Circuit breakers
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lancair360
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One for the screen. One for the autopilot. One for the radio. One for the transponder.
And at least one "spare" breaker since you don't know what you will need a few years from now. When I did my panel I put 2 spare "holes" and 2yrs later wound up badly needing them. I was so thankful I had those spare locations available. 
Marc_J._Zeitlin
Active Member
I think this is more complicated than just "how may CB's do I need for three or four items". You don't say whether this is a new build with a new electrical system design; whether it's a retrofit in an existing plane; or how much ELSE you're wiring/re-wiring at the same time.
I strongly suggest getting "The AeroElectric Connection" book from Bob Nuckolls and understanding the philosophy behind aircraft electrical system architectures, and then choosing an architecture from his choices based on your mission and equipment.
Personally I like fuses over CB's (easily available, far more reliable, cheaper, simpler, etc.), but to each their own.
MY philosophy (and Bob's, to the extent I can tell) is to use a circuit protection device for each power feed line and to almost never have more than one device on any power feed line. Why would you want a short or failure of one device to be able to take down other devices with it? And why would you want to oversize wiring (since the circuit protection device is there to protect the wire, NOT the device).
I use an advanced control module so I have no circuit breakers in my airplane except the electronic ones. Works great
Marc_J._Zeitlin
Active Member
You really need to read the philosophy of aircraft electrical system design in "The AeroElectric Connection". You design out the possibility of safety failures in the architecture of the system - you don't rely on the pilot to make up for design deficiencies. Pullable breakers are bandaids for poorly designed systems.
Yeah, I know I'll get crapped on for saying that, but if it walks like a duck...
Rhino
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It should be noted that Bob Nuckolls doesn't in any way oppose the use of breakers from a safety or functionality standpoint. The only hesitations he has for them is based on space and weight, and the unlikely event that a true fault tripping a breaker will be corrected by resetting it. Spurious tripping of breakers is most often caused by poor power production/distribution system design or construction rather than a true system fault, and breakers should not need to be reset inflight if the system is designed properly. A hard failure causing a breaker to trip inflight isn't going to be corrected by resetting that breaker except in extremely rare circumstances. Nuckolls prefers to use fuse blocks with blade fuses to save weight and space. They aren't as easy to reset, but they provide that ability if absolutely necessary. If you aren't very confident in your system design and construction, there's nothing wrong with using breakers. Just make sure they have the proper specs, just like fuses. But as Marc noted, they shouldn't be needed in a well designed system, and they're unlikely to correct a true system fault. So why waste the weight and space if you don't have to? I will also reiterate Marc's suggestion that you buy a copy of The AeroElectric Connection. It's an invaluable resource.
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With the greatest respect the Skyview Installation manual section 10 (autopilot installation) still calls for the autopilot servo power to be via "a circuit breaker accessible to the pilot while in flight." This is your last line of defence, short of over powering the servos and perhaps breaking the shear screws, in the event that the Skyview system itself goes haywire, in which case you may not have access to a VPX software circuit breaker.
On my little panel I've arranged the breakers in a hierarchy of three rows, ie. : power supply, stuff that can kill you, the rest.
The autopilot CB is in that second category along with with trim, flaps and propeller control.
On my little panel I've arranged the breakers in a hierarchy of three rows, ie. : power supply, stuff that can kill you, the rest.
The autopilot CB is in that second category along with with trim, flaps and propeller control.
Rhino
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Where breakers are recommended to provide correction for runaway conditions, such as in trim or autopilot servos, fuses and switches can provide the same protection with less weight. My personal preference is a hybrid system, with only a few breakers and the rest fuses. Servos are indeed an area where I may opt for breakers simply out of convenience. Every builder must choose the architecture that best fits their circumstances. A simple statement of personal preference, and the reasoning behind it, isn't advocating for blanket compliance with those preferences. It's simply providing builders with the most options, and the considerations that may or may not warrant choosing them. Given that the OP is in the stage of design where those considerations and options are most important, this is the best time to express all of them. So it's not really an argument. It's a buffet, man! Choose what you like best.
Compliant: There is a disconnect button on both sticks, beside the ECBs. So I have ECB access from either EFIS (at most two taps), plus two hard buttons. In addition, the trim forces and servo torque are light enough that I could land safely if needed, from any trim/ap servo condition.
swatson999
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Isn't this a two-fault scenario? An autopilot that "runs away" somehow (?) AND a failure of the A/P Disconnect button?
Rhino
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No, it's three actually. He has breakers on the servos too. However, the A/P disconnect may not protect you if the autopilot system (not servos) goes into runaway, because a runaway system could ignore a disconnect signal. A servo breaker would protect you against any runaway scenario though. The rest isn't absolutely necessary. But it's his plane. He can be as cautious as he wants to be.
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Go with whatever you feel comfortable with, after all , that's what Experimental is about. Me, I tried to follow FAR 23 on this subject because that's the sorta guy I am.
https://www.risingup.com/fars/info/part23-1329-FAR.shtml
I think this sub clause tells me to have a breaker, a fuse or some wire cutters as last resort ;P
(g) There must be protection against adverse interaction of integrated components, resulting from a malfunction.
https://www.risingup.com/fars/info/part23-1329-FAR.shtml
I think this sub clause tells me to have a breaker, a fuse or some wire cutters as last resort ;P
(g) There must be protection against adverse interaction of integrated components, resulting from a malfunction.
swatson999
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I don't think that's what that means (although having each servo on its own fuse is, IMO, a good idea). It means that a failure in one component cannot cause a failure in another. The terminology in aerospace is Fault Containment Region (FCR), and the boundaries of an FCR are determined (partly) via tools such as FMEAs.
John Bright
Member
I'm with Bob Nuckolls re using fuses instead of breakers.
I would add space for more switches and/or breakers than you think you will need but for me fuses are in Bussmann 15600 fuse panels behind the instrument panel.
My setup:
BTW Bob Nuckolls' book Aeroelectric Connection is free to download at http://aeroelectric.com/Connection/
BTW BTW Bob's latest Z schematics and Z schematic revs are not in the book, important to me is Z101. I keep a list of links that may be helpful.
I would add space for more switches and/or breakers than you think you will need but for me fuses are in Bussmann 15600 fuse panels behind the instrument panel.
My setup:
- The AP servos get a 5A pullable breaker that feeds them both. The way I read the IM, Dynon wants a pullable breaker or a fuse and switch even though there is a disconnect button (AP Disengage/CWS). I don't see it now but I read somewhere that if Skyview is booted up while the servos are unpowered there will be a non-resettable error so I used a pullable breaker so there is no switch to remember to turn on.
- The SV-AP-PANEL trim motor section gets a 5A fuse. A smaller fuse could be used but I reduce part numbers by not using fuses smaller than 5A.
- I used a 7-1/2A fuse for the HDX because when I add all the loads described in Table 3 of the IM I get 4-1/2A. Divide this by 0.75 nuisance blowing factor and I get 6A so I go with the next available size up, 7-1/2A.
- The Dynon radio gets a 5A fuse that feeds the transceiver and the control head.
- The SV-XPNDR-261 and SV-ADSB-472 share a 5A fuse.
BTW Bob Nuckolls' book Aeroelectric Connection is free to download at http://aeroelectric.com/Connection/
BTW BTW Bob's latest Z schematics and Z schematic revs are not in the book, important to me is Z101. I keep a list of links that may be helpful.
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