adjustments are necessary until the aircraft no longer skids  left or right during normal cruising flight.  [Figure 6-22]  Rather than using a movable tab on the trailing edge of the  elevator, some aircraft have an adjustable stabilizer. With this  arrangement, linkages pivot the horizontal stabilizer about  its rear spar. This is accomplished by the use of a jackscrew  mounted on the leading edge of the stabilator.  [Figure 6-23]  On small aircraft, the jackscrew is cable operated with a trim  wheel or crank. On larger aircraft, it is motor driven. The  trimming effect and flight deck indications for an adjustable  stabilizer are similar to those of a trim tab.  Autopilot is an automatic flight control system that keeps an  aircraft in level flight or on a set course. It can be directed by  the pilot, or it may be coupled to a radio navigation signal.  Autopilot reduces the physical and mental demands on a pilot  and increases safety. The common features available on an  autopilot are altitude and heading hold.  The simplest systems use gyroscopic attitude indicators and  magnetic compasses to control servos connected to the flight  control system.  [Figure 6-24]  The number and location of  these servos depends on the complexity of the system. For  example, a single-axis autopilot controls the aircraft about the  longitudinal axis and a servo actuates the ailerons. A three-axis  autopilot controls the aircraft about the longitudinal, lateral, and  vertical axes. Three different servos actuate ailerons, elevator,  and rudder. More advanced systems often include a vertical  speed and/or indicated airspeed hold mode. Advanced autopilot  systems are coupled to navigational aids through a flight director.  Because flight control systems and aerodynamic  characteristics vary greatly between aircraft, it is essential  that a pilot become familiar with the primary and secondary  flight control systems of the aircraft being flown. The  primary source of this information is the AFM or the POH.  Various manufacturer and owner group websites can also be  a valuable source of additional information.  Additionally, autopilots can be manually overridden. Because  autopilot systems differ widely in their operation, refer to  the autopilot operating instructions in the Airplane Flight  Manual (AFM) or the Pilot’s Operating Handbook (POH).  The autopilot system also incorporates a disconnect safety  feature to disengage the system automatically or manually.  These autopilots work with inertial navigation systems,  global positioning systems (GPS), and flight computers to  control the aircraft. In fly-by-wire systems, the autopilot is  an integrated component.  Figure 6-22.  A ground adjustable tab is used on the rudder of many  small airplanes to correct for a tendency to fly with the fuselage  slightly misaligned with the relative wind.  Figure 6-24.  Basic autopilot system integrated into the flight  control system.  Figure 6-23.  Some aircraft, including most jet transports, use an  adjustable stabilizer to provide the required pitch trim forces.  Adjustable Stabilizer  Autopilot  Chapter Summary  6-12  Adjustable stabilizer  Nose down  Nose up  Trim motor or trim cable  Jackscrew  Pivot 