, or specification of the material and specific rivets, hole pattern and sizes for antenna doublers

are done IAW standard aircraft maintenance practices (AC43-13-2B in the FAA world). The installer must be presumed to have some basic intelligence. Or does an EASA installer require a drawing even to mount a GNS430? That would beggar belief. In the FAA world they would fall over backwards laughing. I am talking non-pressurised here…

I know (and you will recall from past discussions, years ago) that in EASA world generic procedures like AC43-13-2B are frowned upon by some installers, and even with an N-reg some customers are asked to pay a couple of k for a DER8110 design package, but surely that is going too far especially if there is an example aircraft available for inspection.

Load analysis is everyday work for an installer with a brain, and the factory wiring diagram.

Classic examples of the above, installing stuff which was Socata-certified, might be

  • GNS430/W (S/N 2k+)
  • GNS530/W (S/N 2k+)
  • Shadin fuel totaliser
  • EDM700

etc, and all those went in with just a logbook entry. The MM contains the wiring details – as if they weren’t pretty obvious. This stuff has been done all over Europe, on N-regs and EASA-regs. The big gotcha has been the 2k+ S/N limitation which screwed over a lot of people who wanted a dual GNS installation done and EASA refused purely on the grounds of the s/n.

Even specific types of landing gear pumps were installed in this way, having been originally Socata certified.

Shoreham EGKA, United Kingdom

Personally I’ve never seen this done on EASA or FAA aircraft in over 30 years of GA maintenance. I certainly wouldn’t certify an installation on this basis but if others want to then that’s entirely their risk of course. I wonder what would happen when a CAMO asked to see the approved modification data and instructions for continuing airworthiness?

Avionics geek.
Fairoaks. EGTF

I wonder what would happen when a CAMO asked to see the approved modification data

From the mfg MM, containing wiring diagrams etc. What does one need, really? Just how prescriptive does this have to be?

and instructions for continuing airworthiness?

From the specimen AFMS which comes with the GPS (if it’s a GPS) MM.

You may recall, c. 2005, we discussed doing exactly this with the Artex ELT. It was installed by Socata (under their TC) to my plane which was originally built as N-reg. Then when I bought it, 2002, I moved it to G-reg (stupidly; another story). At that point the UK CAA said ELTs are illegal in the UK and it must be removed! So Air Touring removed it (and it vanished somewhere…). Then, 2005, when I went N-reg, I needed an ELT to meet FAA regs. The original ELT (well, same model) went back in. No need for any paperwork. All approved by the FAA DAR. Later, the unit was upgraded to a dual band unit but that’s another story.

Quite possibly there is an extra bit of work generation in EASA-land, even with TC-approved items. I can see Jesse heading this way at 150kt… so I better get my coat. But I know of certain big avionics shops (not UK) who have done exactly what I describe on EASA reg planes. Dual GNS installation on TBs!

What exactly is incorrect here?

Shoreham EGKA, United Kingdom

When Socata as the TC holder approved the dual GNS that you’re referring to, was it as a Major change to type design applied to the TCDS? If yes, and then you apply the supposedly identical modification (without access to the factory design but using generic data from AC43 or equipment installation manuals) to another aircraft, how can you just issue a logbook entry? If you don’t know the basis of how they approved it, how can you determine the method of signing off the new installation? If you’re N-Reg, then a logbook entry is only applicable to a Minor Alteration. A Major Change or Major Alteration requires the issue of a Form 337 referencing the approved data. How is this addressed in your scenario?

Using a generic AFM supplement provided by Garmin is immediately deviating from the TC holder’s approved design, as they’d have issued their own version, which could contain additional limitations.

If you look at follow-on field approvals under FAA, then you can certainly reference approved data such as an STC that’s been applied to another aircraft, or perhaps wiring diagrams from the maintenance manual, but the 337 needs signing by the FAA to approve the data as being acceptable for the proposed follow-on aircraft.

Avionics geek.
Fairoaks. EGTF

I have no idea how Socata did it. They are a part 21 plus part 145 (faa and easa) company, with a tight relationship to the DGAC who rubber-stamp most things they do (the DGAC-insisted dodgy 25%-error fuel totaliser location being one famous exception ).

One previous thread on this topic is here.

How did Cirrus do it?

Surely, in both cases, the TC holder has the capability to certify anything which they can support with data. That is how aircraft manufacture works. Otherwise, if e.g. Socata wanted to make 100 planes each with a GNS530, they would need to apply to the DGAC 100×.

Shoreham EGKA, United Kingdom
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