Aircraft Electrical Diagram

Aircraft Electrical Systems, I

Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more com and nav radio's, transponder, intercom and other electronic systems.

The electrical system consist of a battery and an alternator or generator on older aircraft to recharge the battery, fuses and switches and lights for indication purposes.

All of this is connected through several meters (kilometers in large aircraft) of wire and connectors, attached to the airframe with insulation material as cushion clamps, tywraps and what not.

Even for the private pilot it pays to have some basic knowledge of the electrical systems of his or her aircraft, which could be a life saver in case of an emergency.

Our story starts with the basics like energy storage and generation, which includes batteries and magnetism, and continues to describe normal operation with some detail on electrical system failures which may happen in flight.

Energy storage

For an aircraft engine to be able to start (not by handpropping) there is a need to store energy and release that in a controlled method. Usually done in a chemical form in the battery, just like reservoir, and being topped up by an alternator or generator driven by the engine. In light aircraft it is usually a 12 volt type, as in a car. More sophisticated aircraft use a 24 volt system because they need more electrical power (for starting turboprops or turbines) without installing a larger and heavier 12 volt battery and thicker wires.

With an 24 volt system you can carry twice the amount of amps in the same wire without any problems.

Battery types

Batteries are the lead acid type or NiCAD (Nickel Cadmium) battery. As in a car, the lead acid battery also generates hydrogen (very explosive) during charging and it needs to be vented overboard. The acid in the battery is very corrosive. Hence the use of NiCADs in larger aircraft which do not have these disadvantages, but these need current limiting and a temperature sensor as they get warm during recharge and a thermal runaway must be prevented.
The capacity of a battery is expressed in A/h, ampere per hour. A battery of 35 A/h is capable of delivering 35 A for one hour or 1 A for 35 hours or any other combination, but it needs to be fully charged to be able to do that, hence the need for recharging during flight.

Recharging

Most modern aircraft use an alternator because these are able to deliver more power and are able to charge the battery with the engine idling, were the generator is unable to (less power per RPM) and is heavier. The main design difference between these two is that an alternator has a rotating electromagnet (rotor) which is energized by the field current (ALT switch) and a stationary coil (stator) delivering the power. The generator has a stationary permanent magnet with field coils and rotating coils within the fixed magnetic field generating the power through a commutator which retifies the alternating voltage/current (AC) to direct voltage/current (DC). The alternator uses diodes to convert the AC to DC.

Both types are voltage regulated and deliver 13,8 volts to the aircraft electrical system and they need an overvoltage protection and indicator. Current protection is by design in an alternator but not in the generator, this one needs a current limiter.

After the engine has started the alternator (or generator) will provide the electrical power for the aircraft and charges the battery. The battery is thus only needed to start the engine, supplementing the alternator in a high load situation and as an emergency source of power in case of engine or alternator failure.

Ground power

Some aircraft have a ground power receptacle, useful when testing the electrical system without running the engine and draining down the battery. Can sometimes also be used for starting the engine when battery capacity is not sufficient. You will find a ground power switch near the master switch with a 'ground' and 'flight' mode. Make sure that the ground power unit is of the same voltage and polarity as the aircraft.

Electrical system

The remainder of the electrical system consists of a master switch, one or more bus bars, ammeter and or voltmeter and fuses or circuit breakers and switches to control the radio's, lights and other electrical devices.

Master Switch

Switches the electrical system on through a heavy duty relay. Usually a split red switch with one side for the alternator and the other for the battery. When switched to on electrical power is supplied via the one or more bus bars to the devices. Each bus bar should have its own circuit breaker so that a short circuit with, for example, a flap motor doesn't take out the radio's or the EFIS.

Bus bar and fuses

Aircraft Circuit Breaker Each device is attached to a bus bar through a circuit breaker or fuse and a switch (which can be the same device). Again making sure that one faulty radio doesn't short circuit the others. As a fault in COM1 should not have any consequences for either COM2 or any other device on the radio bus bar.

The bus bar is normally a thick copper strip connected to group of switches / circuit breakers with the same basic function. Think of an engine instruments, radio/communication, lights and auxiliary bus bar. In a multi engine or an engine with dual alternators, bus bars can be interconnected should an alternator fail for some reason. So that electrical power is always available to some essential devices.

Monitoring Volt and Amps

To measure how the system is operating an ammeter is usually installed and sometimes even a voltmeter. The ammeter indicates the amount of current (flow of electrons) from the alternator to the attached load (left zero ammeter). There are two types in use: the center zero ammeter measures the current to or from the battery. The left zero ammeter indicates only when the alternator is delivering a load. The center zero ammeter has a '+' side, indicating a charging battery and a '-' side indicating that the battery is supplying electrical power.

The voltmeter indicates the voltage of the battery which, when the alternator is not operating, should be 12 volt. This voltage, however, says nothing about the capacity left in the battery, for that you will need to measure the specific gravity of the acid inside the battery. With the aircraft engine / alternator running the voltage should be around 13,8 volt (or 24/28 volts for aircraft with those systems).
If one of the cells (there are six in a 12 volt) battery is defect or short circuited then this will be indicated in the unloaded battery voltage, it will read lower than normal. This will not show if the alternator/engine is running (13,8 regulated voltage) but the charging amps to the battery will be somewhat higher than normal.

Aircraft wiring and ground

All devices in the system need two wires to connect to the DC electrical system. One is connected to the plus and the other to the min or return of the system. Sometimes the aircraft frame is used as ground. Which is not so good. As this will induce a lot of problems with interference in radio's, intercoms, transponders and EFIS by high voltage strobe systems, high power COM transmitter or even the engine ignition (wires and frame can act as pickup antenna). It is wise to have a central ground point to which all return wires are connected and that ground point on the airframe should have one thick wire connected to the battery.