TM 1-1510-262-10
SECTION III. ENGINES AND RELATED SYSTEMS
2-16. ENGINES.
Two PT6A-67 turboprop engines, rated at 1200 SHP (Fig. 2-17 and Fig. 2-18), power the aircraft. Each engine
is equipped with a hydraulically controlled, reversible, constant-speed, four-bladed full feathering propeller. The
engines are reverse low, free turbines, employing four-stage axial compressors and single-stage centrifugal com-
pressors 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 langed propeller shaft. The oil tanks, iller 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 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, and then is routed through two exhaust
ports near the front of the engine. A pneumatic fuel control system schedules fuel low 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, oil pumps, 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 and #2 ICE VANE CONTROL - ON , located on the overhead control panel (Fig. 2-13). The system
incorporates an electrical back-up system which operates identically to the main system. The back-up system is
controlled by two switches placarded #1 and #2 ICE VANE POWER SELECT MAIN STBY , located on the
overhead control panel (Fig. 2-13). 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 on the overhead circuit breaker panel.
(1) The vane delects the ram airlow slightly downward to introduce a sudden turn in the airlow to the
engine, causing the moisture particles to continue on undelected, because of their greater momentum, and be
discharged overboard.
(2) Once the ice vane system is actuated, green annunciators indicate the extended position of the vane
and bypass door, placarded #1 VANE EXTEND and #2 VANE EXTEND 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 or
#2VANE 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
EXTEND or #2 VANE EXTEND annunciator(s) will illuminate.
