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Aircraft Electrical Systems, Basics

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 advanced electronic system of your choice.

The electrical system consist of a battery and an alternator (you will find a generator on older type aircraft) to power the system and recharge the battery. You will also find fuses and switches and lights for indication purposes. A volt and/or ammeter is used for monitoring.

All of this is connected through several meters (kilometers/ miles in large aircraft) of wire and connectors, attached to the airframe with insulation material as cushion clamps, ty-wraps 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.

Electricity Basics

The most basic part of electricity is the electron, to be more precise the free electron. Every atom consist of a nucleus of protons and electrons in orbit around the nucleus, the electrons are held in orbit by attraction of an equal and opposite charge of the protons. The electrons are located in shells around the nucleus.

Conductors

A substance with a good number of free electrons is said to be a good conductor and allowing electricity to flow through freely. Very good conductors are: silver, copper and aluminum. Materials with a few or no free electrons at all are insulators like: rubber, ceramics and plastics. And these are used to create electronic parts or conductors separated from each other preventing a short circuit.

Resistance

The resistance of a conductor is not only defined by the material (specific resistance ρ) used but also by its physical size. Cross sectional area, length and temperature all have their influence. In formula: R = (ρ × L) / A, where ρ is specific resistance, L is the length and A the cross-sectional area ((π / 4) × D2).

Voltage loss

Voltage loss occurs as a current flows through a wire, this depends on the specific resistance of the wire. Devices connected on the other end will therefore see a lower voltage and may not perform as expected or specified. If the wires are too thin they can even heat up and cause a fire. Use a #18 (0,8mm2) wire for anything up to ten amps and ten feet long, if longer it is best to use #16 (~ 1mm2) wire.

If current requirements is around 20 amps you will need #16 wire if the length is within ten feet, if the wires are longer use #14 (2mm2) for lengths up to 15 feet and #12 (3mm2) for wire up to 20 feet. From RBE Electronics we have a wire gauge table on site so you may select the proper wire gauge.

Current & electron flow

The electrical charge of an electron is negative and atoms lacking electrons, are positive charged. The battery has two connecting terminals: a positive pole and a negative pole where the surplus of electrons are. When a circuit is connected across the poles the electrons will flow from the negative pole through the circuit to the positive pole (attracted by the atoms lacking electrons).

For understanding electrical circuits the current flow is from the positive to the negative battery terminal.

Empty battery

This process of moving electrons from the negative pole to the positive pole continues until the chemical action in the battery, and the production of free electrons, is exhausted. The electrical charge on both poles is then equal and the battery is said to be discharged or empty. Until a charging current reverses the chemical process and 'reloads' or recharges the battery.

Batteries needs recharging with the right profile, meaning with constant current or constant voltage with the correct voltages and current for the chemistry you are charging. You will need a special charger capable of doing this to ensure long battery life. Any amount of fast charging will reduce battery life eventually.

Battery charge

The amount of energy or electron charge (or capacity) in a battery is called Coulomb, named after Charles-Augustin de Coulomb. It was agreed that one Coulomb equals one Ampere-second. Or put differently: one Ah (Amp-hour) is equal to 3600 C (Coulomb). Today, sometimes mAh (milli ampere hour) is used as this sounds bigger. Example: a powerbank (battery) of 20000 mAh is equal to 20 Ah or 72000 C.

Current flow

The flow of electrons is from the negative terminal to the positive terminal, but in general speaking terms the current flow is said to be from the positive terminal to the negative terminal. This is a result of a convention long ago when the workings of electrons and protons was not properly understood and scientists wrongly assumed the flow to be from the positive to the negative terminal.

Written by EAI.

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