Aircraft Electrical Diagram

Power Generation, III

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. All of this is connected through several meters (kilometers in large aircraft) of wire.

All matter on earth is made up from molecules and they consist of atoms. Atoms are made of electrons, protons and neutrons. Electricity is about the flow of electrons attracted to protons and repelled by other electrons.

Alternators and generators generate electrons for us to use, but do nothing if the engine is not operating, so we need some form of storage.

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 a 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.

Capacity

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 rectifies the alternating voltage/current (AC) to direct voltage/current (DC). The alternator uses diodes to convert the AC to DC.

Regulation

Both types are voltage regulated and deliver 13,8 volts to the aircraft electrical system and they need an over voltage 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.

The generator needs at least 1200 engine RPM to supply enough electrical power to recharge the battery, where the alternator can do that from idle RPM (below 900).

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.