FAQ's

Prop Governor Maintenance Tips
Every pilot, A&P/IA, and aircraft operator knows that periodic aircraft maintenance is paramount to achieve the required levels of performance, and even more importantly: Safety!

Every component of an aircraft is necessary for the safe operation and airworthiness of the aircraft, however, some are of critical importance. The propeller governor is critical, and because it is small and mostly out of view, while working quietly and inconspicuously in the background, it can be easily forgotten.

Controlling the propeller pitch with varying manifold pressures, flight profiles and many other factors influencing engine power and the pilot’s inputs is a constant and never-ending task, from start-up to shut down.

Lack of maintenance, incorrect installations and many other factors can provide for a very nasty surprise and ruin your day!

To get started, it is necessary to understand how the Propeller Governor works.

The prop governor is basically a refined hydraulic pump, which has the capability to constantly vary the oil pressure being pumped toward the propeller hub which causes the propeller blades to change pitch between fine and course pitch (and any point between the mechanic stops of the propeller). Essentially varying the RPM of the propeller and engine based on flight conditions and operator inputs.

This little hydraulic pump increases the oil pressure provided by the engine (each engine manufacturer has a different oil pressure setting, but usually between 50 and 80 PSI) to somewhere between 275 and up to 450 PSI (depending on the propeller manufacturer and model).

The governor maintains the RPM selected by the pilot by changing the propeller blades pitch. For example, in order to obtain the greatest power during the take-off roll and initial climb out the highest power setting is selected and the governor provides the necessary oil pressure to provide a fine pitch. Once level flight is achieved, the pilot will reduce the RPM and the governor will change the oil pressure to the hub and therefore the propeller blades will go to a more course pitch, which will “grab” more air and provide the desired RPM for the cruise phase of the flight and fuel economy.

Propeller governors can be designed to increase pitch or decrease pitch. The latter is usually used for propellers that are either counterweighted or full-feathering.

Now for the Maintenance Tip: The best maintenance you can provide your engine and propeller governor: Regular Oil Changes! It is the cheapest, most effective maintenance you can provide!

Secondly, always cycle your prop 3 times in the run up area prior to departure. This action, not only proves the governor and propeller is working as prescribed, but also flows fresh oil to the propeller hub.

What happens if a propeller governor fails to perform as designed?
Despite their extremely tight tolerances and complexity, propeller governors are very robust and catastrophic failures are extremely rare. With regular use and due to exacerbating circumstances, such as lack of maintenance, the governor’s parts are subject to wear and tear and will slowly decrease in effectiveness. A vigilant pilot will almost certainly notice the governor starting to “hunt” the selected RPM or possible surging. These are clear indications that the governor needs to be overhauled.

Loss of hydraulic (oil) pressure from the Governor to the propeller hub
As the propeller governor wears, either because it is past TBO or because of poor preventative maintenance issues, or in the rare case of deficient parts; hydraulic/oil pressure can be reduced. This could be due to out of tolerance between critical components or worn out O-rings, gaskets, seals or a combination of all.
Without adequate hydraulic pressure the propeller blades will tend to return to fine pitch (in the case of a pressure to increase pitch governor, which is most common on single engine aircraft), which can cause an overspeed. In turn, this can both damage the engine itself and the propeller, mandating a full teardown inspection (along with the great cost related to these tasks)! It is import to mention that a severe overspeed could cause a catastrophic failure of the propeller and engine if not caught fast enough. This, is an extremely rare event!!
In the case of a twin-engine aircraft, equipped with feathering propellers. This application requires pressure to decrease governors (and an accumulator). In the case of a hydraulic pressure loss the prop will tend to feather or go to its full course position.

Oil Leaks (Non-internal leakage).
If the propeller governor develops a leak through the governor base gasket (which sits against the engine pad) or one of the O-rings or seals (especially the one below the control arm), this can cause oil to leak into the engine compartment, producing a serious fire hazard. In a very extreme situation the engine could lose all oil and seize.

Propeller Governor (TBO) – Time Before Overhaul
Governor TBO’s vary considerably depending on the manufacturer, model and your specific flight conditions. For example, agricultural spraying and aerobatic operations are the hardest on the governor, due to extreme flight operations which are normal in these modes of flight.
In many instances, the TBO is the same as your engine or propeller, or 5 calendar years. Notwithstanding, please check your manufacturer’s recommendation. Most professionals recommend you overhaul your governor at the same time as your propeller or engine. It may sound obvious, but also if your governor is not performing as it should. For example, not being able to hold the selected RPM or noticeable oil leaks.

If you operate an aircraft under FAA Part 135 (pertaining to commercial operations), you must comply with the manufacturers mandated TBO recommendations.

Conversely, if you are a private operator and your aircraft falls under FAA Part 91, the maintenance isn’t mandated. Having stated this, the manufacturers recommended TBO is there for a reason: SAFETY. And that seems like a great reason to comply.