Many systems that keep an aircraft operating run on electricity generated at a frequency you won’t find anywhere else in daily life. Aircraft run on a 400Hz power system, not the 50Hz or 60Hz power that runs the grid, your home, and your work.
400Hz isn’t arbitrary, but the result of a decades-old engineering trade-off that comes down to the main thing aviation cares about: weight.
In this post, we’ll explore what a 400Hz power system is, why aircraft use it instead of standard grid frequency, and why it became the industry standard.
A 400Hz power system is the electrical architecture aircraft use to generate, distribute, and consume AC power at a nominal frequency of 400 cycles per second. On most aircraft, that means three-phase power at 115/200 volts, produced by engine- or APU-driven generators and distributed to everything from avionics to galley equipment. Because the aircraft generates power onboard, its equipment is built around it from the start.
Frequency, measured in Hertz (Hz), describes how many times an alternating current (AC) completes a full cycle per second. Power grids in North America run at 60Hz; most of the rest of the world runs at 50Hz. However, aircraft electrical systems are designed to run at 400Hz, more than six times the frequency of the power in your home.
This isn’t just a different number on a spec sheet. Frequency directly affects the physical size and weight of the electrical equipment built to use it, which is exactly why aviation adopted it.
Every kilogram an aircraft carries costs money over its lifetime: in fuel burn, in reduced payload, in reduced range. Aircraft engineers treat weight as a constraint on par with structural integrity, and the electrical system is not exempt.
Here’s the underlying physics: the core cross-sectional area of a transformer, the iron core that a transformer's windings wrap around, is inversely proportional to the current frequency. Double the frequency, and you can roughly halve the core material needed to handle the same power. At 400Hz, a transformer needs only about one-eighth of the core area required at 50Hz.
Less core material means less iron, less copper, and a lighter, smaller, cheaper-to-fly transformer. Aircraft contain dozens of transformers and motors across their electrical, hydraulic, and environmental control systems:
Multiply the weight savings of transformers across an entire aircraft’s electrical system, and the cumulative savings become significant enough to justify building a completely different standard just for aviation.
The potential weight savings of running at 400Hz show up in three places:
That higher rotational speed also means less rotating mass for the engine’s accessory drive to manage, plus the reduced weight and space the smaller frame claims inside the aircraft.
Exact weight reduction varies by aircraft design and configuration.
There’s a trade-off: running at higher frequency increases eddy current and hysteresis losses inside the iron core, meaning the equipment runs a bit less efficiently and generates more heat per unit of material. That trade-off is exactly why 400Hz became the settled number rather than aviation pushing frequency even higher.
If you tried to power an aircraft with ordinary 50Hz or 60Hz electricity, the mismatch wouldn’t just cause a performance hit, but real damage.
The aircraft’s onboard transformers are built with cores sized for 400Hz. Feed them 50Hz power instead, and they saturate; the core can no longer contain the magnetic flux.
Motors designed to spin at 400Hz-driven speeds would run at the wrong RPM or overheat trying to compensate. Sensitive avionics that use the power frequency itself as a reference would receive the wrong clock signal entirely.
In short: an aircraft electrical system is engineered around 400Hz from the ground up, and there’s no safe way to feed it anything else. That’s true whether the aircraft is in the air, generating its own 400Hz power from engine-driven generators, or parked at the gate with engines off, where it still needs 400Hz power from an external source to run lighting, avionics, and environmental systems during turnaround.
If higher frequency means lighter equipment, why stop at 400Hz? The answer comes back to the efficiency tradeoff mentioned earlier.
Going higher than 400Hz would shrink equipment further, but at a cost:
Staying at 400Hz delivers most of the available weight savings while keeping losses and cooling needs manageable with proven materials and designs, which is why aviation engineers settled on it decades ago and why it remains the global standard today.
The catch is that 400Hz doesn’t exist naturally anywhere outside an aircraft’s own generators. Airports run on standard 50Hz or 60Hz grid power, same as everything else in the surrounding area. So when an aircraft is parked at the gate with its engines shut down, something else has to supply 400Hz power, like a ground power unit (GPU).
If you’re already weighing what kind of 400Hz setup makes sense for your airport, hangar, or MRO facility, our team at Unitron Power Systems is happy to talk through your specific applications and help figure out what fits.