Teaching the Electrical System

[Big Pistons Forever]

As a PE I find that a lack of understanding of light airplane electrical systems is the most common consistently weak area on Flight Instructor Class 4 flight tests.

This is how I teach the electrical system in 3 parts. Since the Cessna 172 is the most common flight training aircraft it is aimed at that airplane although other common trainers will be very similar.

Like almost everything in flight instruction it is a way to teach it, not the only way. If it is of use please borrow any or all ideas, if you have constructive suggestions please add to this thread

Basic Concepts

- think of a water hose. Voltage is like water pressure, amps is like water volume. Too much pressure and things break. Too little pressure and the thin stream won't reach the target. Too little volume and things don't work, too much volume and you drown what you are pointing the hose at.

- When you turn on the master all power comes from the battery. When the alternator is turned on it will provide power to the airplane and as the battery is used up it is refilled from the alternator. Battery voltage is 12 or 24 volts but the electrical system will be 14 or 28 respectively , this is to ensure power goes from the alternator ( higher voltages) to the battery (lower voltage) not backwards

- To protect the electrical system from over voltage there will be a device (often called an overvoltage relay) which will disconnect the alternator from the electrical system if there is too high a voltage sensed.

- To protect components from too high amperage there are circuit breakers for each individual aircraft system (eg a radio or a light). These will pop out showing a white collar in the event of a over current event or short circuit

- High current draw systems like the airplane master switch and the starter switch have the switch connected to an electrical switch called a relay which energizes the system.. This to avoid having to run heavy electrical cables to the cockpit.

Most modern airplanes will have annunciator lights to alert the pilot to electrical system malfunctions. These could include an over voltage light, an alternator fail light or a low voltage light

- Similar systems like radios or lights are grouped into bus bars. However while all the legacy 172's have just a primary bus and an avionics bus the newer R and S models have 2 primary busses and 2 avionics buses, and particularly in the case of he avionics buses split the avionics between the buses to avoid loosing totally loosing com and nav functionality in the event of a failure of one bus.

- There are 3 possible electrical system gauges. Your airplane may have one, two, or three
1) Ammeter. This gauges has a zero in the middle and an arc with a minus sign to the left and a plus sign to the right. A minus indication means power is flowing out of the battery, a plus indication means that power is flowing into the battery from the alternator. Normal operation is with the needle at zero or slightly positive which means the alternator is keeping the battery full.
2) Load meter this meter displays the output of the alternator, Eg 20 means that it is outputting 20 amps of power.
3) Volt meter. This indicates system voltage. Normal voltage should be the alternator output eg 14 or 28.

- Electrical system switches. There are 2, the master switch which connects the battery to all electrical devices in the airplane and the alternator switch which connects the alternator to the airplane electrical system.

So lets now use a practical example You want to turn on all the radios. How did power get to radio number 1

1) We turned on the master switch through the master relay
2) We turned on the alternator switch
3) We engaged the starter which through the starter relay energized the starter motor
4) We turned on the avionics master switch which energized the avionics bus, the radio was then powered and turned on

You now know pretty much everything a pilot needs to know about the electrical system components and how they are connected

Now you need to know as a pilot what is normal operation, abnormal operation and what is an electrical emergency.

By phase of flight

Before start

- What voltage should you see, what should the ammeter be indicating?

After start

- What voltage should you see, what should the ammeter be indicating?

Runup

- How do I test the electrical system ?

Cruise flight

- What indication should I expect to see on the electrical gauges?

Questions for students

- Before start the voltmeter indicates 10 volts

- Before start one of the circuit breakers is found popped out

- After start the ammeter pegs at full scale positive, what has happened ?

- After start you see the ammeter indicating a steady negative 20 amps

- After start the low voltage light is on

- During the runup the ammeter stays in the negative side

- During cruise flight the over voltage light comes on

- During cruise flight you see 15 amps negative on the ammeter

- During cruise flight a circuit breaker pops out

- During cruise flight all the radios go dark

- at anytime you smell an electrical burning smell and/or see smoke coming from behind the instrument panel

[digits_]

This part confused me for quite a bit when doing more advanced training:

- Similar systems like radios or lights are grouped into bus bars.

Very often, it's the exact opposite. Similar systems get put on different buses so that if one bus fails you don't lose all your lights or all your radios etc.

They are often also grouped based on their technical requirements, what they *need* (AC, DC) and not on what they *do*.

While not technically wrong in a c172, i think it instills the wrong concept of what a bus bar does, or why they are being used.

[LineUpAndWeight]

Power isn't simultaneously drawn from the battery and replaced. The battery has two terminals so at any given moment power is either going in or out, but cannot be both. In or out depends only on the voltage on the terminals which determines which way the chemical reactions in the battery are going. More than 14-point-something and power is going in, less that 12-point-something and power is coming out, and at voltages in between no significant amount of power is going either in or out of the battery.

In flight, with the electrical system working the bus voltage is 14-point-something, with about 200mV of ripple. So at all times, power is entering the battery. Either recharging it, if it's depleted, or being wasted as a small amount of heat, if it's fully charged.

Open circuit battery voltages (and healthy charging voltages) depend on temperature; they should be higher when it's cold and lower in summer.

The only time you use battery power in a working system is to start the engine. The rest of the time the power to drive the electrical system is coming 100% the regulator from the alternator That changes of course if the alternator or regulator are defective. IF the alternator output drops below 12-point-something voltage then current is automatically drawn from the battery instead. Until it's depleted.

Fuses and circuit breakers are there to protect the external wiring to devices that use electrical power. They don't protect internal components. Avionics systems have internal fuses and protection if they need it. It's possible to overheat a wire inside a radio and set something alight at current draws well below what will trip the CB. The CB will stop a gross short circuit heating up an external wire enough to set light to anything combustible outside the radio or other enclosure. It won't do much to protect what's inside the radio, which will already be faulty if the CB trips due to an internal fault.


Cessna master switches have three modes; off, battery power only, battery + alternator power. Piper master switches have four modes: off, battery power only, alternator power only, battery + alternator power. This allows you to isolate the battery but leave the alternator providing power in the event of an over-voltage on the bus. Avionics and other equipment is better suited to handling a (mild) overvoltage event than a battery which will overheat and boil if subjected to anything more than about 15 volts for very long. The same situation in an aircraft with a Cessna-style master switch may only be manageable by shutting off the alternator leaving only the limited electrical reserve in the battery available.

Of the three electrical gauges mentioned in the original post, the bus voltmeter is arguably the most useful. 14.something volts - all healthy. 13-point-something volts - likely an alternator issue, battery not charging, but alternator supplying the load. 12-point-something volts - very much an alternator issue. 12-point-something volts and dropping - alternator offline and load supplied only by the battery.

I hope that is interesting.

[Bede]

- All power comes from the battery. As the battery is used up it is refilled from the alternator. Battery voltage is 12 or 24 volts but the electrical system will be 14 or 28 respectively , this is to ensure power goes from the alternator ( higher voltages) to the battery (lower voltage) not backwards

This is an excellent thread but this paragraph is incorrect. Power only comes from the battery when the alternators are not producing power (engines off or field not energized). Once the alternators are producing power, they are at a higher potential than the battery so current will flow from the alternator directly to the load, including charging the battery, not from the battery to the load with the battery being recharged.

Interesting how you mention instructors. These things should be understood by a CPL and perhaps even a PPL candidate.

[digits_]

- To protect components from too high amperage there are circuit breakers for each individual aircraft system (eg a radio or a light). These will pop out showing a white collar in the event of a over current event or short circuit

This is also a source of common misunderstanding. 'components' is a bit vague. The purpose of the circuit breakers is to prevent damage to the wiring. It doesn't protect the individual system. On the contrary, it's usually a way to protect the wiring from a malfunctioning individual system that starts drawing too much power.

Also note that not every single system has a circuit breaker. Multiple systems could theoretically be connected to the same wires. There will be (at least) one circuit breaker per wire loop. But it's possible to have a light and a pump for example powered by the same circuit breaker. Or a motor and a light.

[Big Pistons Forever]

Bede

Thanks for your comment. In a bid to keep the explanation simple I over simplified the description. I have amended that paragraph to more correctly describe the relationship between the battery and alternator.

Digits

The opening paragraph starts with referencing knowledge deficits in flight tests. To make it clearer I have added a sentence specifying the Cessna 172 and similar training airplanes. With respect to the description of electrical busses I expect the candidate to know the system in the airplane not the one on an Airbus. You point about splitting services is valid and I have added that to the paragraph on buses as while all the legacy 172's have just a primary bus and an avionics bus the newer R and S models have 2 primary busses and 2 avionics buses, and particularly in the case of he avionics buses split the avionics between the buses to avoid loosing totally loosing com and nav functionality in the event of a failure of one bus.

With respect to your comment on circuit breakers. Circuit breakers cut power when the current reaches and unsafe level. From a pilot perspective I don't see why you need to know anything more than that.

Finally I once did a initial Class 4 Instructor for flight test. On the walk around I pointed to the alternator drive belt and asked what it did. The answer "turn the propeller". While this is an extreme example I stand by my contention that many instructors are not doing a good job of teaching students on a very important aircraft system and so I am hoping my and others contributions to this tread will help both students and instructors.

[digits_]

With respect to your comment on circuit breakers. Circuit breakers cut power when the current reaches and unsafe level. From a pilot perspective I don't see why you need to know anything more than that.

That's a bit of a weird argument to use in a topic that's hoping to share more theoretical information. I suppose you could dumb everything down to 'flick switch, system work. system not work? take checklist. flick other. switch'.

If your aim is to improve understanding, it takes just as much effort to state that the circuit breakers protect the wiring instead of the more vague 'components'.

Knowing the distinction can be important to explain why one shouldn't reset a circuit breaker in flight. A student might think "this circuit breaker protects my radio. It popped. But I need my radio. If it protects the radio, and it's already broken, what's the harm in trying to reset it". A student who knows the danger is to the wiring, might think "oh no, my radio circuit breaker popped. Better not reset it as the broken radio could melt the wiring and start a fire"

[goldeneagle]

Digits is correct. Breakers are to protect wiring, because putting to much current thru a wire will start a fire.