Propeller Overspeed Check Explained

by Fletcher Sharp | P&WC Senior Field Service Rep for Ag Market (Retired)

The vast majority PT6A powered ag aircraft have a test switch marked Propellor Overspeed Check or Test. Most ag operators I talk with have no idea what that switch does and most responses are, “I don’t ever touch it.” Or, “What does it do and how often should I check it?” I’ll try to explain what happens when you properly (note properly) perform a propeller overspeed check/test.

The PT6A propeller is a bit unique with three totally separate propeller overspeed protection systems. To the best of my knowledge, there has never been an event where all three systems failed. So, the reliability of these protection systems is incredibly robust.

The first prop overspeed backup is in the primary propeller governor, using the balance of speeder springs and flyweights. The primary propeller governor, also referred to as the CSU (Constant Speed Unit), works to control any propeller overspeed up to approximately 102 to 103% Np (prop speed). Should the propeller speed exceed that value, from approximately 104 to 106% Np, the propeller overspeed governor gets involved by dumping oil from the propeller dome back into the reduction gearbox. This moves the propeller blades to a coarser angle, thus adding drag to the propeller’s speed, hopefully slowing down the propeller speed. Should the second system fail, a third propeller overspeed protection inside the main propeller governor takes over. At 106 to 107% Np and above, the flyweights inside the main propeller governor open up a bleed air link allowing pressurized air, referred to as Py pressure, to leak into ambient air. Py pressure comes from the Fuel Control Unit (FCU).

The following is for PT6A engines up thru and including the PT6A-45 series only. (Larger series PT6A engines with Woodward FCUs function in a different manner, but with the same results.) As P3 air enters the FCU of the smaller PT6A engines, it is divided into two components, Px and Py. They have different pressures and between the two they allow a balance to be obtained in evacuated bellows inside the FCU. Evacuated bellows are the “brain” of the FCU on small PT6As. Anything that upsets the air pressure balance will cause the FCU to react. In the case of the Py bleed link of the main propeller governor opening that lets Py bleed off to ambient, the FCU sees this as a significant change in balance inside the evacuated bellows. The governor signals the FCU to reduce fuel flow to the engine, thus slowing down the gas generator/ compressor that reduces propeller speed.

The propeller overspeed governor, located at the 9:00 o’clock position from the pilot’s perspective on the front of the engine, has an electric solenoid attached to it. If it is working properly, the propeller overspeed governor doesn’t function until a propeller overspeed reaches 104% to 106% of selected propeller RPM. It is desirable to check the functionality of the unit at a lower propeller speed. When you turn on the overspeed test switch, it electrically resets the overspeed governor trip values to a much lower propeller speed. While different ag aircraft manufacturers have slightly different procedures for performing this check, they all do the following;

With the propeller control lever full forward and the throttle at idle, turn on the propeller governor test switch and hold it on while advancing the throttle to high power. At some point, because solenoid has been energized, the propeller will start governing at a lower propeller speed by dumping oil from the propeller dome through the overspeed governor and back into the reduction gearbox. This will have the effect of controlling the propeller RPM at the selected lower speed of the overspeed governor. Make note of the propeller RPM where it was governing, then retard the throttle back to idle. Release the propeller overspeed test switch. Repeat the check without activating the overspeed test switch. Note the propeller RPM. It should have returned to whatever reduction gearbox speed setting is maximum for your model of PT6A; 2,200 propeller RPM for most small PT6s, 1,900 propeller RPM for all -100 series PT6As (-112, -135, -135A, -140AG) and 1,700 propeller RPM for all large PT6As in the -60 series.

If a lower propeller RPM was observed with the test switch held on and the normal maximum propeller RPM for the particular engine model was observed when the test switch was released, indicates a successfully tested propeller overspeed governor. If there is no difference in propeller RPMs at the high power setting, the overspeed governor is not functioning.

As propeller overspeed governors wear out, it is possible for the overspeed governor setting to drift upwards to the point where it conflicts with the primary propeller governor, or the primary governor setting can drift downwards. Strong propeller speed fluctuations during takeoff can be attributed to interaction between the primary governor and the overspeed governor. With the two fighting for control, propeller oscillations can occur. Check your OEM’s flight manual for guidance on how often the propeller overspeed governor should be checked.