Aircrafts use 400 Hz power in their electric systems because the power supplies needed are very small and lightweight. This is necessary on aircraft because of the need to control the exact weight of all onboard objects. 400 Hz power electronics are not the most effective, but because they are so lightweight, they are a popular choice on aircraft.
400 Hz have smaller and more lightweight power supplies than do the more common 60 Hz power settings. This means that they take up less space on the aircraft. Even though 400 Hz power supplies are more inefficient, most design engineers are willing to sacrifice electronic efficiency for space and low weight due to the need for an aircraft to mitigate weight and space issues. The effects of weight on an aircraft are exacerbated because additional weight requires more fuel to keep it airborne. This additional fuel further increases the overall weight of the vehicle.
Because modern airplanes use a number of different electronics and electronic generation devices onboard, the choice of 400 Hz power has a serious impact on the overall design of the vehicle. While aircraft electrical designs include AC and DC power generation sources, typically commercial aircraft use AC power supplying 115 volts at 400 Hz to be able to run flight instruments, avionics, lighting and other electrical equipment.