January 2012 Issue
All About Avionics
We take a look at the all-important firewall-forward electrical elements.
When we hear the term "aircraft electronics," it usually conjures up images of complex instrument panels or airframe wiring. What we often don't think of is what goes on forward of the firewall electrically. In our Light Sport-type aircraft the wiring and associated electronics forward of the firewall are every bit as important (and, in some cases, more so) than the rest of the airframe. The biggest reason is that outside of the firewall resides the largest single point of failure in our airplanes, the engine. In a single-engine airplane, it matters little what else you do if you don't pay close attention to the single engine driving the single prop.
Devices such as exhaust gas temperature (EGT) probes need to be adequately mounted, secured and protected.
Like the rest of the industry, components and engines forward of the firewall are becoming increasingly dependent on electrical power; items such as FADEC (full authority digital engine control), ignitions, fuel pumps and more are all core to the operation of the engine. Even modern twin-engine aircraft are becoming more electrically dependent, all the way up to the 100% electrically reliant modern airliners. Heck, it probably won't be long before more and more planes are buzzing around solely on electrical power.
Antennas mounted forward of the firewall need to be adequately secured and grounded.
Almost every airplane (I say almost, because a number of planes still happily fly around without any electrical systems) has a source of electrical power generation attached to the engine. Sure, some older aircraft used a wind generator, but it's neither common nor practical anymore. Most common is a simple alternator (or, historically, a generator), which in design varies little from its automotive brethren. They can be either internally or externally regulated, and I won't get into arguing which one is better; the fact is that both of them work, and both are acceptable. Some engines have gear-driven alternators, some are belt driven, and some even use an integral flywheel-mounted "generating or lighting coil" as seen in the well-proven Rotax engines. In the end, they all perform the same task: providing power for both the engine and other systems in the plane.
Protective boots over heavy wire terminals can be beneficial.
Even with simple installation, the firewall-forward area of your plane can become quite crowded, which makes routing of wires even more important for safety, reliability and future maintenance.
There are a couple of key factors when talking about the alternator and electrical systems. First is a secure ground. I'll repeat it because it's of utmost importance: Make sure you have a good engine-to-airframe ground. That may sound simple, but it's often overlooked and frequently the cause of problems for first-time builders. You see, the alternator is most likely attached to the engine, which frequently is isolated from the airframe via rubber bushings that do not conduct electricity. Therefore, it's common for an engine to have no ground path to the engine other than through various other wires such as an alternator field, P-lead shielding, etc. This can cause all sorts of electrical problems, and the solution is to install a ground strap or wire of sufficient size from the firewall to the engine to handle all probable electrical loads on the engine, which can include the alternator, starter, ignition and more. Make sure this strap or wire is of fairly good size and also goes directly from a reliable contact point on the airframe to a reliable point on the engine, typically the engine case itself. Installing a ground strap should be considered a mandatory item.
You can see some of the many wires that will be on the firewall as well as running throughout the engine compartment.
Routing of spark plug harnesses secured through baffle materials.
Keeping the fire lit is something we rarely think about, but it is of great importance. While piston airplane engines rely on spark plugs (except for a few diesels), the method of producing that spark can vary a bit. Good old-fashioned magnetos generate their own power. Even though it is old technology, the magneto is a stalwart of reliability. While they generate their own power, we still need a method to turn them on or off, which typically is an ignition switch hooked to a wire from the magneto called a P-lead. In case you're wondering why it's called a P-lead, it's because that wire typically ends up going through the magneto and is attached to the primary coil windings, hence the name. This wire is often shielded, so it frequently ends up being the wire that is the sole ground return to the engine when the main ground strap or wire is defective or missing. It's also an electronically noisy wire that we want to run separate from other wires through the firewall to the ignition switch.
This clean installation shows use of protective boots, cushion clamps and clean wire routing.
Another important item that is often overlooked is the spark plug lead. Many times we have found that the automotive-type harnesses are just fine when constructed properly, but if they're not assembled perfectly, they can cause all kinds of weird problems with noise or instrument indications. Of course, keeping the plugs clean and gapped properly is always good practice. If you are using an electronic ignition, it's also important to keep your wire runs to those devices or to their computers and coils as neat and tidy as possible.
Standoffs may be used to better position some sensors and probes.
Most of us use electric starters on our planes that greatly ease the task of getting the fan turning. If everything is wired correctly with the starter and ignition (and fuel pump), things normally go as planned. However, if you forget to install the mandatory ground strap, you'll likely fry the P-leads, alternator leads or any other electrical connections between the airframe and engine, as they are the only ground returns.
The wire to the starter is typically a fairly large and fat wire. Make sure you support it properly, and also make sure you use a good crimper or "the terminal tool" to fasten the terminals to the fat wire. You may choose to solder those terminals, but make sure you have a hot enough iron or torch to do a good job. Note that with the starter solenoid, it's a good idea to install diodes across the posts on the relays to protect other devices from the resultant voltage spike. Diodes for the starter and master relays are available from a number of sources, but we find it most convenient to spend the few dollars and purchase them from Van's Aircraft.
Mounting these sensors directly to the firewall ensures good ground contact.
In years past, we've used mechanical gauges driven by fluid lines in the cockpit. But now the majority of homebuilt aircraft will have electronic instrumentation. Modern instruments use senders that are mounted on or near the firewall, even sometimes on the engine itself. One very important aspect with most of these sensors and transducers is that they normally must have good grounds. Lack of a good ground has flummoxed many a technician trying to troubleshoot gremlins. Some sensors are provided with their own ground, but many of the popular ones rely on the case of the sensor to provide a ground path. This means you need to ensure the case somehow has a ground connection, either through the mount or via a wire.
Alternative mounting for probes and sensors on this composite firewall.
A typical FWF probe and sensor package.
It's also important to mount sensors securely and per manufacturers' recommendations. More than one issue with broken sensors has been the result of improper mounting and connection. For example, if a sensor is mounted on the firewall and connected to the engine, then a flexible line is typically recommended.
Firewall forward view showing the battery box and relays as well as a ground strap on the engine mount.
Many airplanes will have a battery mounted on or near the firewall. There isn't much to go wrong with a battery, but there are some basics to be followed. First, ensure the battery is secure. Second, ensure the terminals are easily identified. A good tip is to physically mark + and – somewhere close to each post. It's not unheard of for a customer to install or purchase a new battery where the terminals are physically reversed on the battery from their old one. This can be true especially for those using recreational ATV or motorcycle batteries. When the owner installs it, the cables attach just fine and then when the master switch is turned on, there's a smoky surprise because the battery is hooked up backward.
The coax cables secured to this engine mount tube are routed through cushion clamps and wire ties.
More secured coax cables bundled with other wires along an engine mount tube.
Other beneficial items are boots to cover the terminals where the wires are attached. This will help keep the errant piece of safety wire from shorting the terminal to a firewall or engine mount. Lastly, it is handy to permanently install a lead for a trickle charger that allows you to plug it in without having to remove the cowl. We like to use the Battery Tender Jr. from www.batterytender.com (it includes a nice lead for the charger that you can install on the battery permanently). There are many others available that work just as well.
An overall view of a clean and tidy engine installation.
One last aspect to talk about when discussing wiring forward of the firewall are the physical considerations, meaning the actual mounting and securing of wires. Many times you'll see pictures of wires that are zip-tied to engine mounts with nothing for protection. This is bad for various reasons, first because the zip tie will eat into the engine mount, and second, because it can also eat into the wire. If you must use zip ties, then at least use some sort of physical tape or other insulation to protect both the wire and the engine mount. Chafe tape, silicone tape, rubber tape and other heat-resistant products usually suffice. Along that line, you need to make sure the zip ties you use are at a minimum the heat-resistant type, but the best are the turquoise-blue colored Tefzel ties that can withstand the highest temps.
When you run out of space on the firewall, you can sometimes use the engine mount tubes for probe mounting. Note the rubber boot on the master relay wire.
Other means of securing wires are the standard cushion type clamps (aka Adel clamps), spring clamps, lacing cord or other methods. Spark plug wires, as previously discussed, often will run through engine baffles, so it's likewise important that they are protected from rubbing on the baffling. Wires such as CHT and EGT thermocouple type wires should be secured as well.
Note the usage of cushion clamps and high-temperature Tefzel ties in this installation.
More high-temp Tefzel ties and protective coverings used for wires near the exhaust.
The FWF ground tab/block.
Overall, the wiring firewall forward is just as important as the instrument panel, except you also need to be wary of increased heat and vibration not present with wires inside the cockpit. For some airplanes there are even oil leaks to contend with. If you do a good job with the wire routing and securing, you're halfway there. The other half is to pay close attention to grounds. Firewall-forward grounds can cause a lot of problems with seemingly unrelated systems in the airplane and can eat up lots of hours troubleshooting. As with almost any endeavor, it's much easier to put in a few hours of extra time to make it right when first doing the job.
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