Propeller Overspeed Governor

T-34C AIRCRAFT SYSTEMS FAMILIARIZATION

CHAPTER EIGHT

WORKBOOK

804.

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.

Fuel Topping

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.

Torque Limiter

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