ATA 61-08-54
Page 8
Original Issue
2021-08-13
4.
DESIGN AND OPERATION INFORMATION
The Avia Propeller aircraft overspeed governors P-8S( )-( ) are base mounted centrifugal overspeed
governors for use with single acting constant speed propeller control systems on turboprop engines.
The propeller overspeed governor is located on the reduction gearbox. 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 given speed setting by detouring high pressure oil back to the reduction
gearbox. The overspeed governor should then govern the propeller at given speed. Ground test of the
overspeed governor is provided by a test valve actuated by a
28-VDC solenoid. A pushbutton, located on the TEST panel at the bottom of the instrument panel in
the cockpit, activates the solenoid and opens the test valve, allowing high pressure
oil to reset the normal overspeed setting to given percentage below normal. A properly functioning
overspeed governor is indicated during ground test by the propeller RPM stabilizing at adjusted lower
propeller speed. Releasing the TEST button resets the
overspeed governor to normal operation and the propeller RPM rises toward the normal governing
RPM.
UNDERSPEED
If the primary propeller governor works properly, the overspeed governor is in underspeed condition.
Propeller speed setting of the overspeed governor is over the primary propeller governor speed setting.
Underspeed condition means, that the force of the preloaded speeder spring (representing requested
speed) is higher than the force from rotating flyweights (representing actual speed) and thus the pilot
valve is forced down (see diagram) covering the control ports in the drive shaft. It avoids pressure oil
to flow from the pitch changing mechanism. The overspeed governor doesn´t enter in control loop of
the primary propeller governor.
