TM 1-1510-224-10 ground  fine  gate,  a  mechanical  linkage  at  the  propeller governor  will  begin  to  bleed  Py  air  from  the  fuel  control unit,  provided  the   PROP   levers   are   positioned   to   the minimum  RPM  position.    This  results  in  a  decrease  in both  engine  N,,  torque,  and  propeller  RPM.    With  the POWER   levers   at   the   ground   fine   gate,   engine   N1 should be within the range of 62% to 67%, and propeller RPM should not be less than 1000 RPM. 2-45.  PROPELLER SYNCHROPHASER. a. Description. The propeller synchrophaser matches left and right propeller RPM as well as propeller phase  relationship.    This  phase  relationship  is  designed to  decrease  cabin  noise,  and  is  not  adjustable  in  flight. A  toggle  switch,  placarded   PROP   SYN   -   ON   -   OFF, installed   adjacent   to   the   synchroscope   on   the   pilot's instrument panel (fig.  2-18), turns the system on/off. Signal  pulses  occurring  once  per  revolution  of  the propeller are obtained from magnetic pickups (located in the   front   of   the   engine   on   the   deice   brush   mounting bracket) when the target (mounted on the aft side of the spinner   bulkhead)   passes   the   magnetic   pickup.      The signal  pulses  are  sent  to  a  control  box  installed  forward of  the  pedestal.    The  control  box  receives  these  signal pulses  and  compares  them  for  pulse  rate  and  relative position.      Differences   in   pulse   rate   and/or   propeller position  cause  the  control  box  to  vary  the  voltage  in  the primary  governor  coil,  which  in  turn  increases  propeller speed until the correct speed and phasing are obtained. A   governor   coil   increases   the   speed   set   by   the propeller  control  lever,  but  never  decreases  the  speed set  by  the  control  lever.    The  maximum  synchrophaser range   is   approximately   20   RPM.      This   limited   range prevents either propeller from losing more than a limited RPM    if    the    other    propeller    is    feathered    with    the synchrophaser  ON. There  is  no  master  or  slave  engine  in  this  system. There   is   a   limited   range   for   synchronizing,   called   the "holding  -range".    There  is  a  maximum  RPM  differential (capture    range),    at    which    the    synchrophaser,    when turned  on,  will  begin  to  synchronize  the  propellers.    For this reason the propellers should be manually synchronized before turning the synchrophaser on. NOTE If  the  synchrophaser  is  ON  but  does not adjust properly, the synchrophaser  has  reached  the  limit of  its  range.    Turn  the  system  OFF, manually   adjust   the   propeller   RPM into   synchronization,   then   turn   the synchrophaser ON. The  propeller  synchrophaser  may  be  used  on  takeoff  at the pilot's option. b. Synchrophaser  Control  Box.    The  control  box, located   forward   of   the   pedestal,   converts   pulse   rate differences   into   correction   commands.      Differences   in pulse  rate,  and/or  propeller  position,  cause  the  control box   to   vary   the   voltage   in   the   primary   governor   coil, which  in  turn  increases  propeller  speed  until  the  correct speed    and    phasing    are    obtained.        The    system    is protected    by    a    5-ampere    circuit    breaker    placarded PROP  SYNC,  located  on  the  overhead  circuit  breaker panel (fig.  2-9). c. Synchroscope.      The   propeller   Synchroscope, located   on   the   pilot's   instrument   panel,   provides   an indication   of   synchronization   of   the   propellers.      If   the right propeller is operating at a higher RPM than the left, a   black   and   white   cross   pattern   spins   in   a   clockwise direction.    Left,  or  counterclockwise,  rotation  indicates  a higher  RPM  of  the  left  propeller.    This  instrument  aids the   pilot   in   obtaining   complete   synchronization   of   the propellers. 2-46.  PROPELLER LEVERS. Two PROP levers on the control pedestal (fig.      2-14),   placarded   PROP,   are   used   to   regulate propeller    speeds.        Each    lever    controls    a    primary governor,  which  acts  to  regulate  propeller  speeds  within the  normal  operational  range.    The  full  forward  position of  the  levers  is  placarded   TAKEOFF,  LANDING,  AND REVERSE  -  HIGH  RPM.      The   full   aft   position   of   the levers is placarded  FEATHER.    When  a  lever  is  placed at  HIGH  RPM,  the  propeller  may  attain  a  static  RPM  of 1700 depending upon POWER lever position.  As a lever is   moved   aft,   passing   through   the   propeller   governing range,    but    stopping    at    the    feathering    detent,    the propeller    RPM    will    correspondingly    decrease    to    the lowest limit (approximately 1200 RPM).  Moving a  PROP lever  aft  past  the  detent  into   FEATHER  will  feather  the propeller. 2-47.  PROPELLER REVERSING. CAUTION Do    not    move    the    POWER    levers below  the  flight  idle  gate  unless  the engine   is   running.      Damage   to   the reverse linkage mechanisms will occur. Propeller   reversing   on   deteriorating surfaces  may  cause  engine  FOD  and propeller erosion from reversed airflow. CAUTION Consideration  should  be  given  to  not reversing  propellers  when  operating in    snow    or    dusty    conditions,    to prevent obscuring the pilot's vision. 2-55