This month I’ve assembled various items from the book that don’t fit neatly into a single topic, but together make up a collection of useful information. Hopefully you’ll find this information helpful while planning and building the electrical system for your Experimental aircraft.
All electrical circuits must have some sort of circuit protection that protects the wires in case of a short circuit. Rather than having the wire catch on fire or melt at an arbitrary location, the circuit protection provides a mechanism to limit the current and also control the location of the failure point when there is a short. A short circuit occurs when the positive wire touches a ground wire or grounded metal airframe.
All circuit protection works in a similar way. If the current is at or below the rated value for the circuit protection device, the circuit stays closed. If the current rises above the rated value, then the circuit protection opens and stops the current from flowing.
There is an inverse relationship between the amount of overcurrent and the time it takes to open the circuit. For example, if 5.5 amps is flowing through a 5-amp breaker, it will take a good amount of time to open the circuit. If 25 amps is flowing through the same circuit, it will open almost instantaneously. This relationship can be seen in Figure 1.
Table 1 below shows the three main types of circuit protection used on aircraft and the pros and cons of each.
Electronic Circuit Breakers
Electronic circuit breakers (ECB) are solid-state devices that provide circuit protection and on/off switching functions. ECB systems for Experimental aircraft are bundled together into a single enclosure that provides a power distribution hub for all of the aircraft’s electrical needs. Using multiple microprocessors provides redundancy and also allows the builder to install independent backup circuits for emergency power, in case the primary power fails.
Vertical Power is the leading provider of ECB systems to the Experimental aircraft market. Their flagship product integrates with many popular EFIS displays, enabling you to monitor and control your entire electrical system on the EFIS.
A typical mechanical circuit breaker has a mean time between failure (MTBF) of about 17,000 hours. A single electronic circuit breaker has an MTBF of about 1,000,000 hours. Further, a mechanical switch is rated for about 30,000 on/off cycles. ECBs are rated for about 2 billion on/off cycles. As you can see, modern solid-state components offer significantly higher reliability than older components.
ECBs provide circuit protection like old-fashioned thermal circuit breakers, but ECBs do a lot more than just detect circuit faults. ECBs are intelligent, configurable, and offer capabilities not otherwise available with old-style breakers. For example, ECBs can detect a burned-out landing light or disable the starter circuit while the engine is running.
ECBs greatly simplify wiring while at the same time provide advanced electrical system capabilities. Wiring is simplified because you don’t have to install circuit breakers, bus bars, relays, trim and flap modules, shunts, e-bus diodes, or other complex wiring right on the back side of the instrument panel. The advanced electrical system capabilities include solid-state power switching and circuit protection, open circuit detection, automatic landing light wig-wag, pilot and copilot trim control, runaway trim protection with backup trim controls on the EFIS, flap control with intermediate flap stops, starter disable when engine is running, flap overspeed alarms, trim and flap position display, overvoltage protection, alternator control, and more.
D-Sub (Subminiature): By far the most common connectors you’ll use on homebuilt aircraft are D-sub connectors. These connectors come in a variety of sizes and capacities. Most avionics use standard size D-sub connectors that support 20-24 AWG wire and come in 9-, 15-, 25-, 37-, and 50-pin configurations. Also available but less used are high-density D-sub connectors (Garmin likes to use these). Unless you see them next to each other, it is hard to tell them apart. Be careful not to get the standard and high-density parts mixed up—it can be easy to do.
Typical 15-pin D-sub connector. Note that numbering is reversed, but matches up once the connectors are mated.
I recommend Conec plastic back shells with thumbscrews. They are high quality, yet reasonably priced, with an excellent strain relief mechanism (strain relief mechanisms secure the wire bundle so that no strain is put on the terminals when the wire is tugged). These are available from major electronic supply dealers like Digi-Key and Mouser. Part numbers:
165X10139XE 9 positions
165X10149XE 15 positions
165X10159XE 25 positions
165X10169XE 37 positions
Mate-N-Lok: These are often used for power wires. Be aware that these connectors do not have any strain relief accommodations. Be sure to use models that have a positive lock feature.
AMP Circular Plastic Connectors (CPC): These are available in a wide range of capacities and sizes, and include a strain relief accessory. The AMP CPC Series 2 uses standard D-sub pins (size 20) and is good for up to about 5 amps. The CPC Series 1 uses size 16 contacts, is good for up to 13 amps, and requires a special crimp and removal tool. A complete set of terminals, crimpers, housings, and back shells is available at Mouser and Digi-Key.
The Series 1 connectors are good for mating high-power wires, and Series 2 connectors are good for mating wires for the instrument panel avionics. They look similar, but use different terminals.
Mil-Spec Circular: Mil-spec connectors are mentioned simply to dissuade you from using them. They are heavy and expensive, and require expensive tools. The benefit is just not there for the homebuilder community.
Diode Isolated Power Inputs
Many modern avionics have diode-isolated power inputs—typically a primary and secondary power input. The diode isolation ensures that each power input is independent of the other power input. Current coming in from one bus cannot flow through the device and out the other pin to the other bus. The diodes shown can be thought of as check valves that ensure current only goes in—but does not come out—that particular power pin. The device will automatically choose between the inputs and select the one with the highest voltage.
The EFIS on the right in the example on page 49 does not have diode-isolated power inputs. It simply is using two power pins to distribute the current. Each pin can carry about 5 amps, and if the device draws 8 amps, the engineers decided to spread the load between two pins, so each carries 4 amps. Therefore, both pins must be fed from the same bus, and from the same circuit.
This can be easy to miss. Be sure to read the manufacturer’s instructions carefully to determine how the input power is handled.
Recommendations On Using 14 or 28 Volts
Today, most Experimental aircraft use a 14-volt electrical system, while most certified aircraft use a 28-volt electrical system. Components for 14-volt systems are easy to find, and often kit manufacturers supply components that only work at a specific voltage (flap motors, for example). For the majority of Experimental aircraft, 14 volts is sufficient.
One reason to use 28 volts is to reduce wire size. The same device draws half as much current at 28 volts than it does at 14 volts. Therefore, you can size the wires smaller and save some weight. In a hypothetical Experimental aircraft with 600 feet of power and ground wire, if you use all 20 AWG wire instead of 18 AWG wire, the weight savings amounts to 1.5 lbs. Based on this analysis, weight alone should not be a deciding factor.
Most Experimental aircraft are small and have relatively short wire runs and relatively low-current devices. In such cases it makes sense to install a 14-volt system. However, you should consider a 28-volt system if you have an air conditioning system, or retractable gear that requires a hydraulic pump.
I also do not recommend installing two different buses with two different voltages. By the time you install the voltage converters and sort through the confusion of multiple voltages across multiple buses, it is better to stick with a common voltage for the entire aircraft.
Primary power distribution cables, engine monitor wires, and other wires like the starter control and alternator field wire need to pass through the firewall. The firewall penetration must be secure so that hazardous gases do not get through, and it provides a barrier in case of an engine compartment fire.
Rather than run everything through one large hole, consolidate wires by function and run two or three smaller holes. There are several ways to do this:
• Drill a hole with a Unibit step drill and install a snap bushing. Run the wires, then fill in the hole with high-temperature silicon. While this does the job, it is not a very secure penetration.
• Install a stainless steel firewall pass-through as shown in the images below. You’ll want to install, at minimum, one for the heavy-gauge wires, and another for the engine monitor and other smaller wires.
The firewall pass-through serves several purposes:
- Reduces the chance of electrical wires chafing on the sharp firewall, which can lead to fires.
- Reduces the chance of fire igniting in a crash.
- Slows the penetration of a fire in the engine compartment.
- Seals the firewall, reducing the chances of carbon monoxide or other gases entering the cabin.
Read the Book
Hopefully this article helps you understand the electrical system in your aircraft. It is an excerpt from my new book entitled Aircraft Wiring Guide. For more information, or to order a copy, visit www.aircraftwiringguide.com.