TM 55-1510-220-10 b. Yaw   Damp   System.      A   yaw   damp   system   is provided   to      aid   the   pilot   in   maintaining   'directional stability  and  increase  ride  comfort.    The  system  may  be used   at   any   altitude   and   is   required   for   flight   above 17,000   feet.      It   must   be   deactivated   for   takeoff   and landing.  The yaw damp system is a part of the autopilot. Operating  instructions  for  this  system  are  contained  in Chapter  3.    The  system  is  controlled  by  a  YAW  DAMP switch adjacent to the ELEV TRIM switch on the pedestal extension. c. Rudder Boost System.  A rudder boost system is provided to aid the pilot in maintaining directional control 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 which  actuate  the  cables  to  provide  rudder  pressure  to help compensate for asymmetrical thrust. (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   which   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 valve switches on the overhead control panel to PNEU & ENVIRO OFF position will disengage the rudder boost system. NOTE Condition  levers  must  be  in  LOW IDLE   position   to   perform   rudder boost check. (2) The    system    is    controlled    by    a    switch located   on   the   extended   pedestal   placarded   RUDDER BOOST ON OFF (fig.  2-8), and is to be turned on before flight.  A preflight check of the system can be performed during the run-up by retarding the power on one engine to idle and advancing power on the opposite engine until the power difference between the engines is great enough to activate  the  switch  to  turn  on  the  rudder  boost  system. Movement  of  the  appropriate  rudder  pedal  (left  engine idling,  right  rudder  pedal  moves  forward)  will  be  noted when     the     switch     closes,     indicating     the     system     is functioning  properly  for  low  engine  power  on  that  side. Repeat  the  check  with  opposite  power  settings  to  check for movement of the opposite rudder pedal.  The system is   protected   by   a   5-ampere   circuit   breaker,   placarded RUDDER    BOOST,    located    on    the    overhead    circuit breaker panel (fig.  2-27). NOTE With brake de-ice on, rudder boost may be inoperative. 2-40.  FLIGHT CONTROLS LOCK. CAUTION Remove control locks before towing     the     aircraft     or     starting engines.      Serious   damage   could result   in   the   steering   linkage   if towed by a tug with the rudder lock installed. Positive locking of the rudder, elevator and aileron control    surfaces,    and    engine    controls    (power    levers, propeller  levers,  and  condition  levers)  is  provided  by  a removable  lock  assembly  (fig.    2-19)  consisting  of  two pins, and an elongated U-shaped strap interconnected by a chain.  Installation of the control locks is accomplished by inserting the U-shaped strap around the aligned control levers  from  the  copilot's  side;  then  the  aileron/elevator locking  pin  is  inserted  through  a  guide  hole  in  the  top  of the   pilot's   control   column   assembly,   thus   locking   the control wheel.  The rudder is held in a neutral position by an L-shaped pin which is installed through a guide hole in the   floor   aft   of   the   pilots   rudder   pedals.      The   rudder pedals  must  be  centered  to  align  the  hole  in  the  rudder bellcrank  with  the  guide  hole  in  the  floor.    Remove  the locks in reverse order, i.e., rudder pin, control column pin, and power control clamp. 2-41.  TRIM TABS. Trim tabs are provided for all flight control surfaces. These tabs are manually activated, and are mechanically controlled    by    a    cable-drum    and    jack-screw    actuator system,  except  the  right  aileron  tab  which  is  of  the  fixed bendable type.  Elevator and aileron trim tabs incorporate neutral, non-servo action, i.e., as the elevators or ailerons are   displaced   from   the   neutral   position,   the   trim   tab maintains an "as adjusted" position.  The rudder trim tab incorporates    anti-servo    action,    i.e.,    as    the    rudder    is displaced from the neutral position the trim tab moves in the  same  direction  as  the  control  surface.    This  action increases control pressure as rudder is deflected from the neutral position. 2-37