TM 55-1510-220-10 opposite  engine  becomes  extinguished.    If  torque  drops further,  to  a  reading  between  9  and  14%,  oil  is  dumped from   the   servo   of   the   affected   propeller   allowing   a feathering  spring  to  move  the  blades  into  the  feathered position. Feathering also causes the No.1 AUTOFEATHER   or   No.2   AUTOFEATHER   annunciator light of the feathered propeller to extinguish.  At this time, both the No.1 AUTOFEATHER and No.2 AUTOFEATHER  lights  are  extinguished,  the  propeller  of the  defective  engine  has  feathered,  and  the  propeller  of the    operative    engine    has    been    disarmed    from    the autofeathering capability.  Only manual feathering control remains for the second propeller. b. Propeller Autofeather Switch.  Autofeathering is    controlled    by    an    AUTOFEATHER    switch    on    the overhead  control  panel  (fig.    2-12).    The  three-position switch  is  placarded  ARM,  OFF  and  TEST,  and  is  spring loaded  from  TEST  to  OFF.    The  ARM  position  is  used only during takeoff and landing.  The TEST position of the switch,    enables    the    pilot    to    check    readiness    of    the autofeather  systems,  below  88%  to  92%  N1,  and  is  for ground checkout purposes only. c. Autofeather   Lights.      Two   green   lights   on   the caution/advisory annunciator panel, placarded AUTOFEATHER No.1 and AUTOFEATHER No.2.  When illuminated, the lights indicate that the autofeather system is   armed.      Both   lights   will   be   extinguished   if   either propeller   has   been   autofeathered   or   if   the   system   is disarmed by retarding a power lever.  Autofeather circuits are  protected  by  one  5-ampere  circuit  breaker  placarded AUTO FEATHER, located on the overhead circuit breaker panel (fig.  2-27). 2-45.  PROPELLER GOVERNORS. Two governors, a constant speed governor, and an overspeed governor, control propeller RPM.  The constant speed governor, mounted on top of the reduction housing, controls   the   propeller   through   its   entire   range.      The propeller control lever operates the propeller by means of this   governor.      If   the   constant   speed   governor   should malfunction    and    request    more    than    2000    RPM,    the overspeed  governor  cuts  in  at  2080  RPM  and  dumps  oil from   the   propeller   to   keep   the   RPM   from   exceeding approximately  2080  RPM.    A  solenoid,  actuated  by  the PROP GOV TEST switch located on the overhead control panel (fig.  2-12), is provided for resetting the overspeed governor  to  approximately  1830  to  1910  RPM  for  test purposes.      If   the   propeller   sticks   or   moves   too   slowly during     a     transient     condition     causing     the     propeller governor   to   act   too   slowly   to   prevent   an   over   speed condition,  the  power  turbine  governor,  contained  within the   constant   speed   governor   housing,   acts   as   a   fuel topping   governor.      When   the   propeller   reaches   2120 RPM,  the  fuel  topping  governor  will  limit  fuel  flow  to  the gas  generator,  thereby  keeping  the  propeller  below  2120 RPM.    During  operation  in  the  reverse  range,  the  power turbine  governor  is  reset  to  approximately  95%  of  the propeller RPM be< fore the propeller reaches a negative pitch angle.  This insures 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    therefor,    will    always    sense    an    underspeed condition  and  direct  oil  pressure  to  the  propeller  servo piston  to  permit  propeller  operation  in  beta  and  reverse ranges. 2-46.  PROPELLER TEST SWITCHES. Two  two-position  switches  on  the  overhead  control panel .  2-12), are provided for operational testing of the propeller  systems.    Placarding  above  the  switches  reads PROP GOV TEST.  Each switch controls test circuits for the   corresponding   propeller.      In   the   test   position,   the switches are used to test the function of the corresponding overspeed    governor.        Refer    to    Chapter    8,    for    test procedure.  Propeller test circuits are protected by one 5- ampere circuit breaker placarded PROP GOV located on the overhead circuit breaker panel (fig.  2-27). 2-47.  PROPELLER SYNCHROPHASER. a. Operation. The propeller synchrophaser automatically   matches   the   RPM   of   the   right   propeller (slave   propeller)   to   that   of   the   left   propeller   (master propeller) and maintains the blades of one propeller at a predetermined   relative   position   with   the   blades   of   the other propeller.  To prevent the right propeller from losing excessive RPM if the left propeller is feathered while the synchrophaser   is   on,   the   synchrophaser   has   a   limited range    of    control    from    the    manual    governor    setting. Normal     governor     operation     is     unchanged     but     the synchrophaser   will   continuously   monitor   propeller   RPM and  reset  the  governor  as  required.    A  magnetic  pickup mounted    in    each    propeller    overspeed    governor    and adjacent  to  each  propeller  deice  brush  block  transmits electric   pulses   to   a   transistorized   control   box   installed forward  of  the  pedestal.    The  right  propeller  RPM  and phase will automatically be -adjusted to correspond to the left.  To change RPM, Change 2   2-40