is known as an altitude hold function. The signals control the  pitch servos, which move to correct the error. An altitude  select function causes the signals to continuously be sent to  the pitch servos until a preselected altitude has been reached.  The aircraft then maintains the preselected altitude using  altitude hold signals. Modern general aviation AFCS are fully integrated with  digital attitude heading and reference systems (AHRS) and  navigational aids including glideslope. They also contain  modern computer architecture for the autopilot (and flight  director systems) that is slightly different than described  above for analog autopilot systems. Functionality is  distributed across a number of interrelated computers and  includes the use of intelligent servos that handle some of the  error correction calculations. The servos communicate with  dedicated avionics computers and display unit computers  through a control panel, while no central autopilot computer  exists.  [Figure 10-113] Many aircraft have a tendency to oscillate around their  vertical axis while flying a fixed heading. Near continuous  rudder input is needed to counteract this effect. A yaw damper  is used to correct this motion. It can be part of an autopilot  system or a completely independent unit. A yaw damper  receives error signals from the turn coordinator rate gyro.  Oscillating yaw motion is counteracted by rudder movement,  which is made automatically by the rudder servo(s) in  response to the polarity and magnitude of the error signal.  A flight director system is an instrument system consisting of  electronic components that compute and indicate the aircraft  attitude required to attain and maintain a preselected flight  condition. A command bar on the aircraft’s attitude indicator  shows the pilot how much and in what direction the attitude  of the aircraft must be changed to achieve the desired result.  The computed command indications relieve the pilot of many  of the mental calculations required for instrument flights,  such as interception angles, wind drift correction, and rates  of climb and descent. An aircraft autopilot with many features and various autopilot  related systems integrated into a single system is called an  automatic flight control system (AFCS). These were formerly  found only on high-performance aircraft. Currently, due to  advances in digital technology for aircraft, modern aircraft  of any size may have AFCS. Essentially, a flight director system is an autopilot system  without the servos. All of the same sensing and computations  are made, but the pilot controls the airplane and makes  maneuvers by following the commands displayed on the  instrument panel. Flight director systems can be part of an  autopilot system or exist on aircraft that do not possess full  autopilot systems. Many autopilot systems allow for the  option of engaging or disengaging a flight director display.  AFCS capabilities vary from system to system. Some of the  advances beyond ordinary autopilot systems are the extent  of programmability, the level of integration of navigational  aids, the integration of flight director and autothrottle  systems, and combining of the command elements of these  various systems into a single integrated flight control human  interface.  [Figure 10-112] It is at the AFCS level of integration that an autothrottle  system is integrated into the flight director and autopilot  systems with glide scope modes so that auto landings are  possible. Small general aviation aircraft being produced with  AFCS may lack the throttle-dependent features. Flight director information is displayed on the instrument that  displays the aircraft’s attitude. The process is accomplished  with a visual reference technique. A symbol representing  the aircraft is fit into a command bar positioned by the  flight director in the proper location for a maneuver to be  accomplished. The symbols used to represent the aircraft  Figure 10-112.  The AFCS control panel commands several integrated systems from a single panel including: flight directors, autopilots,  autothrottles, autoland, and navigational aids. Mode selections for many features are made from this single interface.  10-66$^{MACH}$ Flight Director Systems Yaw Dampening Automatic Flight Control System (AFCS) HOLD IAS HOLD IAS ACO INS BACK BEAM MAX CLIMB MAX CRUISE TRK HDG HDG HOLD TURN GLIDE PITCH HOLD MACH HOLD VOR LOG LAND IAS HOLD VERT SPEED ALT HOLD ALT ACQ AUTOLAND CA T 3 GO GROUND 1 7 0 0 0 1 1 0 4 0 0 2 2 4 1 3 0 2 2 0 AP2 AP1 AP2 FD2 FD1 AT1 AT2 AP1 AUTOTHROTTLE SPEED SELECT KNOTS ALTITUDE SELECT FEET