To use RVSM, aircraft must have certified altimeters, and autopilots must meet more accurate standards. Lear developed radio direction finders, autopilots, and the first fully automatic aircraft landing system. Modern autopilots use computer software to control the aircraft. Aircraft and spacecraft autopilots are now part of the electronic controller. Modern autopilots, coupled with flight management systems, can enter and fly holding patterns automatically. Autopilots exist that automate all of these flight phases except taxi and takeoff. The aircraft was descending with two engaged autopilots and engaged auto-thrust. There are three levels of control in autopilots for smaller aircraft. Several vendors are currently producing PX4 autopilots and accessories. Several vendors are currently producing Slugs autopilots and accessories. Free software autopilots provide more flexible hardware and software. Open Source autopilots provide flexible hardware and software. Several vendors are currently producing Paparazzi autopilots and accessories. This computer autopilots spaceships from one part of the galaxy to another. Autopilots have a fail passive requirement.