Bob Lazar, who worked at Area 51, said that he had been involved in inspecting crashed UFOs to determine how they were constructed and their propulsion system.
He said there were nine crashed UFOs stored at area 51 and one had been test-flown. I don't believe that as aliens would have a more advanced system to fly them than we could understand, possibly through thought or finger-tip control. There wouldn't have been a simple key switch.