5-Times a Charm

[Stevetin]

Hello All, I'm new to Sonex Builders. I'm the fifth owner of a Waiex (A), #0030, but the first to get beyond the tail section. I'm currently installing avionics and the electrical system. I do have a question regarding the master switch, actually the master relay. The AeroVee installation booklet calls for a relay to be wired directly to the battery as a master switch (then wired to an on/off switch on the instrument panel). My question is: Are other builders doing it this way or are builders using different master switch configurations? My concern is that the automotive type relays are normally only used for a starter motor or for activating glow plugs in diesels. They are energized for only a few seconds at a time.

When using this type of relay for a master switch it's activated for hours at a time, my concern is that the relay will fail in flight and then it's good-bye electrical system. Aircraft Spruce sells a White Rogers brand relay for experimenters but again, I'm concerned. As an FYI, years ago I had a total in-flight electrical failure while flying a 172 at night. I'm thankful that the mags kept me humming along until I could land.
Thanks!
Steve Paradise

[Jgibson]

There are two types of relays.
There is a 'momentary' 'starter' relay and continuous duty relay that feeds everything else.
IF you were to use a 'starter' relay in a continuous mode, most certainly it would fail. Your master switch should energize the continuous duty. Your starter relay is then able to be energized during startup.
Hope this helps.

Joe Gibson

[Stevetin]

Thank you Joe, makes sense. The two relays that I was looking at from Aircraft Spruce are both continuous duty. I'll probably go with the more expensive of the two ($70). I think that I'm going to wire a 75 amp pull switch in parallel to the relay as an emergency backup...just in case!
Steve

[Bryan Cotton]

Welcome Steve! There is a good thread on the master contactor here:
viewtopic.php?f=6&t=2009

I went with a mechanical switch that I actuate through the firewall. It's documented there. Others have mentioned relays that draw well less than an amp.

[sonex1374]

Steve,

The purpose of a master contactor (e.g. battery contactor or master relay) is to isolate the battery from the airplane under certain conditions. One is to prevent the airframe from draining the battery thru small parasitic loads, as this could lead to a dead battery unless you flew often, and a dead battery all the time will shorten the battery's life (and is pretty inconvenient for the pilot). The second use is to isolate the battery from supplying large amounts of current that can cause a fire risk. This is a concern in a crash-type situation where the crunching of structure could cause wires to short circuit - if the battery is still "connected" to the electrical system, the stored energy is pretty good at causing sparks and an airframe fire. The FARs require certified airplanes to be capable of disconnecting this stored energy by simple pilot action as a safety measure; experimentals follow suite as a matter of good practice.

You mentioned having a 75 amp pull switch as a "just in case" addition. Nothing wrong with having an alternate circuit to power the electrical system, but battery contactors are pretty reliable, and the failure rate of them is pretty darn low. I suspect that your pull switch will never get used. If you have critical things that must have power to keep flying, you'll want to really think through your electrical system design to ensure the power stays on thru all conceivable situations. It takes quite a bit of thought to craft a system that can handle various component failures, and a good place to start dissecting all this is the info contained in the book "AeroElectric Connection". It's the go-to reference for experimental electrical system design, and is highly recommended reading!

Best of luck on your new Waiex project!

Jeff

[Stevetin]

Thank you Bryan and Jeff for your feedback. Bryan, I'll research the link that you sent me, thank you. Jeff, regarding the 75 amp pull switch, I agree that I hope to never need it but since I've already lost all electrical power once, at night, I'm a little gun shy. This project has been a lot of fun and keeps me out of the bitter cold in the winter and the extreme heat in the summer (like now).

[Jgibson]

One reason many Part 23 planes mount the main contactor relay on the battery box is so that there is minimal 'hot' points between the battery and relay.
There is usually an appropriately short cable from battery to the relay. When the master is killed, minimal chance (opportunity) for a sheared or chafed wire to start a fire in the event of a crash.
I learned the 'value' of the arrangement the hard way. (VERY hard way).
The aircraft manufacturers spend a LOT of time and a LOT of money building certified planes because of our litigious society and if it works for them...

Joe Gibson

[NWade]

Hi Steve,

There are a few “single points of failure” in the electrical systems/schematics that most people implement... Your contractor could fail, your buss bar could get loose & fall off of your fuse or circuit breaker stack, you could have a critical wire in the alternator system come loose, etc. All of these are low-probability failures if proper practices are followed.
There are two ways you can address this:

1. Put a lot of thought, time, money, and weight into adding redundancy everywhere (since eliminating only one failure point that’s low-probability doesn’t really improve your overall odds).
2. Accept that there may be a rare failure and have a way to safely handle the various failure modes.

For my aircraft, I decided that the magnetrons will keep my AeroVee engine turning so for me the critical issue is situational awareness - keeping the airplane flying and being able to make a good landing at a nearby airport. Thus I mounted an MGL Infinity ASX-1 in my panel (it has since been superseded by the Vega ASV-1). This is a 2.25” round instrument that provides Airspeed, Altitude, and VSI. It requires just a few milliamps and can run on 8 volts - so I wired it up to a 3-way switch. It can run on ship’s power *or* a 9V battery (which I’ll simply swap out each annual). With this setup I can lose all main electrical power for any reason (contactor, battery, alternator, CB failure, etc) and still have both a running engine and enough instrumentation to get me safely on the ground without a panic. All while not having extra wires or components in the airplane that I’m carrying around as extra weight, complexity, and maintenance items (as they themselves could cause failures or short-circuits).
For non-MGL folks the Dynon Pocket Panel is another good “backup instrument” option.

Take care,

—Noel
Sonex #1339