TM 55-1510-222-10 Section III.  ENGINES AND RELATED SYSTEMS 2-16.  ENGINES. The aircraft is powered by two PT6A-67 turboprop engines, rated at 1100 SHP (fig. 2-10).  Each engine is equipped    with    a    hydraulically    controlled,    reversible, constant-speed,   four-bladed   full   feathering   propeller. The  engines  are  reverse  flow,  free  turbines,  employing four-stage axial compressors and single-stage centrifugal  compressors  in  combination,  driven  by  the gas  generator  turbine.    The  gas  generator  turbine  and the  free  power  turbine  are  in  line  with  each  other  and have     opposite     rotations.          The     power     turbine     is connected   through   planetary   reduction   gearing   to   a flanged  propeller  shaft.    The  oil  tanks,  filler  cap  and dipstick are an integral part of the engine. 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  turbines,  then  is  routed  through  two  exhaust ports  near  the  front  of  the  engine.    A  pneumatic  fuel control system schedules fuel flow to maintain 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, oilpumps, refrigerant compressor   (right   engine),   starter/generator,   and   the tachometer  generator.    The  reduction  gearbox  forward of  the  power  turbine  provides  gearing  for  the  propeller and    drives    the    propeller    tachometer    generator,    the propeller overspeed governor, and the propeller primary governor. 2-17.  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. 2-18.  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. 2-19.  FOREIGN OBJECT DAMAGE CONTROL. The    engine    has    an    integral    air    inlet    screen designed  to  obstruct  objects  large  enough  to  damage the compressor. 2-20.  ENGINE ICE PROTECTION SYSTEMS. a. Inertial   Separator.      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 #1 & #2 ICE  VANE  POWER  SELECT  MAIN  STBY,  located  in the overhead control panel.  The system incorporates an electrical  back-up  system  which  operates  identically  to the  main  system.    If  the  main  system  fails,  placing  the switch  in  the  STBY  position  will  actuate  the  back-up system. Electrical protection is provided through two 5- ampere  circuit  breakers  placarded  ICE  VANE  CONTR MAIN  and  ICE.   VANE  CONTR  AUXILIARY,  located  in the overhead circuit breaker panel. (1) The vane deflects the ram airflow slightly downward to introduce a sudden turn in the airflow to the engine,  causing  the  moisture  particles  to  continue  on undeflected,  because  of  their  greater  momentum,  and be discharged overboard. (2) Once the ice vane system is actuated, the extended   position   of   the   vane   and   bypass   door   is indicated  by  green  annunciators,  placarded  #1  VANE EXTEND    and    #2    VANE    EXTEND    located    in    the caution/advisory panel.  If for any reason the vane(s) do not  attain  the  selected  position  within  33  seconds,  an amber #1 VANE FAIL or #2 VANE FAIL annunciator(s) illuminates on the caution/ advisory panel.  In this event, the  appropriate  #1  or  #2  ICE  VANE  POWER  SELECT switch should be placed in the STBY position.  Once the vane    is    successfully    positioned,    using    the    standby (STBY) system, the amber annunciator(s) will extinguish and   the   applicable   green   #'l   VANE   EXTEND   or   #2 VANE EXTEND annunciator(s) will illuminate. 2-20