I remember being told that the airplane was equipped with a 28 volt, direct current, alternator driven electrical system. “Is that good?” I thought, as a student pilot without a background in mechanical stuff. It could have been a 1.21 gigawatts electrical system and I wouldn’t have known any difference. I’m sure I learned about and passed tests on basic electricity as a kid, but whatever I learned didn’t stick with me to a practical level when it came time to use it. As with any topic in aviation, you can learn it on the surface, and then learn it to a deeper level, and then keep learning it at seemingly endless levels of understanding. When it came time to know the systems on my airplane, I knew I had to understand deeper than “low volts? Better get some more.”
Watts An Amp?
Volts and watts and amps. What are they, and what is the difference? “They’ll all shock you,” I thought. The best way to visualize these fundamentals of electricity is to think in terms of water pipes. Volts (voltage) are the pressure of the pump moving the water. Amps (current) are the flow rate of the water, and volts times amps equals the total power of the system, or watts. Let’s take the example of a landing light in this Cessna 172. The “electrical pressure” being generated in the system is 28 volts. A common landing light uses 50 watts of power. So how many amps does that draw? Volts x Amps = Watts. Watts/Amps = Volts. Watts/Volts = Amps. Those are all the same equation, just solving for the different values. Plug in our numbers and you have 50 Watts/28 Volts=Amps. 50/28=1.8 Amps. You could math this all day to realize that a lower voltage system would have more amps flowing, as would a higher wattage light. I’m pretty amped to understand this relationship.
It also helps to understand what the alternator is doing. As the engine turns, it drives a belt attached to the alternator. An alternator works by using the rotation of the belt to rotate a magnetic core inside wiring that produces electrical current. It might be obvious to those who have a familiarity with automotive systems, but it helped me to understand that the battery helped to start the engine (via the starter), but then the engine ran the alternator that both powers the airplane’s electrical system and keeps the battery charged. The 28 volt system, then, means that the alternator is producing 28 volts of voltage. That’s enough to run the electrical system and keep the 24 volt battery charged. Older aircraft systems used a 14 volt alternator with a 12 volt battery. A higher voltage system reduces the current, making the system more efficient, and without needing increased wire size. Shocking, right?
The Hero Of Ignition
Then there are magnetos. I still think of the X-Men character every time. These little guys are what provide the spark to the spark plugs, which ignite the fuel and make the engine run. Before I got the concept, they seemed like magical little superheroes because they didn’t need the battery to function. The fog lifted on that one when I realized that they are basically miniature alternators. Every time the engine turns, it drives the magnetos to make a spark. So the battery causes the starter to start the engine, and then the engine’s rotation causes both the alternator to make electricity for all the electrical components and the magnetos to produce a little spark for the engine to keep burning fuel. Why didn’t I think of that?
Get On The Bus
The electrical components in the airplane (such as lights, electrically powered instruments, radios, etc) are on electrical buses. You’ve probably heard of the primary bus and the avionics bus. A bus is just a common wire that runs a series of components. I think of it like a power strip, where several items are plugged into the same strip, or bus. Some airplanes allow you to isolate a bus either using a bus circuit breaker or a switch, like an avionics master switch with a split rocker (two sides) for two different buses.
A Short On Circuits
Most checklists tell you to check that circuit breakers are in, but what would they be doing out? Circuit breakers are basically automatic switches. If the current that it’s component draws is greater than the circuit breaker is rated for, it pops out, opening the circuit, and effectively turning that component off. Take a circuit breaker with a 5 on it, for example. Let’s say it’s the circuit breaker for the autopilot. If something that the autopilot does pulls more than 5 amps of current, the breaker will pop out. So do you push it back in? A particular airplanes POH will discuss its particular troubleshooting steps for the electrical system, but you’ll find that it’s not recommended to reset (push back in) a circuit breaker more than once. One time might have been an anomaly of higher current, but continuous popping of the circuit breaker usually means there is an issue with its electrical component and continuing to allow excessive current can start a fire.
The ammeter shows the health or performance of the charging system. Usually, a functioning instrument that reads zero might lead you to think nothing is happening, but in this case, zero is usually good. The indication shown is the rate of charge or discharge of the battery. A zero indication means the alternator is doing its job to provide power to the electrical system. If the ammeter shows a discharge (the needle is below zero), then the battery is actively supplying power to the system. This could mean that the alternator isn’t functioning properly. A positive indication on the ammeter (the needle above zero) indicates that the battery is receiving a charge from the alternator. You might see this just after starting the engine, for example, when the battery has been solely supplying the electrical power until the engine is on and turning the alternator. In this case, the battery needs a little top off from the alternator and the ammeter shows a positive indication.
You might be thinking, “I just wanted to fly a plane, not become an electrician.” I’m not a mechanic or an electrician, so these things don’t come as naturally to me as they do to some, but gaining a deeper understanding of the airplane’s electrical systems – or any of the system for that matter – make you a better pilot and better able to troubleshoot any problems that might arise in flight. I recently found a great podcast called Airplane Owner Maintenance by Dean Showalter. He gives some great insight from the perspective of an airplane mechanic for pilots wishing to know more about their airplane.
I’ll leave you with this week’s flight simulator video, where we play around with some of the electrical components while flying around scenic Hawaii to answer the question, “what happens if I turn off my master switch in flight?” Watch it here:
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