Why in general larger aircraft make use of both AC and DC electrical systems.?
AC motors and controls can be made smaller and lighter, especially when driven at 120V and 400 hz. Almost all, if not all, systems that are intended for normal use with normal power will be AC. DC, on the other hand, is the old stand-by when down to just battery power. If a pilot arrives at a dead airplane, with no external power (AC), how will he get the thing running if it requires A/C? With one or two batteries, one DC fuel boost pump, a DC starter on the APU, DC fire detection and DC fire fighting systems, he can safely get the airplane up and running, and if a fire breaks out during the process, he can fight the fire. If there is a loss of all AC generators, then there needs to be some DC powered stuff from the battery (basic flight instruments, basic lighting, etc.) to allow continued flight while attempts are made to restore the AC. On the Boeings, the fallback really important stuff will be found on the hot battery bus, battery bus, DC transfer bus, and standby DC bus. The Airbus probably does the same thing using different names.
Geeez!....RickH must write text books for aircraft mechanics. Great answer Rick.
