The Aircraft Electrical System makes, supplies, and controls electrical power to aircraft. The complete system can be considered to be made up of different equipment circuits and power supply circuits.
The system has automatic and manual control features. The system also has protection features. The electrical system makes and supplies AC and DC power to the aircraft.
A standby AC and DC system give normal and emergency power. Built-in test equipment (BITE) and alternate source selection make the system reliable and easy to keep.
Basics of Electricity
- Voltage – Voltage is electrical pressure and it’s measured in terms of volts. Its symbol is “V” or “E”.
- Current – Current is movements of electrons and it’s measured in amps and expressed as “A” or “I” in drawings.
- Resistance – Resistance is the opposition offered to current flow. It is measured in Ohms (Ω), and its symbol is “R” or “Ω”.
- Power – Power is the rate of doing work. It is measured in Watts and expressed as “W” or “P”.
Conventional flows of current are from +ve to -ve. The current will not flow unless there is a complete circuit and there is a voltage (pressure) to drive electrons.
Each equipment circuit provides electricity to various components. They all take their supplies from bus bars that are supplied by generators and /or batteries.
Power supply circuits include generator circuits and battery circuits with transformers and rectifiers. They supply electricity to the bus bars.
Generators can be driven by the aircraft engines; by a Ram Air Turbine (RAT); by a hydraulically driven motor and by the APU (Auxiliary Power Unit). The normal source of supply is by generators driven by the aircraft engines.
- Transformer Rectifier Units (TRUs)
- Inverters – Rotary and Static
- Circuit breakers (C/Bs)
- Proximity switches
A generator is a machine that converts mechanical energy into electrical energy by the process of electromagnetic induction.
There are two types of the generator – AC generator, and DC generator. In both AC and DC types of generator, the voltage induced is alternating. The major difference between both generators being in the method by which the electrical energy is collected and applied to the circuit externally connected to the generator.
A battery is a device made up of a number of cells that depend on battery utilization. The cells convert chemical energy into electrical energy.
A battery may be of the primary cell type or secondary cell type.
Both types of cells exchange the electrons due to the chemical action of an electrolyte and electrode materials.
The difference between the two cells is in the action that occurs during discharge.
Primary cells destroy the active materials of the cell, however secondary cells convert the active material into from which they can subsequently be electrically reconverted into the original materials. The action of re-conversion is more commonly known as charging.
The batteries selected for use in aircraft therefore employ secondary cells and are either of the lead-acid or nickel-cadmium type.
The output from generators and batteries is supplied to bus-bars. Then all electrical services take their supplies from the bus-bars.
Aircraft electrical services can be split into the following groups:
Vital Services – Services would be required after an emergency. Components take their supply from the “hot” battery bus or vital battery bus. The emergency lights are also powered from this bus with their own battery back-up.
Essential Services – Services required to ensure a safe landing in an in-flight emergency. The bus-bars are connected in such a way that they can be fed from a generator or battery. Usually called a DC essential bus and an AC essential bus.
Non-essential Services – Services that can be isolated in an in-flight situation eg, galley supplies, in-flight entertainment, etc. They can also be subject to load shedding. The actual name may vary but there will be a non-essential ac bus and a non-essential dc bus.