Learn how aircraft generate electrical power using alternators, APUs and emergency sources, with clear AC/DC, battery and failure explanations.
In aviation and real-world flying, aircraft generate electrical power mainly with engine-driven alternators or generators that convert mechanical rotation into electricity. Small aircraft usually supply DC systems, while transport aircraft commonly generate AC. Batteries provide starting and temporary backup; APUs and emergency generators cover periods when the main source is unavailable.
How does an aircraft generator work?
An aircraft generator converts shaft rotation into electrical energy by electromagnetic induction. The engine turns the unit through a belt, gears or an accessory gearbox; moving a magnetic field relative to electrical windings creates current.
On many piston aircraft, an alternator produces AC internally and rectifier diodes convert it to DC for the aircraft buses and battery. Older designs may use a DC generator, which usually provides less useful output at low engine speed.
Turbine aircraft drive generators from their accessory gearboxes. Some use starter-generators, which work as electric motors during engine start and become generators once the engine is running. A generator control unit regulates voltage and disconnects a source if its output becomes unsafe.
Conventional transport aircraft often use an integrated drive generator to maintain fixed-frequency AC despite changing engine speed. Other designs generate variable-frequency AC instead. Our explanation of why transport aircraft commonly use 400 Hz power covers the frequency and conversion side in more detail.
Electrical sources by aircraft type
The normal electrical source depends mainly on the aircraft’s engines, equipment and required redundancy.
| Aircraft type | Typical normal source | Typical distribution | Alternative or backup sources |
|---|---|---|---|
| Small piston aircraft | Engine-driven alternator or older DC generator | Usually low-voltage DC | Battery; sometimes a standby alternator |
| Turboprop or light jet | DC starter-generator or AC generator | DC, AC or a mixed system | Second generator, batteries, external power and sometimes an APU |
| Transport aircraft | One or more engine-driven AC generators | Commonly three-phase AC, with DC produced by transformer rectifiers | APU generator, batteries, external power and an emergency generator or ram-air turbine |
Generated power passes through contactors, bus ties and circuit protection before reaching individual loads. Transformer rectifier units convert AC to DC, while inverters can produce AC from a DC source. Essential and standby buses keep critical instruments, communications and flight controls supplied when the main buses are lost.
The familiar 115-volt, 400 Hz AC and 28-volt DC arrangement is common on transport aircraft, but it is not universal. Some newer types use variable-frequency AC or higher-voltage systems for large electrical loads. For a simpler example, see our breakdown of the Cessna battery, alternator and bus layout.
Is the battery the main aircraft electrical source?
An aircraft battery supplies electricity, but it is not normally the continuous source once a generator is online. It converts stored chemical energy into electrical energy for starting, initial cockpit power and short-term backup, then absorbs energy again while being recharged.
Battery endurance is not a universal number. It depends on battery condition, temperature, charge state and which equipment remains connected. Load shedding can make the difference between retaining essential instruments until landing and exhausting the battery early.
A piston engine fitted with independent magnetos may continue running after the electrical system fails, but radios, lights, electrically operated flaps and some fuel pumps may be lost. Engines using electronic ignition, fuel injection or full-authority digital control can have different backup requirements, so the aircraft’s approved documentation governs.
What powers an aircraft when the engines are stopped?
With the engines stopped, an aircraft uses its battery, an APU-driven generator or compatible external power. The available choice depends on the aircraft: most small piston types have no APU, while airliners generally avoid operating on batteries alone for extended ground servicing.
An auxiliary power unit is a small gas turbine that can turn a generator independently of the main engines; our explanation of APU electrical power shows how it supports starting and ground operations. A compatible ground power supply can energise the aircraft buses without consuming battery charge or running the APU. Ground power is supplied to the aircraft rather than generated aboard it.
What happens if an aircraft generator fails?
A failed generator is normally disconnected so unstable voltage or frequency cannot damage equipment. Multi-generator aircraft can transfer affected buses to another engine generator or the APU, often automatically; a single-alternator aircraft usually falls back to its battery.
A running engine does not prove that its generator is supplying power. Common causes of apparent or actual generation failure include:
- The alternator or generator switch, field circuit or line contactor is not connected.
- A drive belt, gearbox, starter-generator or generator control unit has failed.
- Overvoltage, overcurrent or frequency protection has taken the source offline.
- Demand exceeds available output, particularly at low engine speed on some piston aircraft.
- A failed rectifier, transformer rectifier, bus tie or distribution component leaves part of the aircraft unpowered even though generation continues elsewhere.
Emergency arrangements vary by type. They may include a standby alternator, batteries, a ram-air turbine that drives a generator or hydraulic pump, or a dedicated emergency generator.
- Confirm the failure: Check voltage, load indications, warning lights and the aircraft’s electrical system display rather than relying on one dark instrument.
- Use the approved checklist: Restore or reset a source only as the POH, AFM or QRH directs. Repeatedly resetting a tripped circuit breaker can worsen an electrical fault or fire risk.
- Select another source: Connect the remaining generator, APU or permitted bus tie when the aircraft’s procedure allows it.
- Shed non-essential loads: Preserve battery capacity and keep essential flight, navigation and communication equipment powered.
- Plan for finite backup time: If normal generation cannot be restored, treat the remaining electrical endurance as limited and follow the aircraft-specific landing procedure.