MOUNTAIN VIEW, Calif.—"Aircraft should fly themselves!" is scrawled on a whiteboard at Reliable Robotics' offices here.

Recently two staff members watching on monitors tracked the flight path of a small airplane flying itself above northern California, about 40 miles away. The Cessna Caravan had a pilot aboard on standby, along with a flight-test engineer, but was otherwise on its own. They were testing a new collision-avoidance system and new radar that helps the plane see and avoid other aircraft.