Aircraft Maintenance

Overspeed conditions are the result of unloading the propeller blades as in the case of a descent, or the pilot selecting a lower prop rpm. When an overspeed condition is sensed, the increased flyweight force overcomes speeder spring force and the pilot valve is moved to a position whereby oil is allowed to drain from the propeller. As a result, blade angle increases (due to propeller counterweight force) and rpm is thus reduced.

Beta mode
Propeller beta mode is defined as any blade angle between idle and full reverse. In beta mode, the pilot can manually adjust prop blade angle to control reverse thrust power during aircraft deceleration. A design problem must be overcome to allow the pilot to manually select blade angle. The governor must be held in an underspeed condition to eliminate interference between pilot command and speeder spring command.

Output voltage on governor is measured peak-to-peak.

To solve this problem, governors that are installed on Garrett engines have a piston assembly attached to the speeder spring that can alter speeder spring pressure. The governor control shaft runs through the middle of this piston. On one side of the piston, there are spring tension washers and on the other side of the piston, high-pressure oil (200 psi) is supplied to act against the force of the spring washers. In normal operation, the presence of this high-pressure oil will compress the spring tension washers. Beta mode is entered as the propeller pitch control uncovers ports allowing the oil to drain away. As a result, the spring tension washers move the piston into a position that causes an increased speeder spring force. This is a simulated underspeed condition, which will not allow the governor to interact with the propeller until approximately 107 percent of rated rpm. In this way, the prop is allowed to travel through the zero thrust range and into reverse. Actual control of the propeller at this point is accomplished by the pitch control sleeve that surrounds the beta tube in the engine. It is a means of mechanically controlling propeller blade angle and rpm without interaction from the governor. As propeller blade angle approaches the desired setting, pressure ports in the beta tube are uncovered, which will maintain the blade angle. When further travel into reverse is desired, the power lever resets the sleeve in the pitch control, allowing oil to move the propeller deeper into reverse.

Synchronizing On multi-engine applications, synchronizing and synchrophasing between two engines is accomplished through the use of a speed bias coil inside the governor. The coil is located between the pilot valve and the spinning flyweights. When energized, the coil exerts a magnetic force on the flyweights causing the pilot valve to move in small amounts. The synchronizing computer receives speed signals from both engines and varies the coil voltage of each governor to attain synchronization between both engines.

This is a typical Garrett engine governor with two part numbers.

Look Mom, no linkage
Perhaps the most interesting Garrett engine governor is found on the TPE-331-8 (or -10) as installed on the Cessna Conquest II. This particular governor (P/N 897160-X) has no mechanical linkage for actuation. The fuel control computer and an actuator in the governor accomplish control of engine speed. The actuator in the governor is actually a torque motor that can alter speeder spring pressure and, thus, rpm. The governor has an integral speed pickup that sends rpm indications to the fuel control computer. The fuel control computer compares the actual engine rpm with the rpm selected by the pilot. It then sends power to the torque motor to change speeder spring pressure and rpm as required. In a similar fashion to other Garrett engine governors, beta control is achieved by simulating an underspeed condition within the governor. When ground idle is chosen by the pilot, a signal is sent to the governor that resets the piston/ speeder spring to 105 percent of rated rpm. This allows the pilot to control blade angle without interference from the governor.

Troubleshooting
As is the case with any multi-engine aircraft, isolating suspected governor problems can be easily accomplished by swapping the governors side-to-side on the airframe.