TM 55-1510-222-10 N₁RPM,    which    in    turn    prevents    the    propeller    from exceeding  approximately  1802  RPM.    During  operation in the reverse range, the power turbine governor is reset to   approximately   95%   of   propeller   RPM   before   the propeller  reaches  a  negative  pitch  angle.  This  ensures that  the  engine  power  is  limited  to  maintain  a  propeller RPM of somewhat less than that of the constant speed governor     setting.          The     constant     speed     governor therefore,   will   always   sense   an   underspeed   condition and  direct  oil  pressure  to  the  propeller  servo  piston  to permit propeller operation in beta and reverse ranges. 2-44.  LOW PITCH STOP. Low  pitch  propeller  position  is  determined  by  a mechanically  monitored,  hydraulic  low  pitch  stop.  The propeller   servo   piston   is   connected   by   four   spring- loaded  sliding  rods  to  the  Beta  collar  mounted  behind the  propeller.    A  carbon  brush  block  riding  in  the  Beta collar  transfers  the  movement  of  the  collar  through  the propeller   reversing   lever   to   the   Beta   valve   of   the governor.    The  initial  forward  motion  of  the  Beta  valve from  its  rigged  position  blocks  off  the  flow  of  oil  to  the propeller.      Further   motion   dumps   the   oil   from   the propeller    into    the    reduction    gear    box    sump.        A mechanical  stop  limits  the  forward  motion  of  the  Beta valve.  Rearward movement of the Beta valve from its rigged position does not affect normal propeller control. When the propeller is rotating at a speed lower than that selected  on  the  governor,  the  governor  pump  provides oil  pressure  to  the  servo  piston  decreasing  pitch  of  the propeller  blades  until  the  feedback  of  motion  from  the Beta collar pulls the Beta valve into a position blocking the supply of oil to the propeller, thus preventing further pitch changes. 2-45.  GROUND FINE. CAUTION Propeller speeds below 1000 rpm are not  authorized,  unless  the  propeller is feathered. Lifting the power levers and moving them aft, past the  flight  idle  stop,  will  place  the  power  levers  into  the ground  fine  position.    Approximately  half  way  back  to the   ground   fine   gate,   a   mechanical   linkage   at   the propeller governor will begin to bleed P_yair from the fuel control unit, provided the propeller levers are positioned to the feather detent.  This results in a decrease in both engine   N_Itorque   and   propeller   rpm.   With   the   power levers at the ground fine gate, engine N₁should be within the range of 62% to 67%, and propeller rpm should not be less than 1000 rpm. 2-46. PROPELLER AUTOFEATHER TEST SWITCHES. A switch in the overhead control panel (fig.  2-13) is    provided    for    operational    testing    of    the    propeller systems.  The AUTOFEATHER TEST OFF ARM switch is  used  to  ARM  the  autofeather  function,  and  allow  for an  operational  ground  test.    The  TEST  position  of  the switch,   enables   the   pilot   to   check   readiness   of   the autofeather systems, below 89% N₁. 2-47.  PROPELLER SYNCIIROPHASER. 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 in the pilot's instrument panel, 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  between  them  for  pulse  rate  and relative    position.        Differences    in    pulse    rate    and/or propeller  position  causes  the  control  box  to  vary  the voltage   in   the   primary   governor   coil,   which   in   turn, increases  propeller  speed  until  the  correct  speed  and phasing is 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   type   system.      There   is   a   limited   range   for synchronizing,  called  the  "holding  range"  which  serves to allow one propeller to be feathered without the other decreasing any appreciable RPM.  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 propeller 2-44