TM  55-1510-221-10 2-43.  FEATHERING  PROVISIONS. Both  manual  and  automatic  propeller  feathering systems   are   provided.   Manual   feathering   is   accom- plished  by  pulling  the  corresponding  propeller  lever aft  past  a  friction  detent.  To  unfeather,  the  propeller lever  is  pushed  forward  into  the  governing  range.  An automatic   feathering   system,   will   sense   loss   of torque   and   will   feather   an   unpowered   propeller. Feathering  springs  will  feather  the  propeller  when  it is  not  turning. a.   Automatic   Feathering.   The   automatic feathering  system  provides  a  means  of  immediately dumping  oil  from  the  propeller  servo  to  enable  the feathering  spring  and  counterweights  to  start  feath- ering  action  of  the  blades  in  the  event  of  an  engine failure.  Although  the  system  is  armed  by  a  switch  on the   overhead   control   panel,   placarded   AUTOFEA- THER  -  ARM  -  OFF  -  TEST,  the  completion  of  the arming  phase  occurs  when  both  power  levers  are advanced above 90% N₁  - at which time both indica- tor  lights  on  the  caution/advisory  annunciator  panel indicate  a  fully  armed  system.  The  annunciator panel   lights   are   green   and   are   placarded   No.1 AUTOFEATHER    (left engine) and No.2 AUTOFEATHER   (right   engine).   The   system   will remain  inoperative  as  long  as  either  power  lever  is retarded  below  90%  N₁   position,  unless  TEST  posi- tion  of  the  AUTOFEATHER  SWITCH  is  selected  to disable  the  power  lever  limit  switches.  The  system  is designed  for  use  only  during  takeoff  and  landing  and should  be  turned  off  when  establishing  cruise  climb. During   takeoff   or   landing,   should   the   torque   for either  engine  drop  to  an  indication  between  16  - 21%,  the  autofeather  system  for  the  opposite  engine will  be  disarmed.  Disarming  is  confirmed  when  the No.1  AUTOFEATHER  or  No.2  AUTOFEATHER annunciator  light  of  the  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.  Feath- ering  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  AUTOFEA- THER   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.  Autofeather- ing  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% N_l , and is for ground checkout purposes only. c. Autofeather  Lights.  Two green lights on the caution/advisory   annunciator   panel   are   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  autofea- thered  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.  2-26). 2-44.   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  pro- peller  governor  to  act  too  slowly  to  prevent  an  over- speed   condition,   the   power   turbine   governor,   con- tained   within   the   constant   speed   governor   housing, acts  as  a  fuel  topping  governor.  When  the  propeller reaches  106%  of  N₂   RPM,  the  fuel  topping  governor limits the fuel flow to the gas generator, reducing N₁ RPM,  which  in  turn  prevents  the  propeller  RPM from   exceeding   approximately   2120   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  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  therefore,  will  always  sense an   underspeed   condition   and   direct   oil   pressure   to the  propeller  servo  piston  to  permit  propeller  opera- tion  in  beta  and  reverse  ranges. 2-45.  PROPELLER  TEST  SWITCHES. Two  two-position  switches  on  the  overhead  con- trol   panel   (fig.   2-12),   are   provided   for   operational testing   of   the   propeller   systems.   Placarding   above 2-39