A flight management system is a fundamental part of a modern aircraft in that it controls the navigation. The flight management system (FMS) is the avionics that holds the flight plan, and allows the pilot to modify as required in flight. The FMS uses various sensors to determine the aircraft's position. Given the position and the flight plan, the FMS guides the aircraft along the flight plan. The FMS is normally controlled through a small screen and a keyboard. The FMS sends the flight plan for display on the EFIS, Navigation Display (ND) or MultiFuction Display (MFD). The modern FMS was introduced on the Boeing 767, though earlier navigation computers did exist. Now, FMS exist on aircraft as small as the Cessna 172. In its evolution an FMS has many different sizes, capabilities and controls. However certain characteristics are common to all FMS.