TM 55-1510-219-10 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, 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.  2-26). NOTE With  brake  deice  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.    2-17)  consisting  of two pins, and an elongated U-shaped strap interconnected by     a     chain.          Installation     of     the     controls     lock     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  jackscrew actuator  system,  except  the  right  aileron  tab  which  is  of the  fixed  bendable  type.    Elevator  and  aileron  trim  tabs incorporate   neutral,   anti-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. a. Elevator Trim Tab Control.  The elevator trim tab control wheel placarded ELEVATOR TAB DOWN, UP, is on the left side of the control pedestal and controls a trim tab on each elevator (fig.  210).  The amount of elevator tab    deflection    in    degrees    from    a    neutral    setting,    is indicated by a position arrow. b. Electric Elevator Trim.  The electric elevator trim system is controlled by an ELEV TRIM PUSH ON PUSH OFF switch located on the pedestal, dual element thumb switches  on  the  control  wheels,  a  trim  disconnect  switch on   each   control   wheel   and   a   circuit   breaker   on   the overhead  circuit  breaker  panel.    The  PUSH  ON  -PUSH OFF  switch  must  be  in  the  ON  position  to  operate  the system.      The   dual   element   thumb   switch   is   moved forward   for   trimming   nose   down,   aft   for   nose   up,   and when  released  returns  to  the  center  (off)  position.    Any activation  of  the  trim  system  through  the  copilot's  trim switch can be cancelled by activation of the pilot's switch. Operating  the  pilot's  and  copilot's  switches  in  opposing directions   simultaneously   results   in   no   trim   action.      A preflight  check  of  the  switches  should  be  accomplished before  flight  by  moving  the  switches  individually  on  both control  wheels.    No  one  switch  alone  should  operate  the system;  operation  of  elevator  trim  should  occur  only  by movement    of    pairs    of    switches.        The    trim    system disconnect   is   a   bi-level,   push   button,   momentary   type switch, located on the outboard grip of each control wheel. Depressing the switch to the first of two levels disconnects the autopilot and yaw damp system, and the second level disconnects the electric trim 2-37