Section III. ENGINES AND RELATED SYSTEMS
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
2-21. ENGINE ICE PROTECTION SYSTEMS.
a. Inertial Separator.
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.
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.
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.
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