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 (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 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.
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.
A substance with a good number of free electrons is said to be a good conductor and allowing electricity to flow through. 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 separate conductors from each other preventing a short circuit.
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 lenght and A the cross-sectional area ((π / 4) × D2).
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 lenghts upto 15 feet and #12 (3mm2) for wire upto 20 feet. From RBE Electronics we have a wire gauge table on site.
Current and electron flow
The electrical charge of an electron is negative and the proton is positive. The battery has two connecting terminals: a plus pole (protons) 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 plus pole (attracted by the protons).
For understanding electrical circuits the current flow is from the plus to the negative battery terminal.
This process of moving electrons from the negative pole to the plus 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 empty. Until a charging current reverses the chemical process and 'reloads' 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.
The amount of energy or electron charge in a battery is called Coulomb, named after Charles-Augustin de Coulomb. It was agreed that one Coulomb equals one Ampere per second. Or put differently: one Ah (Amp-hour) is equal to 3600 C (Coulomb).
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 assumed the flow to be from the plus to the negative pole.