TM 1-1510-225-10 2-70 appear  in  the  sight  gauge,  you  may  assume  that  the selected  ferry  fuel  tank  is  empty.    As  each  ferry  fuel tank   reaches   empty,   close   that   selector   valve   and either select the other tank and/or turn off the transfer pump(s).      When   the   ferry   fuel   system   is   depleted, securely   close   all   selector   valves   and   turn   off   the transfer pumps. Section V.  FLIGHT CONTROLS 2-35.  FLIGHT CONTROL SYSTEM. The     aircraft's     primary     flight     control     system consists  of  conventional  rudder,  elevator,  and  aileron control    surfaces.        These    surfaces    are    manually operated from the cockpit through mechanical linkage using  a  control  wheel  for  the  ailerons  and  elevators, and   adjustable   rudder/brake   pedals   for   the   rudder.   Both  the  pilot  and  copilot  have  flight  controls.    Trim control    for    the    rudder,    elevators,    and    ailerons    is accomplished through a manually actuated cable-drum system for each set of control surfaces.  The autopilot has    provisions    for    controlling    the    position    of    the ailerons, elevators, elevator trim tab, and rudder. 2-36.   CONTROL WHEELS. Elevator and aileron control surfaces are operated  by  manually  actuating  either  the  pilot's  or copilot's   control   wheel.      Refer   to   Figure   2 -22.      A control    wheel    is    installed    on    each    side    of    the instrument    panel.        Switches    are    installed    in    the outboard  grip  of  each  wheel  to  operate  the  elevator trim   tabs.      A   microphone   switch;   an   autopilot/yaw damp/electric   trim   disconnect   switch;   and   a   pitch synchronize and control wheel steering switch are also installed  in  the  outboard  grip  of  each  control  wheel.   The  outboard  grip  of  the  copilot's  control  wheel  also has a GO AROUND (GA) switch.  A transponder ident switch  is  installed  on  the  forward  side  of  the  inboard grip  of  each  control  wheel.    Installed  in  the  center  of each  control  wheel  is  a  digital  electric  clock.    A  map light  switch  is  installed  on  the  inboard  grip  of  each control wheel. 2-37.  RUDDER SYSTEM. a.   Rudder  Pedals.    Aircraft  rudder  control  and nose  wheel  steering  is  accomplished  by  actuation  of the  rudder  pedals  from  either  the  pilot's  or  copilot's station.        The    rudder    pedals    may    be    individually adjusted,  forward  or  aft,  to  provide  adequate  legroom for the pilot and copilot.  Adjustment is accomplished by  pressing  the  lever  alongside  the  rudder  pedal  arm and  moving  the  pedal,  forward  or  aft,  until  the  locking pin engages in the selected position. b.  Yaw    Damper    System.        A    yaw    damper system   is   provided   to   aid   the   pilot   in   maintaining directional   stability  and  increase  ride  comfort.    The system may be used at any altitude, but is required for flight  above  17,000  feet.    It  must  be  deactivated  for takeoff and landing.  The yaw damper system is a part of  the  autopilot.    A  Yaw  Damper  (YD)  switch  located on the autopilot control panel controls the system.  The yaw  damper  may  also  be  disconnected  by  pressing the   control   wheel   autopilot/yaw   damper/electric  trim disconnect    switch,    placarded    AP    DISC    &    TRIM INTRPT,  to  the  first  level.    Operating  instructions  for this system are contained in Chapter 3. c.   Rudder   Boost   System.    Rudder  boost  is provided   to   aid   the   pilot   in   maintaining   directional stability   resulting   from   an   engine   failure   or   a   large variation of power between the engines.  Incorporated in the rudder cable system are two pneumatic rudder- boosting   servos   that   actuate   the   cables   to   provide rudder  pressure  to  help  compensate  for  asymmetrical thrust.  Rudder boost is not required for flight. NOTE Rudder   boost   may   be   inoperative   when brake deice is on. (1)   During   operation,  a  differential  pressure valve  accepts  bleed  air  pressure  from  each  engine.   When    the    pressure    varies    between    the    bleed    air systems,  the  shuttle  in  the  differential  pressure  valve moves toward the low-pressure side.  As the pressure difference reaches a preset tolerance, a switch closes on   the   low-pressure  side  that  activates  the  rudder boost  system.    This  system  is  designed  only  to  help compensate    for    asymmetrical    thrust.        Appropriate trimming  is  to  be  accomplished  by  the  pilot.    Moving either or both of the  BLEED AIR VALVES switches on the  copilot's  subpanel  to  the    INSTR  &  ENVIR   OFF position will disengage the rudder boost system.