Avionics master switch - TB20

Hello Edouard and welcome

“Avionics master” can mean different things.

At the simplest, it is a switch labelled “avionics master” or similar. Do you not have such a switch? That is just a switch, which could be switching the avionics supply directly or via a relay.

I think most TBs had the more proper setup, like this

where the avionics master switch operates 3 relays, each one controlling each avionics bus. These are mounted on a circuit board somewhere under the centre stack I think. It is very possible that Socata don’t supply this board; they have reportedly said they no longer make custom assemblies (but have since denied having said so).

But I would be surprised if you didn’t already have all this in your plane. So this is a mystery.

Later TBs had an addition whereby all the avionics relays were disconnected while the starter motor was energised.

BTW I have a quantity of the relays for sale. No paperwork but I did buy them direct from the manufacturer Britax Signalisation in France and have their paperwork. Details here.

That’s amazing… maybe that really was factory standard. Unfortunately there isn’t anything like a decent list of what changes were done at which serial number.

The circuit board can probably be legally retrofitted, because it existed on slightly later but otherwise identical aircraft. See here. There is some question mark about the paperwork required. In that thread you have one avionics installer who says Yes and another who says No… well sort of. I think the answer is Yes.

The bigger Q is whether you can manufacture it. IMHO if you make a duplicate of the Socata part then that is OK. I know for a fact this is done in Europe, for EASA-reg planes. For example one German company has manufactured a part similar to the factory one (which wasn’t available) and signed it off, etc. OTOH I am sure most companies would be too scared. But you would need the details, how and where to mount it, etc. Technically it is trivial. The latest TB GT schematic of the avionics relay board is here and it is easy to make. You can get PCBs made in China for peanuts so it would be a professional job – for a few hundred €. Unless you have sight of the Socata one, you won’t be able to make it exactly the same however.

You need the Socata TB MM (Maintenance Manual) CD. This came up on Ebay a while ago, dated Feb 2006 which is after the date when Socata stopped issuing changes. Drop me a line with your address…

But mainly you need to find a decent and sensible avionics shop… They will have to do a fair bit of rewiring because you probably have all the avionics running off a single circuit breaker. Maybe the MM CD shows this (for some early S/N) but I don’t have the time or the knowledge of how to look for it.

OK I will send you the CD, free of course

I removed your address since that should not be on a public site.

If you have a good shop, and there is somebody who is into electronics (most avionics shops don’t have anyone who is into electronics) they can easily build the PCB with the 3 or 4 relays on it. I have searched the MM for a diagram of the PCB but can’t find it, and I can’t fin any photos on google.

I think you might be getting a little to complicated here, I have just installed an avionic master switch on moth my Cessna’s , it is simply a switch CB, of course you need two of them to provide redundancy.

Could be, but in that case why is this an issue for the avionics shop?

I don’t think your shop has any idea of what they are doing, ESN.

You don’t need an EASA Form 1 for a switch. Well, lots of avionics shops will say you do

You probably do need an EASA Form 1 for the avionics relay PCB, if you need to fit one of those. God knows where you will get one of those from…

What S/N is your TB20?