by Richard Morey
In conversations with designated pilot examiners and FAA safety representatives, it is clear that most pilot applicants, be they for private, commercial, instrument or even flight instructor certificates, do not understand aircraft systems. Pilots do not need to have an airframe and powerplant mechanic’s background to pass their checkrides or flight reviews. Pilots do need to have a basic understanding of systems in order to be safe and competent pilots. As in most things, the more you know about it, the better off you will be. This article will cover basic aircraft systems on a basic level. The systems discussed are those found in a Cessna 172, and should be applicable to other similar aircraft. After reading this article you should have a basic understanding of how the electrical and ignition systems function. In a future article, I will discuss the pitot static, vacuum and brake systems.
Definitions: In order to understand how systems work, it is important to understand what different terms mean. To that end, the following information will prove useful.
Electromagnetic Induction: When electricity passes through a wire, it creates electromagnetic energy as a byproduct. If that electromagnetic energy goes through a conductor such as steel, a magnet is created.
Solenoids are electric switches that use electromagnets to turn on and off. Solenoids are used as remote switches and are placed on the engine side of the firewall. A small amount of electricity closes the solenoids’ heavy contactors, allowing high amperage current to stay in the engine compartment and out of the cabin.
Master Switch. A physical switch which routes power to the battery solenoid and the alternator control unit.
Ignition Switch. Routs spark either to a ground or spark plugs, and routs power to the starter solenoid.
Ground Power Receptacle, where maintenance plugs in a battery cart or ground power unit for a boost start. It bypasses the master switch in Cessnas and WILL NOT charge the battery.
Alternator: A device that takes mechanical energy from the engine, and a little electricity from the battery, through an alternator control unit, to create more electricity by spinning a coil of wire through the magnetic field.
Starter: An electric motor with clutch assembly which turns the engine when the ignition switch is in the “start” position.
Alternator Control Unit: A device which meters electricity to the alternator field coils, regulating alternator output.
Magnetos: Devices which take mechanical energy and turns it into a high voltage, low amperage spark. A magneto always creates spark when turned in the proper rotation.
Spark Plug: A device screwed into an engine’s cylinder head which ignites the fuel air mixture.
Voltage: How much push or force electricity has to move it through a conductor such as a wire.
Amperage: The volume of electricity moved.
Electrical System: Made up of the battery, alternator, solenoids, switches, and an alternator control unit. When the battery side of the master switch is turned to the “on” position, a low amperage electricidal flow is directed from the battery to the battery solenoid. If there is sufficient voltage in the battery, the solenoid closes. The “click” a pilot hears on preflight when the master switch is turned on is the battery solenoid closing. If no click is heard and aircraft systems, such as the fuel gauges and turn coordinator do not respond, then either something is preventing the electricity from reaching the solenoid, or the battery is discharged to the point that there is not enough voltage to close the solenoid. A broken wire, a malfunctioning master switch or a defective solenoid would cause the solenoid not to close. In this case it is far more likely that the battery has low voltage, commonly referred to as a “dead battery.” If the battery is dead, then the recommended course of action is for the battery to be removed and charged. See my article on the charging system in Midwest Flyer Magazine (https://midwestflyer.com/?p=16065) for a detailed discussion of the charging system. If you choose to ether prop start the aircraft or use a battery boost to start, expect the alternator not to work. More on this later.
The right side of the master switch is labeled alternator. Turning this side of the switch to on closes a circuit which sends a small flow (amperage) of electricity to the Alternator Control Unit (ACU). The ACU then sends electricity to the alternator field coils. The field coils when energized create electromagnets within the alternator. The center of the alternator is turned via a belt and pully arrangement, with pullies on the propellor hub and alternator. The alternator only works when the engine is running, as the engine provides the mechanical energy to turn the alternator. Coils of wire are turned through the electromagnetic field, creating electricity. The alternator control unit monitors the alternator output and regulates that output to match the aircraft’s requirements. The ACU will shut down if a voltage discrepancy (generally an overvoltage) is detected. It is important to note this and monitor the ammeter to make sure the alternator is charging. If a discharge is noted on the ammeter once the engine starts, the cause may be too low of engine rotation per minute (RPM) for the electrical load, a blown circuit breaker, or an alternator control unit that sensed an anomaly and took itself offline. If the engine is at idle, add power to increase RPM and see if the ammeter now shows a charge. If that does not work, check the alternator circuit breaker. If the circuit breaker is blown, I would recommend you not reset it, as doing so may result in an electrical fire. Instead, discontinue the flight as soon as practical and have maintenance check out the aircraft. If the alternator circuit breaker is not blown, try turning off the alternator side of the master switch, waiting a minute, then turning the switch back on. This will reset the alternator control unit if it took itself offline.
The take-away on alternators:
1.An alternator takes mechanical energy from the engine, and a small amount of electricity from the battery to create a larger amount of electricity.
2.The alternator control unit gets electricity from the battery through the alternator side of the master switch.
3.The alternator control unit will turn itself off if it senses abnormal electrical output. The alternator control unit can be reset by turning off the alternator side of the master switch – waiting – then turning the switch back on.
4.Only reset an alternator circuit breaker if it is a matter of life or death.
5.A dead battery results in no alternator output. Propping or boost starting an aircraft with a dead battery is not recommended for this reason.
The Ignition System
The ignition system is made up of the ignition switch, the p leads, the magnetos, the spark plug wires also known as the ignition harness, and the spark plugs.
The ignition switch on a modern, single-engine Cessna aircraft, is key-operated and has five positions, Off, Left, Right, Both and Start. In the Off position, the ignition switch grounds the spark from both Left and Right magnetos. In the Right position, the Left magneto’s spark is grounded, and the Left magneto’s spark goes to the proper spark plug. In the Left position, the Right magneto’s spark is grounded, and the Left magneto’s spark goes to the proper spark plug. With the ignition switch in the Both position, the sparks from both magnetos go to their respective spark plugs, that is top and bottom spark plugs of whichever cylinder is in sequence to fire. The Start position engages the starter solenoid, allowing the starter to turn the engine over. Spark from both Left and Right magnetos go to the spark plugs. P leads are used to ground the magnetos through the ignition switch, and spark plug wires are used to take the spark from the magneto to the spark plug. The purpose of the spark plug is to ignite the fuel/air mixture in the cylinder of the aircraft engine.
It is imperative that pilots understand that a magneto always generates a spark when turned in the proper direction of rotation! Always! Where that spark goes is dependent on the integrity of the “p leads” and the ignition switch. For instance, if the p leads are not connected or broken, then the spark generated by the magnetos will go to the spark plugs. When moving the propeller, ALWAYS push on the blunt edge and move the propeller backwards. If you persist on moving the propeller forwards, eventually the engine will start! This happened to me many years ago and I do not recommend it!
A “runup” is often referred to as a mag check. This is only partially correct. A runup does check the function of the magnetos, and the integrity of the ignition switch, the p leads, the spark plug wires, the spark plugs and the engine’s cylinders. By grounding out one magneto, the function of the other magneto/ignition system can be assessed. Excessive RPM drop and/or a rough running engine, means something is amiss. Common causes of an engine running rough on one magneto include, but are not limited to, having a fouled spark plug, a nonfunctional spark plug, and a bad ignition wire. To clear a fouled spark plug, turn the ignition switch to BOTH and keep the RPM at the recommended runup level. Lean the mixture until a slight rise in RPM is noted, then run the engine at this setting for one minute. After a minute, leave the mixture lean and repeat the ignition system check. If oil fouling was the cause, then the engine should run smoothly with an acceptable drop in RPM. If this action does not clear up the problem, do not fly the aircraft and have maintenance investigate.
If during the ignition system check no RPM drop is noted when a single magneto is selected, there is an issue with the ignition switch or p lead. The magneto is not being grounded. Having a “hot” magneto is not a safe condition. Sometimes wiggling the key in the ignition switch will cure this. If the problem persists, throttle to idle and turn the magnetos to the Off position. If the engine continues to run, then indeed you have a hot magneto. Do not fly the aircraft in this condition and have maintenance look at the aircraft.
Take away on the ignition system:
1.A magneto is a spark generator. It creates a spark every time it is turned over in proper rotation.
2.The spark goes either to the spark plug or through the ignition switch to an electrical ground.
3.Mag checks are in reality checking the entire ignition system.
4.If you have to turn the propeller, ALWAYS turn the propeller backwards, unless you are intentionally prop-starting the aircraft.
I do not expect you as a reader and pilot to memorize this material. The take aways are sufficient for practical use. Understanding of systems will help pilots be safer pilots, add peace of mind to flights, and help in troubleshooting issues that may arise.
Be safe!
EDITOR’S NOTE: Richard Morey was born into an aviation family. He is the third generation to operate the family FBO and flight school, Morey Airplane Company at Middleton Municipal Airport – Morey Field (C29). Among Richard’s diverse roles include charter pilot, flight instructor, and airport manager. He holds an ATP, CFII, MEII, and is an Airframe and Powerplant Mechanic (A&P) with Inspection Authorization (IA). Richard has been an active flight instructor since 1991 with over 15,000 hours instructing, and more than 20,000 hours total time. Of his many roles, flight instruction is by far his favorite! Comments are welcomed via email at Rich@moreyairport.com or by telephone at 608-836-1711. (www.MoreyAirport.com)
DISCLAIMER: The information contained in this column is the expressed opinion of the author only. Readers are advised to seek the advice of their personal flight instructor, aircraft technician, and others, and refer to the Federal Aviation Regulations, FAA Aeronautical Information Manual, and instructional materials concerning any procedures discussed herein.
© Copyright 2024. All rights reserved!