400 Hertz is common on Aircraft because the generator and motors and wiring can all be lighter for the same power.
The one we picked up at the surplus auction was for a radar unit. Since we were only using it for lights and a coffee pot, it worked out great; couldn't use it on anything that had a regular AC motor, though, because those are wired for 60 Hertz. We also got a small trailer to mount it on, and the lights. It worked great - we had a house fire a few weeks later, at night, and were able to use the floodlights for the recovery and to help the arson investigator (and keep warm with some hot coffee while we waited). The next summer, we had a tornado blow through the next town over and do considerable damage, again at night, and we put it to good use there to help clean up.
The cockpit simulators I worked on had to use 400 Hertz, because we used actual aircraft electronics. Sometimes there were slight modifications to work with our computers, but a lot of the stuff we used was right off the shelf from the military maintenance shop.
