not all of the passenger aircraft flying today have an autopilot system. older and smaller general aviation aircraft especially are still hand-flown, and even small airliners with fewer than twenty seats may also be without an autopilot as they are used on short-duration flights with two pilots. the installation of autopilots in aircraft with more than twenty seats is generally made mandatory by international aviation regulations. there are three levels of control in autopilots for smaller aircraft. a single-axis autopilot controls an aircraft in the roll axis only; such autopilots are also known colloquially as "wing levellers," reflecting their limitations. a two-axis autopilot controls an aircraft in the pitch axis as well as roll, and may be little more than a wing leveller with limited pitch oscillation-correcting ability; or it may receive inputs from on-board radio navigation systems to provide true automatic flight guidance once the aircraft has taken off until shortly before landing; or its capabilities may lie somewhere between these two extremes. a three-axis autopilot adds control in the yaw axis and is not required in many small aircraft.