Page 25 - June 2022

The hard-to-find Prop Gov/Idle Stop CB ... with a pull cap.
What happens if the mechanism that operates the “mechanically activated oil shut-off valve” fails to function on the PT6A-20’s Primary Propeller Governor (PPG)? The answer: The LPS is totally lost and the only limit to low blade angle travel becomes the metal-on- metal LPS at -10°. Does “scary and damaging landing” come to mind? It should! That’s what would happen if power and airspeed were both sufficiently low at the same time.
Aha! What do I see coming over the horizon to rescue us from the clutches of this non-existent LPS? Why, it’s Secondary Low Pitch Stop. Now that there are two different Low Pitch Stops, the normal one is referred to as the Primary LPS and the backup is the Secondary LPS: PLPS and SLPS.
Sorry, but it’s time for a little technical talk. The valve that regulates the flow of oil into or out of the
JUNE 2022
propeller dome to either decrease or increase blade angle goes by the name of “pilot valve.” It moves due to a combination of forces: An upward lifting force caused by spinning flyweights and a downward pushing force caused by a spring called the “speeder spring.” The force of the flyweights depends on propeller speed and the force of the speeder spring depends on the position of the propeller levers in the cockpit. When the speeder spring’s force is greater than the flyweight force, the pilot valve moves downward and opens the valve to allow more oil into the propeller dome. This is what the PPG does whenever it senses an underspeed condition. The additional oil causes the blade angle to decrease, giving less rotational resistance, so an onspeed condition is reestablished.
In the same manner, in an overspeed condition the upward force of the flyweights is greater than the
KING AIR MAGAZINE • 23