Understanding how your aerial machine works is just as important as knowing how to manipulate the controls to fly it. An airplane is really a whole collection of important systems flying in very close formation. The pilot must observe and manage these systems. Aircraft electrical systems come in all shapes and sizes, but we will confine this discussion to the typical two- and four-place light airplanes that we use for training.
It is important to remember that your airplane has two electrical systems. One of these powers the engine ignition only. The magnetos are the heart of the engine ignition system (See "The Magneto Check," January AOPA Flight Training). The other electrical system is for the airframe. Every electrical appliance in your airplane, other than the engine ignition, operates from the airframe system. The team of the battery and generator (or alternator) forms the heart of the airframe electrical system.
Some airplanes have a 12-volt battery, and others have a 24-volt battery. If you ever need to get a jump start, it's important to know which system your airplane uses. These batteries are lead/acid types just like those found in automobiles, but in small planes, they are not as robust. Heavy duty means heavy weight, and small planes must be kept as light as possible. A 12-volt airplane battery will not even come close to the cranking power for starting that a 12-volt car battery provides. That's one reason why preheating an airplane engine on cold days can be so important for starting - it's easy to run down your small airplane battery quickly during a hard start.
The generator is the part of your electrical team that keeps the battery charged and carries the total electrical load most of the time. Generators come in two flavors: direct current (DC) and alternating current (AC), generally referred to as alternators. Direct current, like the power you get from a battery, flows in one direction. Alternating current, similar to but at a much lower voltage than you find in your home, reverses direction at regular intervals. Regardless of which type of generator is installed on the airplane, however, the end result is direct current being supplied to the airplane electrical system because the AC from an alternator is converted to DC for the airplane. In case you are wondering, larger airplanes do use both AC and DC power.
Generators require more engine rpm to operate than alternators and usually don't kick in until about 1,500 rpm. Alternators provide electrical power at lower engine rpm, often as low as idle. Most automobiles and modern airplanes have alternators. For the purpose of this discussion, let's use the term generator when the information applies to both generators and alternators.
The battery and the generator must be connected to the airframe electrical items in the airplane through some sort of a switching system. Some airplanes have a single switch, often called the master switch, which connects the battery and generator to the airframe electrical system. Other airplanes have two switches. One of these two switches turns the generator on and off, and the other switch connects the battery to the system. This double switch gives you a little more flexibility in the control of the electrical system and more options in some electrical emergencies. The actual switch placards installed in your airplane may not use the words I have used. The pilot operating handbook (POH) and your flight instructor can clarify this for you.
Now we need to do a quick review of electrical basics as they relate to your cockpit management of the electrical system. Volts equal electrical pressure. The generator will produce a few more volts (pressure) than your battery produces. This results in a voltage pressure of about 14 or 28 volts, depending on which system your airplane is equipped to operate on. Think of voltage as being similar to water pressure. Amps is the term we apply to electrical flow. Think of amps as being like the volume of water flowing through a hose. Volts and amps (pressure and flow) are what determine the amount of electrical power we use, or are allowed to use.
We can measure the electrical system performance through the monitoring of volts and amps and there are several ways to do this. Some airplanes have voltmeters, and some don't. Some airplanes have low-voltage warning lights. Most airplanes have some way to measure power flow � amps - but the methods used to do this vary. Amps are measured on an ammeter, and these can be hooked up to give you different information. Some ammeters simply use a plus and minus scale. A plus needle means the generator is supplying power, and a minus needle means it is not.
Other ammeters are actually load meters. Load meters read the actual amps (current flow) being used by all the electrical equipment on your aircraft. On these airplanes, you will read high amps if you have a lot of electrical equipment turned on and low amps if most the equipment is turned off. The Piper Cherokee that made the off-field landing has a load-meter-type system. When the pilot turned off the master switch, his load meter indicated zero. It is very important to know and understand what your electrical system annunciator lights and meter indicators are telling you. All airplane indicating systems are not the same.
Circuit protection is designed to prevent us from overloading the airplane wiring. I said that electrical pressure (volts) and electrical flow (amps) are much like water. If we put too much water pressure in too small a hose, the hose will burst. If we want more water, we need a bigger hose. In the electrical system, voltage is the pressure, and wire is the hose. Although the analogy is close, there is a difference in the way water flow and electrical flow occur. Water pressure pushes the water through the hose. Electrical flow is pulled through the wire. When you turn on an electrical appliance, you apply a load that pulls the electricity from the generator. If the load you apply requires a greater electrical flow (amps) than the wire can support, the wire will burst (melt) just like the overloaded water hose. Fuses and circuit breakers are designed to prevent the wire from melting and the generator from being overloaded.
Fuses are actually designed to melt, or burn out, in order to prevent wiring damage. Circuit breakers are automatic switches that trip open (open means that the circuit is turned off) to remove an overload from the wiring. The pilot can manually trip some types of circuit breakers to remove power from an electrical item or group of items. Other types of circuit breakers will only trip open if an overload occurs. Part of every normal preflight is to assure that all appropriate circuit breakers are set and fuses are in place. Spare fuses are an important preflight item.
The generator will always have some sort of overload protection. This will be provided by a high-capacity fuse or circuit breaker. Some alternators may have two circuit breakers. One is for the overload protection and the other is a smaller one for the alternator field circuit. A trip of either of these alternator circuit breakers will cause the alternator to shut down. Other circuit breakers or fuses protect the wiring to the various electrical items installed in your plane. Remember, it's the wiring that is protected, not the electrical appliance. Some electrical appliances have built-in overload protection circuits, and some don't. It is certainly possible for an electrical item to short-circuit, causing smoke and fumes, yet not pull enough current to trip the circuit protection. This is why we have electrical fire and smoke procedures. The common procedure for replacing burned fuses or resetting tripped circuit breakers is to replace or reset one time only. If it trips again, leave it alone.
A while back I was administering a flight review in a Cessna 152. We had just completed a full-flap stall and during the recovery found that the flaps would not retract. The flap motor circuit breaker had not tripped. I moved the flap control switch again and noticed that it was hot. We then detected a burning odor and saw smoke in the cockpit. The flap motor has the circuit protection, but it was the flap switch that had shorted out. Our only choice was to turn off the alternator and battery switches and return to the airport with no electrical system. The smoke stopped and all ended well, but if the smoke had not stopped, using the fire extinguisher would have been our next action. Does your airplane have a cockpit-approved fire extinguisher installed?
It is important to understand your airplane systems and their related procedures. Review the procedures that are spelled out in your POH. Make sure that you understand the electrical malfunction procedures. Checklists are used to guide you through procedures for systems that you understand. They should not be used as a learning tool if and when a real emergency occurs.
Electricity deserves your respect. I have seen aluminum foil jammed into fuse holders and regular household wire used to make electrical modifications. Your electrical system must be treated with knowledge and respect by the pilot and maintained by a licensed technician. Do you remember the old story about Benjamin Franklin flying a kite under a thunderstorm with a metal house key attached to it? As it turns out, old Ben was not the one holding the kite string. He had his son fly the kite. That must be why God allowed us to invent copilots - someone has to be the lightning rod.
When he's not working as manager of health, safety, and environmental operations at an Oklahoma manufacturing plant, Earl Downs can be found at his flight school, Golden Age Aviation. He owns an Aeronca 7AC Champion.
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