Aircraft use 400 Hz alternating current instead of the 50 or 60 Hz found in household outlets because higher frequency electricity allows transformers, motors, and generators to be dramatically smaller and lighter. In aviation, every kilogram matters, and switching to 400 Hz shrank critical electrical components to a fraction of their original size while delivering the same power output.
How Higher Frequency Reduces Weight
The core physics is straightforward. The power a transformer can handle is proportional to its operating frequency, all else being equal. A transformer running at 400 Hz can deliver roughly eight times more power than one of identical size running at 50 Hz, because 400 is about eight times 50. Flip that around and you get the real benefit for aviation: a transformer that needs to deliver a fixed amount of power can be built about eight times smaller and lighter at 400 Hz than at standard utility frequencies.
The same principle applies to electric motors. When aviation first adopted 400 Hz power, a motor that had been the size of a watermelon could be replaced by one the size of a coffee can while doing the same work. That kind of weight savings, multiplied across dozens of motors, transformers, and power supplies throughout an aircraft, translates directly into greater cargo capacity and lower fuel burn.
The key tradeoff is transmission loss. Higher frequencies lose more energy as heat in long wires. On the ground, where power lines can stretch for kilometers, that penalty makes 400 Hz impractical. Inside an aircraft, where wire runs are measured in meters rather than miles, those losses are negligible. The weight savings far outweigh the slight inefficiency in transmission.
Why 400 Hz and Not Some Other Frequency
When aviation first adopted electricity, aircraft ran on DC power. As engineers shifted to AC for its advantages in voltage conversion and motor design, they had to pick an optimal frequency. Going higher than 400 Hz would have increased transmission losses in the wiring to a point where efficiency suffered. Going lower would have made the components heavier without meaningful gains in transmission efficiency. 400 Hz hit the sweet spot: light enough components, low enough losses for the short distances inside a fuselage.
A special generator was designed to produce this output, and the standard stuck. Airports worldwide adopted the same 400 Hz system, including standardized plugs and cables, so that any aircraft from any country could land and receive ground power at any airport. This made 400 Hz one of the first truly global power standards in any industry.
How Aircraft Generate 400 Hz Power
Aircraft generators are synchronous machines, meaning their output frequency is directly tied to how fast the shaft spins. The relationship follows a simple formula: frequency equals shaft speed (in RPM) multiplied by the number of magnetic poles, divided by 120. Engineers select the pole count and use a gearbox called an integrated drive generator (IDG) to keep the shaft spinning at a constant speed regardless of how fast the jet engine is running. This produces a steady 400 Hz output even as engine throttle changes throughout a flight.
When an aircraft is parked at a gate with its engines off, it obviously can’t generate its own 400 Hz power. Airports solve this with ground power units that convert standard 50 or 60 Hz utility power into 400 Hz. These units plug into the aircraft and keep cabin lights, air conditioning, and avionics running without requiring the plane to burn fuel on the ground.
The Shift Toward Variable Frequency
While 400 Hz remains the dominant standard, newer aircraft are moving away from fixed-frequency power. The Boeing 787 Dreamliner, for example, uses variable-frequency starter generators that produce power anywhere from 360 to 800 Hz, depending on engine speed. This eliminates the heavy, complex gearbox that traditional systems need to maintain a constant shaft speed.
The result is a simpler, lighter, and more reliable generator. The 787 also removed the traditional pneumatic bleed air system, replacing it with electrically powered alternatives. This “more electric aircraft” approach reduces weight further and cuts down on maintenance-intensive pneumatic ducting. Modern onboard electronics are designed to accept variable-frequency input and regulate it internally, making the fixed 400 Hz standard less critical than it once was.
That said, 400 Hz remains the universal ground power standard at airports worldwide, and most aircraft systems are still built around it. Variable frequency generation is best understood as an evolution of the same principle that drove 400 Hz adoption in the first place: keep the electrical system as light and efficient as possible for the unique constraints of flight.