TM 55-1510-220-10 Section III.  ENGINES AND RELATED SYSTEMS 2-17.  ENGINES. The aircraft is powered by two .PT6A-41 turboprop engines  (fig.    2-11).'The  engine  has  a  three  stage  axial, single  stage  centrifugal  compressor,  driven  by  a  single stage  reaction  turbine.    The  power  turbine,  a  two  stage reaction   turbine,   counter-rotating   with   the   compressor turbine,  drives  the  output  shaft.    Both  the  compressor turbine    and    the    power    turbine    are    located    in    the approximate    center    of    the    engine    with    their    shafts extending  in  opposite  directions.    Being  a  reverse  flow engine, the ram air supply enters the lower portion of the nacelle and is drawn in through the aft protective screens. The  air  is  then  routed  into  the  compressor.    After  it  is compressed,   it   is   forced   into   the   annular   combustion chamber,  and  mixed  with  fuel  that  is  sprayed  in  through 14  nozzles  mounted  around  the  gas  generator  case.    A capacitance  discharge  ignition  unit  and  two  spark  igniter plugs are used to start combustion.  After combustion, the exhaust  passes  through  the  compressor  turbine  and  two stages   of   power   turbine   then   is   routed   through   two exhaust ports near the front of the engine.  A pneumatic fuel  control  system  schedules  fuel  flow  to  maintain  the power    set    by    the    gas    generator    power    lever.    The accessory  drive  at  the  aft  end  of  the  engine  provides power to drive the fuel pumps, fuel control, the oil pumps, the   refrigerant   compressor   (right   engine),   the   starter- generator,  and  the  turbine  tachometer  transmitter.    The reduction  gearbox  forward  of  the  power  turbine  provides gearing    for    the    propeller    and    drives    the    propeller tachometer transmitter, the propeller overspeed governor, and the propeller governor. 2-18.  ENGINE COMPARTMENT COOLING. The    forward    engine    compartment    including    the accessory  section  is  cooled  by  air  entering  around  the exhaust   stack   cutouts,   the   gap   between   the   propeller spinner   and   forward   cowling,   and   exhausting   through ducts in the upper and lower aft cowling. 2-19.  AIR INDUCTION SYSTEMS - GENERAL. Each engine and oil cooler receives ram air ducted from an air scoop located within the lower section of the forward   nacelle.      Special   components   of   the   engine induction  system  protect  the  power  plant  from  icing  and foreign object damage. 2-20.  FOREIGN OBJECT DAMAGE CONTROL. The engine has an integral air inlet screen designed to     obstruct     objects     large     enough     to     damage     the compressor. 2-21.  ENGINE ICE PROTECTION SYSTEMS. a.  Inertial Separator. CAUTION After    the    ice    vanes    have    been manually   extended,   they   may   be mechanically   actuated   only.      No electrical    extension    or    retraction shall   be   attempted   as   damage   to the actuator may result.  Linkage in the nacelle area must be reset prior to operation of the electric system. An   inertial   separation   system   is   built   into   each engine air inlet to prevent moisture particles from entering the    engine    inlet    plenum    under    icing    conditions.        A movable  vane  and  a  bypass  door  are  lowered  into  the airstream  when  operating  in  visible  moisture  at  5°C  or colder,     by     energizing     electrical     actuators     with     the switches,    placarded    ICE    VANE    RETRACT    EXTEND, located  on  the  overhead  control  panel.    A  mechanical backup system is provided, and is actuated by pulling the T-handles  just  below  the  pilot's  subpanel  placarded  ICE VANE  No.1  ENG  No.2  ENG.    Decrease  airspeed  to  160 knots   or   less   to   reduce   forces   for   manual   extension. Normal airspeed may then be resumed. (1) The vane deflects the ram airstream slightly downward  to  introduce  a  sudden  turn  in  the  airstream  to the engine, causing the moisture particles to continue on undeflected,  because  of  their  greater  momentum,  and  to be discharged overboard. (2) While in the icing flight mode, the extended position of the vane and bypass door is indicated by green annunciator lights, No.1 VANE EXT and No.2 VANE EXT. (3) In  the  non-ice  protection  mode,  the  vane and  bypass  door  are  retracted  out  of  the  airstream  by placing  the  ice  vane  switches  in  the  RETRACT  position. The  green  annunciator  lights  will  extinguish.    To  assure adequate oil cooling, retraction should be accomplished at 15°C and 2-21