An aircraft electrical system delivers power to the instruments, radios, lights, flight controls, pumps, computers, and passenger systems that keep a flight safe and practical. Modern aircraft usually have several power sources so one failure does not remove all electrical power. The main sources are engine-driven generators, batteries, and often an auxiliary power unit.
Understanding how power moves through buses helps pilots and technicians predict what will keep working during normal and abnormal situations.
Most large aircraft use AC power from generators, then convert some of it to DC power for equipment that needs steady direct current. Buses act like organized power rails that distribute electricity to groups of loads, such as essential avionics or cabin lighting. Backup sources such as batteries, the APU, and a ram-air turbine can feed selected buses when the main generators are not available.
Redundancy is built in so critical loads can receive power from more than one path.
Key Facts
- Electrical power is calculated by P = VI, where P is power in watts, V is voltage, and I is current.
- Most large aircraft generators supply 115 V AC at 400 Hz because higher frequency allows lighter motors and transformers.
- Batteries store chemical energy and usually supply DC power for starting, emergency loads, and short-term backup.
- A rectifier changes AC to DC, while an inverter changes DC to AC.
- A bus is a distribution point that connects one or more power sources to many electrical loads.
- Redundancy means critical systems have alternate power paths, such as Generator 1, Generator 2, battery, APU, or ram-air turbine.
Vocabulary
- Generator
- A generator converts mechanical rotation from an engine or APU into electrical energy for aircraft systems.
- Battery
- A battery stores electrical energy chemically and provides DC power when generators are off or unavailable.
- Bus
- A bus is a common electrical connection point that distributes power to multiple aircraft loads.
- Auxiliary Power Unit
- An auxiliary power unit is a small onboard turbine engine that can provide electrical power and air when the main engines are not running.
- Ram-Air Turbine
- A ram-air turbine is a small deployable turbine that uses airflow to generate emergency electrical or hydraulic power.
Common Mistakes to Avoid
- Confusing voltage with power is wrong because voltage is electrical potential difference, while power depends on both voltage and current using P = VI.
- Assuming the battery can power everything is wrong because aircraft batteries are sized mainly for starting, essential loads, and limited emergency time.
- Treating AC and DC buses as interchangeable is wrong because different equipment requires specific current types and voltages.
- Ignoring bus priority is wrong because essential buses are designed to keep critical flight and communication systems powered before nonessential cabin or comfort loads.
Practice Questions
- 1 An avionics bus supplies 28 V DC to equipment drawing 12 A. What electrical power is the bus delivering in watts?
- 2 A battery rated at 24 V provides 40 A to essential loads for 15 minutes. How much energy is delivered in watt-hours?
- 3 During cruise, both engine-driven generators fail but the aircraft still has a charged battery, an APU that can be started, and a ram-air turbine. Explain which loads should be powered first and why redundancy matters.