Section III. ENGINES AND RELATED SYSTEMS
The aircraft is powered by two PT6A-67
turboprop engines, rated at 1200 SHP each (fig. 2-13).
Each engine is equipped with a hydraulically controlled,
reversible, constant-speed, four-bladed, full-feathering
propeller. The engines are reverse-flow free turbines,
and each employs a four-stage axial compressor and a
single-stage centrifugal compressor in combination,
driven by the gas generator turbine. The gas generator
turbine and the two power turbines are in line and have
opposite rotations. The power turbines are connected
through planetary reduction gearing to a flanged
propeller shaft. The oil tank, filler cap and dipstick are
an integral part of the 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 the air 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
(fig. 2-14). The accessory drive at the aft end of the
engine provides power to drive the fuel pumps, fuel
control, oil pump, 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 which enters around
the exhaust stack cutouts and through the gap between
the propeller spinner and forward cowling, and exhausts
through louvers in the upper forward and aft cowling.
2-18. AIR INDUCTION SYSTEMS - GENERAL.
Each engine and oil cooler receives ram air
ducted from separate air inlets located within the lower
section of the forward nacelle. Induction system
components protect the power plant from icing and
reduce the possibility of 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
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 and #2 ICE
VANE CONTROL - ON, located on the overhead control
panel (fig. 2-15). The system incorporates an electrical
back-up system which operates identically to the main
system. The back-up ice vane system is controlled by
two switches placarded #1 and #2 ICE VANE POWER
SELECT - MAIN - STANDBY, located on the overhead
control panel (fig. 2-15). If the main system fails, placing
the switch in the STBY position will allow use of the
back-up system. Electrical protection is provided
through two 5-ampere circuit breakers placarded ICE
AUXILIARY, located on the overhead circuit breaker
b. Engine Ice Protection Systems Operation.
The vane deflects the ram airflow slightly downward to
introduce a sudden turn in the airflow to the engine.
Because of their greater momentum the particles
continue undeflected and are discharged overboard.
Once the ice vane system is actuated, the
extended position of the vane and bypass door is
indicated by green annunciators, placarded #1 VANE
EXT and #2 VANE EXT, located on 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 and/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 #1 VANE EXT or #2 VANE
EXT annunciator(s) will illuminate.
c. Engine Anti-Ice System.
(1) Air inlet. A small duct, which faces into
the exhaust flow in the left exhaust stack of each engine,
diverts a small portion of the engine exhaust gases to
the engine air inlet lip. The gases are circulated through
the engine air inlet lip and then exhausted through a duct
in the right exhaust stack. The continuous flow of hot
engine exhaust gases heats the engine air inlet lip,
preventing the formation of ice. Two switches,
placarded ENG INLET LIP HEAT #1 and #2 (located on
the overhead control panel, fig. 2-15), operate solenoid
valves in the lip heat