In today’s environment of glass cockpit this and EFIS that, it is common for pilots who are building kit planes to get into heated discussions about the design of their electrical systems. Kits come with varying degrees of completed systems—from nothing to complete wiring harnesses and everything in-between. There are several good books on the topic of light aircraft wiring, starting with the well-illustrated Tony Bingelis classics and continuing with the well-thought-out Bob Nuckolls book (The AeroElectric Connection) that is still being updated. In fact, for the builder who is new to electrons and wiring, there is almost so much noise in the systems world that the result is information overload. You can read the forums, talk to experts, peruse the books and still be left somewhat dazed and confused by the mountains of information. In the end, one might still ask, “So, how shall I wire my airplane?” This is a good question that, in my mind, goes back to the fundamental query: What do I want to protect myself from?
Separating the electrical system into multiple busses is a good way to ensure redundancy. If a short occurs on a bus bar, the pilot can shut it off and power critical equipment from another bus.
If we rule out all types of failures (and I mean all types of failures), then electrical design is exceedingly simple: Hook a wire from the source (battery or alternator) to the loads (radios, lights, cappuccino maker…), and you are done. Just about anyone can do that. Unfortunately, life is not that simple, as anyone who has taken pilot training or read a particularly scary “I learned about flying from” article in which the cockpit either went dark or lit up with smoke and fire can attest. Addressing potential failures is where we have to make things a little more complicated, so it pays to think first about the failures that concern us. Basically, there are two: a short circuit that causes a large number of electrons to flow from the source and back through the ground without doing any work except melting wires and starting fires, and an open circuit, whereby the electrons cannot flow to (or through) your devices at all. Open-circuit failures could include the lack of a source to feed electrons as well, so let’s broaden the second category to include a lack of electrons to the devices. When all is said and done, electrical problems can almost all be resolved into one of these two buckets.
Organizing circuit breakers by bus (in this case, essential, main and avionics) makes it easy for the pilot to understand what equipment is powered from which source. It also makes bus-bar installation simple.
Where There’s Smoke…
Let’s first talk about short circuits, because they can be terribly dramatic. We all know that if we smell smoke in the cockpit, we should immediately power everything down and, if that doesn’t solve the problem, turn off the master switch. Having a fire extinguisher at the ready is a great idea, and landing as soon as you can find a suitable runway is even better. As the old saying goes, it is much better to be down here, wishing you were up there, than vice versa. All that is a reasonable response to a short that causes smoke and fire, but how can we design our systems to prevent such dramatic events and save our adrenaline for when it really counts? Well, the first thing is to use the proper-sized wire (which means sized for the fuse/circuit breaker protecting your circuit) and make sure that the circuit protection itself is appropriate. That should take care of your branch circuits; a wire rubs on a sharp edge, wears through the insulation, shorts to ground and pop goes the breaker. Nonevent. The same thing can (and should) be done for the big bus bars. It is not unheard of for a bar to be shorted to ground for a number of reasons (poor clearances, for instance), or a conductive foreign object to bounce into the wrong place at the wrong time. (Did you ever find those few dropped nuts, washers and pop-rivet mandrels? And where did that 3/8-inch combination wrench go?)
Properly securing wires will go a long way toward preventing shorts. Using grommets to pass them through ribs and bulkheads provides extra insulation, and securing with Adel clamps ensures that they won’t move around and chafe.
You can protect against the fire monster due to a bus short with a large breaker or fuse upstream of the bus. Yes, if it pops or blows you are going to lose the bus, but that is a lot better than letting it arc-weld itself to the fuselage skin; besides, we’re going to talk later about how you recover from the loss of the bus. It is worthwhile to review your system and make sure that anything that can reasonably short to ground is protected by some method of fusing. What can’t be protected? Well, the little bit of wire from the battery to the master contactor, for instance. Often this will be double-insulated, or it might even be a solid bar that can’t move around. If this goes to ground, you are looking at a bad situation, so design in a way that it can’t happen.
Our other failure, where electrons can’t flow through your devices, can come about due to a lack of voltage on the bus (maybe it shorted to ground and the circuit protection opened up), a loss of the branch circuit or a source that has gone away (the battery or alternator is dead). We can summarize this as a lack of a source or an open circuit. The best way to prevent these occurrences (assuming that you want the device to function) is to provide an alternative source and an alternative path.
In this picture of an under-construction wire bundle, you can see nylon edging installed in the lightening hole to prevent the bundle from coming in direct contact with the metal structure. This edging, sometimes referred to as “caterpillar,” is handy and easy to use in round holes that are too large for a grommet.
Alternative power sources might be a backup battery or a backup alternator. I suppose you could try solar cells or a hand-cranked generator, but let’s stay with what we know for now (or with what we know works). I haven’t figured out how to carry food for a cage full of gerbils and the number of exercise wheels they’d need to drive the generator, but sealed lead-acid and AGM batteries are good and reliable these days, and having a second one onboard is pretty simple. Yes, they will add weight, but modern electronics draw little power, so a small battery can easily provide enough juice to get you somewhere safe. The little standby alternators that fit onto the vacuum pump pad of a typical Lycoming are also great sources of electricity in a compact package. If you are worried about the backup battery’s reliability, this is a way to provide some redundancy.
In recent years, the idea of providing alternate power paths for your devices has entered the design concept of new avionics. Most advanced EFIS boxes, as well as some GPSes and radios, now provide two or three power inputs that are isolated internally by diodes and can provide input power to the unit’s power supply. Simply put, the box doesn’t care which of the feeds is good; it is happy as long as one of them provides sufficient input voltage and current to operate. The diode isolation means that if one of the sources is shorted to ground, it is essentially ignored; the other source provides power. This scheme is the simplest form of redundancy from a pilot’s point of view because it takes no action at all to reconfigure power to the device if a bus goes down. Once you have secured the bus (made sure it is not shorted), the rest is automatic. These multiple power sources can be very useful if you like turning on your EFIS before engine start. With a single power feed, most boxes will gray out and reboot when you hit the starter switch, due to the massive power draw and subsequent voltage drop on the main bus. If you have a separate (isolated) power source, such as an auxiliary battery, and it is connected to one of the other power feeds, the device will simply take its power from that source while the main bus is undervolting, and nothing goes offline.
The purple wire was not properly secured in this example. It rubbed on the angle bracket to create a gouge in the insulation and shorted to the structure. It was simply a sensor wire, so it resulted in no tripped circuit. It did, however, render the OAT probe useless.
If you have electrical or electronic devices that you consider critical but do not have multiple power inputs, you can build an easy external version of the same thing using two diodes, sized properly to accept the necessary current. Simply “Y” them together so that their output sides are joined. Next, go to the device’s power input and have the input sides of the diodes go to separate power sources.
As mentioned, the beauty of diode-isolated power supplies lies in the fact that the pilot doesn’t have to do any switching in the case of a bus failure, short of isolating the bad bus from its source. In fact, if your circuit protection is properly designed, even this is taken care of when the breaker pops or the fuse blows. You can keep on your merry way with the alternate power source plugging along. If the failure is instead in a source (most likely your alternator has gone on strike), then you probably want to shed some loads to preserve the battery power for critical functions until you can land.
In older big airplanes, there are checklists and procedures to help the crew shed the appropriate loads. In modern big airplanes, the computers will do a lot of this reconfiguration for you. For light airplanes such as we generally build (even complex IFR machines), you can buy automated electrical control units to do this for you, or you can take a look at the loads you are likely to have and turn off the things you don’t need. Most Experimental airplanes have some lights, pitot heat and their avionics. How hard is it to turn off the lights and heat (if you’re not in the clouds) and turn off one or two radios that you don’t need to save power? It takes longer to write that than to do it.
Engine-compartment wiring should be carefully secured and insulated from metal components such as the engine mount to prevent chafing and short circuits, especially when dealing with large starter wires.
For a day/VFR machine, all of this is really unnecessary. Build simple and light, and if the electrons go away, or start leaking all over the cockpit floor, turn it all off and go home. The engine should be fine. For IFR operations, I like to have dual busses with all critical loads dioded to both, and non-critical loads powered off of what becomes (by default) the main bus. I like to be able to power the other bus (call it emergency, essential or Fred…it doesn’t care!) from the main (through a diode) or the battery; if you have an aux battery, be able to power it from that as well. Source switching is a matter of one switch per battery. As long as you can monitor the voltage on each bus, it is easy to tell whether it’s working.
“Shorts” and “opens” pretty much sum up the potential electrical failures that are likely to ruin your day. Electrical system design can be a mystery if you don’t take the time to understand the simple goals you are trying to achieve and the low number of failures that you are trying to protect against. Don’t let the size of the books scare you. This is really just plumbing with electrons. Like any other aspect of building an airplane, it’s something that you might not know how to do until you have to learn it. Don’t make it any more complex than it has to be, and keep those electrons happy and where they belong.