Why do aircraft use 400 Hz electrical power?
Aircraft use 400 Hz AC power because the higher frequency lets transformers, generators and motors be made smaller and lighter than equivalent 50 or 60 Hz equipment. In Aviation & Real-World Flying, that weight saving matters, while 400 Hz remains low enough to keep wiring losses, heat and interference manageable.
What does 400 Hz save on an aircraft?
The main saving is mass in equipment containing magnetic components. For a given voltage and power rating, raising the frequency reduces the magnetic core material required in a transformer. Motors and generators can also operate at higher rotational speeds, reducing the torque—and often the physical size—needed for a given power output.
This is valuable aboard an aircraft because every kilogram affects payload, fuel burn and range. The benefit does not mean that 400 Hz electricity is inherently more powerful, nor that electrons move faster; it means suitably designed equipment can achieve the required rating with less mass.
The reduction is not a simple eight-to-one comparison with 50 Hz equipment. Cooling, insulation, mechanical strength and electrical losses still determine the final size of a real component.
Why use 400 Hz instead of an even higher frequency?
400 Hz is a practical compromise between lighter equipment and acceptable electrical losses. Increasing frequency beyond it can shrink some magnetic components further, but the disadvantages become harder to control:
- Skin and proximity effects increase effective conductor resistance.
- Hysteresis and eddy-current losses generate more heat in magnetic cores.
- Reactance becomes more significant across long cables and inductive loads.
- Electromagnetic interference, insulation design and power conversion become more demanding.
For conventional transport aircraft, 400 cycles per second became a useful balance: far above domestic 50 or 60 Hz supplies, but still practical for distributing substantial power around an airframe.
Do all aircraft systems run on 400 Hz power?
No—aircraft normally use several electrical systems, and many light aircraft do not distribute 400 Hz AC at all.
| Electrical supply | Typical role | Key caveat |
|---|---|---|
| 115/200 V, three-phase, 400 Hz AC | Motors, pumps, heating, galley equipment and power conversion | Common on conventional transport aircraft; 115 V is phase-to-neutral and 200 V is phase-to-phase |
| 28 V DC | Avionics, controls, lighting, batteries and essential buses | May come from batteries or transformer rectifier units supplied by the AC system |
| 14 or 28 V DC | Main distribution in many light aircraft | The alternator generates AC internally but rectifies it to DC |
| 50 or 60 Hz AC | Passenger or utility sockets | Usually produced by an inverter rather than taken directly from the main 400 Hz network |
| Variable-frequency AC | High-power systems on some newer aircraft | The permitted frequency range and voltage depend on the aircraft design |
A typical Cessna therefore differs markedly from an airliner; our explanation of the Cessna 172 cockpit and its alternator-based electrical system shows the light-aircraft arrangement.
How do aircraft generators maintain 400 Hz?
Generator frequency is determined by rotational speed and the number of magnetic poles, expressed as f = poles × rpm / 120. Because an engine's shaft speed changes, many conventional airliners use an integrated drive generator, or IDG, containing a constant-speed drive that keeps the generator at the speed required for 400 Hz.
An APU generator is also driven at a controlled speed. Our guide to how the APU supplies electrical and pneumatic power explains why it can power the aircraft before the engines are running. External power must likewise provide the correct voltage, three-phase sequence and frequency; aircraft protection normally prevents an unsuitable source from connecting to the buses.
Some newer designs avoid a mechanical constant-speed drive and generate variable-frequency AC instead. Equipment that needs fixed-frequency AC or DC then receives conditioned power through converters. This can reduce mechanical complexity, but it means that “aircraft use 400 Hz” is a common convention rather than a universal rule.
What does 400 Hz mean in a flight simulator?
In most flight simulators, 400 Hz appears as the result of bringing a valid generator online rather than as a frequency the pilot selects. Detailed airliners may show generator voltage and frequency on an electrical systems page, while simpler aircraft model only battery, alternator or generator switches.
A practical A320 start illustrates the source changes: external power or the APU initially feeds the buses, followed by the engine-driven generators. The normal A320 power-up and checklist sequence covers that handover.
A mistake we see constantly is assuming that a running engine automatically means every electrical bus is powered. If displays remain dark, check the battery, external power or APU, generator switches, bus ties and avionics master as applicable; our blank-avionics troubleshooting steps for Microsoft Flight Simulator address that follow-up problem.