Propeller Control for Turbo-Prop Engines

Propeller Control for Turbo-Prop Engines By Dan Ankarlo April 1999 Controlling propeller RPM is only one of the functions of a governor installed on the turboprop engine. Indeed, other more complicated functions are performed by the governor as...

Troubleshooting service difficulties
Now that we have a basic understanding of the governing system, we can proceed with troubleshooting techniques. Woodward Governor of Rockford, Illinois has long been an industry leader when it comes to engineering governors. Governor failure in the field is a very rare occurrence and often the governor is cited for a problem that is caused by another component. In typical cases, governor malfunction is caused by neglect or by incorrect adjustments made in the field. After overhaul, governors are recalibrated to very exacting standards on the test bench. RPM tolerances are usually plus or minus 5 rpm; flow tolerances are specified to the tenth of a quart per minute; and air section bleed rates are measured in inches of water differential pressure. It is impossible to accurately adjust these settings in the field without proper calibration equipment. A good substitute, if a problem exists on a multi-engine aircraft, is to swap the governors from side to side. If the suspected problem follows the governor then the governor can be sent in for investigation. However, if the problem stays with the engine, other components (i.e. the propeller or fuel control unit) should be investigated. This preliminary step can save money and valuable time either by confirming the problem or ruling the governor out.

Propeller surging
The most common and least understood problem reported is that of propeller RPM surging. Surging causes can vary widely. In extreme cases of radical surge (100-400 rpm) the installation should be checked for airframe approval. Our facility has seen a number of instances where the wrong part number overspeed governor was installed and would cause interference with the primary governor (CSU). This happens as a result of the overspeed governor diverting oil prior to the primary CSU reaching 100% rpm. When the OSG diverts oil, the rpm decreases and the primary CSU attempts to increase rpm again. As rpm increases, the overspeed governor once again diverts oil to complete the cycle and thus the surging problem is exaggerated.

The PT-6 governor controls several parameters simultaneously.

Minor rpm surges can be caused by a number of factors. On occasion a governor is sent in for surging problems and it is discovered that the metering valve has become magnetized. The magnetic attraction between the metering valve, flyweights, and speed bias coil can cause small changes in valve position triggering a surge condition. The most reasonable explanation for a magnetized valve is an unreported lightning strike. Other causes for minor surging include flattened spots on the flyweights or torque fluctuations induced by the fuel control unit. In an attempt to maintain a constant rpm the governor will exaggerate these types of problems and a surging condition will result. The first step in isolating a surging problem is to check to see if the condition exists with or without the synchronizing system turned on. In all cases, a verification should be done with the synchronizer off. The next step is to swap the governors side to side to see if the problem follows the suspected unit. If the governors of a multi-engine aircraft are changed side to side and the problem remains on the same side of the aircraft, then other causes should be investigated. Often times torque fluctuations caused by the FCU will keep the governor busy attempting to stabilize rpm and a surge will result. In the case of a high time governor, the most likely causes for surging would be either worn flyweights or low pressure relief. Low pressure relief settings coupled with a high time (and high internal leakage) engine can result in surging as the governor attempts to maintain a constant rpm. There have been cases where a surging governor on one side of an aircraft was changed with the opposite governor and the surging condition disappears. This can be attributed solely to engine "gremlins."

Overspeed governor problems
Another commonly reported problem is inability to attain 100% prop rpm after a ground test is performed on the overspeed governor. In this case, the reset solenoid is stuck in the reset position and prop rpm is limited to typically 92%. The first check is to disconnect the cannon plug to the reset solenoid to ensure no electrical power is being supplied to the solenoid inadvertently. Tapping lightly on the solenoid will sometimes cause it to return to normal position. Many previously canceled flights have been continued after a technician has tapped on a reset solenoid allowing the prop to return to 100%. If the solenoid is stuck in reset mode due to foreign contamination inside, the governor will have to be removed and sent to a repair facility. Field repair or cleaning of a solenoid internally is not permitted.

 The reset eccentric is a sensitive setting and field adjustment is not recomm ended. Tapping lightly on the reset solenoid will sometimes cause it to return to normal position.

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