T-34C AIRCRAFT SYSTEMS FAMILIARIZATION
PROPELLER OVERSPEED GOVERNOR
The propeller overspeed governor is located on the reduction gearbox (Figure 8-2). It limits
propeller RPM should the primary propeller governor fail. Constructed in the same manner as
the primary governor with speeder spring, flyweight assembly, and pilot valve, and driven by
gears from the propeller shaft, the overspeed governor constantly monitors propeller RPM.
Should the primary propeller governor fail and the prop begin to overspeed, the overspeed
governor will ACTIVATE at 2332 RPM by detouring high pressure oil back to the reduction
gearbox. The overspeed governor should then govern the propeller at 2332 RPM. Ground test
of the overspeed governor is provided by a test valve actuated by a 28-VDC solenoid. A push-
button, located on the TEST panel at the bottom of the instrument panel in each cockpit,
activates the solenoid and opens the test valve, allowing high pressure oil to reset the normal
overspeed setting to 10% below normal. A properly functioning overspeed governor is indicated
during ground test by the propeller RPM stabilizing at 1950-2150 RPM with engine torque at
approximately 400-500 ft-lbs. Releasing the TEST button resets the overspeed governor to
normal operation and the propeller RPM rises toward the normal governing RPM of 2200.
The fuel topping section of the primary propeller governor is located in a chamber at the rear of
the governor. Its purpose is to control power turbine overspeed should there be a failure or
malfunction of the propeller pitch change assembly. A Py air line from the FCU connects to a
valve in the fuel topping section. The valve is spring-loaded to the closed position. Mechanical
linkage from the primary propeller governor flyweight assembly opens the valve when the
propeller reaches 2398 RPM. The resulting drop in Py air pressure causes the FCU to reduce fuel
flow to the combustion chamber and turbine RPM decreases. Surging of propeller RPM may
occur until corrective action by the pilot is taken.
The torque limiter is located on the reduction gearbox. The torque limiter limits engine power
output to 550 SHP (1315 ft-lbs. of torque). An air line from the FCU routes Py air to one side of
a shuttle valve inside the torque limiter. The valve is spring-loaded to shut off the Py air line.
Torquemeter oil pressure from the torquemeter in the reduction gearbox is routed to the other
side of the shuttle valve. When torquemeter oil pressure corresponds to 1315 ft-lbs. torque, the
valve is moved to open an orifice which bleeds Py air pressure and reduces fuel flow. Engine
power output decreases back to 1315 ft-lbs. torque and the spring closes the valve. Surging of
propeller RPM may occur until corrective action is taken by the pilot.
Emergency Power Lever
The emergency power lever allows manual control of the FCU in the event of a pneumatic
(P3/Py) air line leak, called "rollback." Should a leak occur, the drop in air pressure will cause
the engine to decelerate to below idle RPM and the FCU will no longer respond to the PCL. The
EPL mechanically forces the FCU to deliver fuel regardless of the pneumatic input. The normal
position for the EPL is DISCONNECT, where it is held in a detent by a spring. When pulled up
POWER PLANT CONTROL SYSTEM 8-5