All aircraft must have some form of electrical supply to power the aircraft subsystems. The supply of electricity is executed by a combination of generators and batteries. Most modern aircraft require both AC and DC supplies. The typical AC voltage is 115 volts at 400 Hz, but there are higher-voltage AC supplies. Typically, the DC voltage supply is 28 volts. The electrical-supply control must ensure safety and comply with mandatory requirements.
The following systems are associated with electrical power:
• engine starting and operation; management of the fuel system
• lighting – both internal and external (Figure 15.27 shows external requirements)
• flight deck instrumentation
• communication and navigation
Figure 15.27. Aircraft lighting requirements
:r side lights
• avionics system
• flight-control system using the PCU
• passenger services for civil aircraft
• APU: emergency electrical power generation and supply
• armament management, electronic defensive and countermeasures for military aircraft
Typically, the electrical supply is generated at the primary and secondary levels. Engine-driven generators supply the primary power. The secondary supply serves before an engine starts and is a standby in an emergency situation. The secondary supply is generated from batteries, the APU, or an auxiliary system such as RAT.
The below-floorboard equipment bay houses items such as batteries, chargers, power controllers, transformers, and inverters (see Figure 15.27).
The weight of an electrical system depends on the load requirements. The cable weight is significant. An avionics system can be 0.4 to 4% for civil aircraft and 0.5 to 5% for military aircraft.