TPE-331 Prop Governing Systems
Theory and Troubleshooting Tips
by Dan Ankarlo
Fundamental differences between Garrett (AlliedSignal) and Pratt & Whitney turbine engines dictate differences in propeller governor function. Whereas the Pratt & Whitney PT6A is a free turbine design; the Garrett engine is direct-drive from the engine to the propeller. This would seem to make the job easier for a governor installed on a Garrett engine than it would a Pratt & Whitney design; however, some special issues must be dealt with. An examination of the basic governor functions will reveal critical differences between the two models.
AlliedSignal and Woodward Governor both manufacture governors installed on Garrett TPE-331 engines. It is interesting to note that the AlliedSignal model of governor contains parts manufactured by Woodward (the base and body sections), and each manufacturer assigns its own part number to the assembly. This can lead to confusion regarding part numbers in the field.
For example, an AlliedSignal governor can be assigned three different part numbers, which are installed on two different dataplates. One of the part numbers is a base/body assembly number as assigned by Woodward. The second part number is the Woodward number for the complete governor assembly. The third part number is the corresponding AlliedSignal number for the assembly. In addition, a governor installed on a TPE-331 engine can also apparently have two different serial numbers. One serial will be for the base/body assembly, and the other serial will be for the completed assembly. Regardless of how many different numbers are stamped into the dataplate(s), both manufacturers' units operate in fundamentally the same way.
Garrett engine speed is controlled directly by prop blade angle. High blade angles (high load) equate to lower engine rpm, while lower blade angles (low load) equate to higher engine rpm. The governor is supplied with engine oil (typically at 95 psi). The governor contains an internal pump and pressure relief system that boosts this pressure to approximately 485 psi and supplies it at capacities of up to 10 quarts per minute. A set of spinning flyweights inside the governor sense engine rpm and direct the pilot valve to move in reaction to changes in rpm other than the desired setting.
In both Garrett and Pratt applications, servo oil (oil supplied to the prop piston) moves the blades toward the low blade angle, or high rpm position. This is designed so that a loss of engine oil pressure will automatically allow the propeller to move toward the feathered position (through the use of prop blade counterweights and feathering spring). The pilot valve position therefore determines whether oil will be ported to the propeller (increasing rpm) or to drain from the propeller (decreasing rpm). A speeder spring directly connected to the flyweights and pilot valve determine the position of the valve and thus a corresponding rpm. Speeder spring pressure is altered through the governor control shaft and associated linkage to the cockpit.
During "on speed" condition, the forces of the spinning flyweights equal the force of the speeder spring, and, as a result, no oil is ported to or from the propeller. A constant rpm results until an upsetting force changes the balanced state. It is important to realize that in reality, the governor is continually porting oil to the propeller to compensate for internal leakage between the gearcase and propshaft.
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