Aircraft Electrical System [Components & Diagrams]
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If you are into having all kinds of “bells and whistles” in your aircraft like lights, navigation tools, radios, and an electric starter, then you probably have an electrical system installed! Today, you will rarely find an aircraft without a full electrical system setup, and some of them are rather complicated.
Small airplanes are usually powered by a 14-volt or 28-volt DC (Direct Current) electrical system. This system also provides and maintains a charge for 12-volt or 24-volt flooded or wet cell batteries (sometimes NiCd or Li-Ion).
Alternators (or generators in older systems) provide the electrical current for your bus bars, lights, electronic controls, and also charge your battery. Your battery helps with starting your engine and acts as a backup source in the event of alternator or generator failure.
Following are the components of the aircraft’s electrical system (and I will discuss some of them in detail below):
- DC alternator or DC generator.
- Battery.
- Switches.
- Circuit breakers or fuses.
- Relays.
- Voltage regulator.
- Ammeter or loadmeter.
- Bus bars.
- Wires.
Some of the uses of DC electrical system include, but are not limited to:
- Starter motor.
- Fuel pump.
- Navigation, communication, and monitoring tools (radio, GPS, warning lights, etc).
- Lights (position, anti-collision, landing, taxi, cabin, gauges, turn indicators).
- Wing flaps.
- Landing gear.
- Pilot heat.
Here is a nice video on the aircraft electrical system overview:
I will divide this article into the following sections:
- Power generation.
- Power accumulation.
- Power distribution.
Power generation
The aircraft electrical system is designed to operate at 14-28 volts. Many planes now are taking advantage of 28-volt electrical systems.
The transformation of mechanical energy into electrical is done by two similar systems called a DC alternator or a DC generator. This transformation process is called electromagnetic induction.
Generators are more common for older aircraft. Both systems produce Direct Current (DC) and are designed to power aircraft’s electrical loads. Charging the aircraft’s battery is also done by either one of them.
DC alternator and DC generator are usually engine-driven systems that have the ability to generate electricity. They start by producing a current, which is constantly changing its value and polarity (also known as an Alternating Current or AC) and transforming it into a current that flows in a single direction (also known as a Direct Current or DC):
DC Generator
Components of a typical 24-volt aircraft DC generator include:
The basic generator system produces AC (or Alternating Current) that needs to be converted into DC (or Direct Current) to be compatible with your airplane’s DC loads. It does so by using a modified slip ring arrangement (also known as commutator):
This system of rings allows the transformation of AC current into 14-volt or 28-volt DC. Even though DC generators in most aircraft are replaced by DC alternators, some older and lighter aircraft are still powered by generators.
DC Alternator
DC alternators are generally lighter than DC generators and are commonly found in modern, piston-engine aircraft. Another main advantage of an alternator over a generator is that it is able to produce a sufficient amount of power even at lower engine RPM.
DC alternator consists of an armature winding to produce AC current and a rectifier to convert it into Direct Current (or DC):
The alternator actually produces three distinct AC sine waves:
These waves are then converted to DC (or Direct Current) by a rectifier system consisting of diodes. As you see, even though DC alternators and DC generators basically serve the same purpose, their working principle is entirely different!
DC controls and switches
The alternator or generator circuit is turned ON by the Master Switch:
There is also a control unit (also known as a voltage regulator) that regulates the charging voltage that the alternator or generator produces. It looks like this:
Here is a simple diagram of the Generator Control Circuit:
And another one of the Alternator Control Circuit:
In most aircraft, you will find an ammeter to monitor output. It measures the flow of energy from the alternator to the bus. Here is a simple diagram:
Power accumulation
The second most important component of the aircraft’s electrical system is a battery. The battery in aircraft is used to provide a startup energy boost for the engine and as a secondary power source in the event of generator or alternator failure.
Since the battery capacity is limited, you should always be checking your gauges to make sure it is being charged by your alternator or generator. Aircraft batteries come in three main chemical variations:
- Lead Acid.
- NiCd (Nickel-Cadmium).
- Li-Ion (Lithium-Ion).
The battery capacity is measured in Amp Hours or one-hour capacity. If you have two 12-volt (24-volts combined) 200AH (400AH in total) batteries (that your aircraft system expects), this means that you can use:
40 hours of battery life @ 10 amps
or
10 hours of battery life @ 40 amps
That is if you discharge it fully, which is NOT recommended! Regular Lead Acid batteries should be discharged no more than 50% and Deep Cycle – about 80%.
Charging of the battery is done by an alternator or generator system. Batteries should NOT be discharged MORE than the manufacturer’s recommendation and NOT undercharged as well.
Undercharging and overcharging batteries will shorten their lifespan and efficiency (there are many tools available to avoid this problem). This is why you need to make sure that your batteries have a full charge in them before taking off.
Maintenance of the battery generally comes down to battery type. Lithium-Ion Deep Cycle batteries are virtually maintenance-free.
Batteries get connected to an electrical distribution bus through the solenoid. It’s a device similar to a relay, only it controls high current circuits (while relay controls low current circuits).
When Master Switch (for the battery) is flipped, the solenoid activates the battery circuit. Here is a diagram:
Power distribution
The main purpose of the airplane electrical system is to generate power, distribute power, and store it in case of an emergency. Here is an overview of the power distribution system:
Bus Bars
The components of the aircraft’s electrical system are divided into multiple buses. The most common types of buses are:
- Primary (or Main) Bus. Lights.
- Avionics Bus. Instrument panel.
Electrical Bus (or Bus Bar) is a common wire which connects multiple electrical components like lights, radio, etc., into one circuit, and uses its own circuit breaker to control energy flow. It works in a way similar to a power strip that we have in our house.
After the engine has started and the alternator (or generator) kicked in, you need to turn Avionics Master Switch ON:
This switch powers electronic devices located on your dashboard and protects your sensitive equipment from being damaged by excessive voltage flow during startup.
Here is a complete diagram of how bus bars are connected in your aircraft:
Circuit breakers or fuses
Before power gets to your actual device from a bus bar, it needs to pass through a circuit breaker for protection. Circuit breakers are more commonly used than fuses due to the complications which come with changing them.
Some older aircraft models still use fuses. Every circuit breaker is marked with a number related to how many amps it will let your equipment use before breaking the connection:
If the electrical component is starting to pull more current than the AMP rating of the circuit breaker, the breaker will pop and open a circuit. The circuit will pop and disconnect your equipment also when an electrical fault is detected.
The most common faults that trigger the disconnection of the electrical circuit, are:
- Circuit overload. When too much power is being pulled from the circuit.
- Short circuit. When wires create an unwanted connection.
This is why at the beginning of every flight:
You should always check your circuit breakers
The whole purpose behind having a circuit breaker or a fuse is NOT to allow excessive current to get to your electronic devices. When one of your circuit breakers pops, the main rule is:
NOT to reset it twice!
Why? Because THERE IS A REASON behind the popped circuit and it should be taken care of before further damage is done.
The popped breaker could mean anything from:
Wiring problem, short circuit, equipment damage
All the way to:
Someone before you used circuit breaker as a switch
In the last case, everything should be fine and it is a rather common situation when you turn things OFF by popping a circuit breaker. You reset it and it stays this way (nothing is being damaged).
In the first case, if the switch pops RIGHT BACK, do NOT attempt to do anything until professional checks your equipment! That includes force-pushing your circuit breaker!
** Warning! Even though it seems logical, holding your circuit breaker in the connected position will NOT solve your problem! Pushing the wrong voltage through an electrical system can overheat your wires and cause a fire!
Ammeter / Loadmeter
An ammeter is an electronic device that lets you monitor the performance of the aircraft’s electrical system:
If the alternator is working properly and charging your battery (while providing sufficient power to the aircraft), the arrow will be in the plus (+) section. If the battery is NOT being charged or is being used INSTEAD of the alternator (or generator), the arrow will be in the minus (-) section.
The loadmeter shows the load that is being placed on the alternator, with zero being NO load (meaning failed or disconnected alternator) and numbers being actual indicators in amps:
** Reference. Diagrams in this article are taken from this resource (check them out!)
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