Basic Aircraft Electrical Systems

Some very simple single engine aircraft do not have an electrical system installed. The piston engine is equiped with a Magneto ignition system, which is self powering, and the fuel tank is situated so it will gravity feed the engine. The aircraft is started by means of a flywheel and crank arrangement or by "hand-proping" the engine.

If an electric starter, lights, electric flight instruments, navigation aids or radios are desired, an electrical system becomes a necessity. In most cases, the system will be DC powered using a single distribution bus, a single battery and a single engine driven generator or alternator. Provisions, in the form of an on/off switch, will be incorporated to allow the battery to be isolated from the bus and for the generator/alternator to be isolated from the bus. An ammeter, loadmeter or warning light will also be incorporated to provide an indication of charging system failure. Electrical components will be wired to the bus-bar incorporating either circuit breakers or fuses for circuit protection. Provisions may be provided to allow an external power source such as an extra battery or a Ground Power Unit (GPU) to be connected to assist with the engine start or to provide power whilst the engine is not running.

Advanced Aircraft Electrical Systems

More sophisticated electrical systems are usually multiple voltage systems using a combination of AC and DC buses to power various aircraft components. Primary power generation is normally AC with one or more Transformer Rectifier Unit (TRU) providing conversion to DC voltage to power the DC busses. Secondary AC generation from an APU is usually provided for use on the ground when engines are not running and for airborne use in the event of component failure. Tertiary generation in the form of a hydraulic motor or a RAT may also be incorporated into the system to provide redundancy in the event of multiple failures. Essential AC and DC components are wired to specific busses and special provisions are made to provide power to these busses under almost all failure situations. In the event that all AC power generation is lost, a static Inverter is included in the system so the Essential AC bus can be powered from the aircraft batteries.

Robust system monitoring and failure warning provisions are incorporated into the electrical system and these are presented to the pilots when appropriate. Warnings may include, but are not limited to, generator malfuntion/failure, TRU failure, battery failure, bus fault/failure and circuit breaker monitoring. The manufacturer will also provide detailed electrical system isolation procedures to be utilized in the event of an electrical fire.

In compliance with applicable regulations, components such as Standby Flight Instruments and Aircraft Emergency Floor Path Illumination have their own backup power supplies and will function even in the event of a complete electrical system failure.

Provisions are virtually always provided for connecting the aircraft electrical system to a fixed or mobile Ground Power Unit (GPU).

Threats

· Generator Failure

· Bus Failure

· Component Failure

· Electrial System Fire

Effects

· Loss of some or all of primary power generation capability

· Loss of all components and systems powered by the failed bus

· Loss of an individual component

· Potential loss of aircraft should the fire become uncontrolable, loss of busses; systems or components due to the fire or as a result of electrical isolation procedures; smoke and/or fumes