Section III. ENGINES AND RELATED SYSTEMS
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
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
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
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
2-20. ENGINE ICE PROTECTION SYSTEMS.
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.
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.
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.