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...

Problems with engine power
Problems with loss or lack of engine power can be diagnosed easily on the aircraft. Lack of engine power can be caused by internal PY leaks within the governor, however, in most instances the governor is not at fault. Isolation of this type of problem is simple. Cap off PY air line and run the engine. Extreme caution should be exercised during engine run. Without the pneumatic air section functioning, the possibility exists of engine overspeed. If engine power still cannot be obtained with the PY capped off; the fuel control unit should be examined next. In many cases, the reset eccentric on the back side of the governor is adjusted by field technicians to adjust engine power in beta mode. Although done frequently, this practice is not recommended due to the sensitivity of the setting. During bench recalibration of a governor this setting is adjusted typically to 4 psi of differential airbleed across an orifice. There is no tolerance for this setting and it is impossible to accurately set it on the airframe. Our facility frequently receives governors for recalibration in which the reset air has been field adjusted to the point where the engine will not develop power in beta mode.

Synchronizing problems
Governor synchronizing problems can easily be investigated on the aircraft. The most common problems involve an internal short to the speed bias coil. The first step in troubleshooting these types of problems is to check output of the speed pickups. If the synchronizing box isn't receiving accurate signals then the system won't work. The next step is to ensure both governors are receiving electrical power. If they are, then checks should be made for electrical shorts on the governors themselves. Type II synchronizing governors have decals on the electrical cover that indicate which two of the three pins on the cannon plug are used. Check these pins against the cannon plug case for an electrical short.

There is an old saying that a teaspoon of oil can completely cover an airplane. Oil leaks at the front of the engine are commonly misdiagnosed as being governor leaks. International Governor was recently involved in a situation whereby the governor was blamed for a leak on a PT6-65 engine. The field technician wouldn't consider the possibility of the leak occurring elsewhere. After installation of five (yes five) different governors on the same engine; the technician finally conceded and took the time to investigate further. As it turned out, the source of the leak was a prop shaft seal. When investigating oil leaks, the easiest method is to isolate the governor. This can be done by wrapping the governor in a clear plastic bag. After engine run, if the outside of the bag is covered with oil and there is no oil inside the bag; the governor should be nonsuspect.

The most common type of oil leak on a PT6 governor is a beta valve leak. This can be isolated by wrapping the valve in masking tape and running the engine. As the beta valve moves in and out, of course, the tape will become crushed but the oil will pool inside and the leak can be verified. At International Governor, we have the most technologically advanced test stand available for PT6 governors. That is, the stand simulates the engine parameters through closed loop circuitry (a positive feedback system). Nothing, however, can actually duplicate the dynamics taking place on an actual engine. This is why it is so important to isolate an oil leak prior to sending a governor in for repair.

The decal on the electrical cover indicates which pins should have continuity.

Governor overhaul
The common factor in all governor service difficulty problems is that of neglect. Because of the high level of engineering and reliability of the governors; they tend to be overlooked. The recommended calendar TBO for all turbine governors (primary and overspeed) is six years. Woodward is continuously in the process of refining their governors by making engineering changes to them. Having the governors overhauled at the six year time interval will ensure that the most current configuration of governor will be re-installed on the engine for continued optimal performance. It should be noted that some circumstances will involve premature overhaul of the turbine engine prop governor. These cases are prop strike, lightning strike, and suspected metal in oil. It is not only mandatory by service bulletin to overhaul; but prudent as well. (Copies of Woodward SB 33580 can be obtained by calling International Governor at 303-464-0043). The internal components of a typical PT6 prop governor are fragile if mistreated. Severe and costly damage can be done by continued operation without overhaul after any of the above mentioned incidents. Our facility has seen a number of cases during this past year whereby governors have been run after such an incident. In several of the cases, the metering valve thrust bearing has come apart and injected ball bearings into the engine. The end result is a mandatory metal in oil inspection for the engine. Cost between $30K-40K. These inspections could have been avoided if proper precautions had been taken.

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