Light Aircraft Association

To fit a relay, or not to fit a relay : that is the question.

All (permit) aircraft wiring diagrams I've ever seen seem to just have a switch from the main bus to the avionics bus, so that the avionics can be isolated for startup to avoid the nasties involved in the operation of the starter (and, to a lesser extent, turning on the generator).

I've discovered that some certified aircraft use a relay, and not just to handle a heavier load than the switch could cope with.
It appears the relay is normally closed (connected) and switching the avionics master switch to OFF actually energises the relay, DISconnecting the avionics!
The idea is if the avionics master switch breaks then the relay remains closed/connected, feeding the avionics with power.

Don't know about anybody else, but I've seen far more problems with relays than I ever have with switches: more external connections to fail, dirty contacts inside, cooked coils, sticky pivots, to name but a few. To put a less reliable piece of kit (relay) in to guard against failure of a more reliable piece (switch) seems barmy.

Also makes me wonder about the "starting an engine on a cold winter day" situation. Turn the key, starter turns about fast enough to start the engine but the voltage fluctuates so avionics relay clicks in and out. This would not only feed 'dirty' power to the avionics (the very thing the avionics master is supposed to prevent!), but the flicking of the relay coil and contacts is going to make that power feed even 'dirtier'.

Think I may have just answered my own question, but what do people recommend:
- a relay installation as described above
- a more conventional use of a relay just to offset sheer current requirements of a complex avionics stack (2 x radios, GPS, xponder, intercom, TCAS etc)
- a nice meaty (10-20 amp?) switch doing the whole job.

Or
- turning each bit of kit on/off individually? Light and simple.

Steve Brown wrote:Or
- turning each bit of kit on/off individually? Light and simple.

Doh! Forgot that one - and it was the way we used to do it in my last aircraft too!

Some radios have the on/off switch on the volume control, and turning the radio off requires rotating to the minimum volum setting first. This causes considerable wear on the resistive element, and can result in scratchy and intermittent reception after may operations. An avionics switch of some sort makes a lot of sense.

Frank Parker

A point to bear in mind yes but perhaps it may be better for the set to be powered up with the volume down, thus maybe minimising the voltage surge to the power output stages on switch on.
As an example, I always turn the volume on my home HI-FI amp to minimum before I turn on the power. If the vol is say halfway round there is often a big click in the speakers that makes me wince (and that amp has a power-on delay circuit).
I agree it can be a pain to turn on lots of kit but individual turn ons give a chance to evaluate if the item is powering up ok, rather than in bulk & maybe risk missing a failed unit.

Rob
I'd use a decent switch.
If you have the discipline to turn on the mags etc etc, before you start up what's the problem with having one more switch ? A solenoid isn't exactly light either....
dh

As with most things, use the KISS principle - Keep It Simple St---d! Personally, I wouldn't use a design that meant a relay coil was energised to keep the electrics off. Switches should always be much more reliable than relays (less moving parts etc. etc.).
Tony Nowak

Just because things are in a GA or certified design does not necessarily mean it is a good idea. Much of the current electrical design thinking goes back decades, and hasn't changed due to regulatorary issues or pilot familiarity reasons. This is depsite fantastic improvements in the actual avioncs devices themselves.

Start from here: No switch and no relay = simplest, most reliable, and lightest.

Any modern avionics that can't survive starting noise on its power feed is not worth buying today (and probably wouldn't pass regulatory testing anyway).

That just leaves the 'convenience' of having an avionics switch as a decider, which only the pilot can decide on.

My plane does not have an avionics switch, nor any relay either.

It works fine.

Cheers

Simon C
~~~~~~
G-MOOV

P.S. You'd be much better off getting a decent battery, looking after it properly, and renewing it regularly (after 3-years max)

“- turning each bit of kit on/off individually? Light and simple.”

Defiantly the way to go. If you switch each item on and set it up in tern you are less likely to miss something. If you have an avionic switch you run the risk of blowing everything up. I have a friend with a 172. He routinely switches on the master and the avionic master to program the GPS. He then forgets to switch the avionics master off and starts the engine. He has blown the DME twice and a cost of several £1000!

If I bought an aircraft with an avionic master I would remove it.

Rod1

Found this http://www.aeroelectric.com/articles/avmaster.pdf this afternoon.

I guessed that Bob Nuckolls would have something to say about it - just didn't find it until today.

I'd already decided that the relay was overkill.

The diagram in the article makes for interesting perusal. 2 "avionics" masters... but it does then label the "avionics bus" as "essential bus", so really warrants a reappraisal of the whole wiring design approach!
I do wonder about the naming of the 'Main power distribution bus' when it only has 7 systems off it and the essential bus has 8!

One other thing that occurs to me is the article is aimed at the EAA / US builders, who can fly IFR / at night etc.
For our current daytime VFR rules complete failure of every electrical system (and the vacuum system as well, come to that) means just turn off the master and look for an airfield.