Page 26 - June 2022

The OSG and SLPS prop test switches.
downward force of the speeder spring so the pilot valve moves upward and allows some oil from the propeller to return into the engine’s nose case. Less oil, larger blade angle, more rotational resistance, the prop slows down and the onspeed condition is restored.
One quick, last thing before we leave the technical talk: To get the oil from a pump inside the PPG to the holes in a spinning propeller shaft is a challenge. If the fit of this connection (officially called a “transfer gland”) is too tight, unnecessary energy is consumed in turning the propeller. If the fit is too loose, the oil that escapes into the nose case and never finds its way into the propeller dome can be so bad that blade angle cannot be properly flattened when needed. The designers’ goal is to have a rather tight-fitting valve but with a very small oil leakage back into the engine’s nose case. This helps to lubricate the stationary-to-moving components.
At first thought, it would appear that when a perfectly balanced onspeed condition exists, no oil enters or exits the propeller dome. Hence, the blade angle remains constant, rotational resistance does not change, and all is well. But it doesn’t work that way. Why? Because of the transfer gland’s slight leakage. Just enough oil must get by the pilot valve to compensate for that leak. If the pilot valve totally shuts off new oil to the prop, the existing prop oil would seep back into the engine, causing blade angle to increase and RPM to decrease. To summarize:
24 • KING AIR MAGAZINE
Whenever blade angle is remaining constant, the pilot valve is letting just enough oil get by it to balance the leakage caused by the transfer gland.
Now ... where were we? Oh yes. Since the failure of the PLPS would never be noticed until we came “off of the governor” in the landing, we would have no way of knowing that our PLPS was no longer there. (If somehow we were aware of the PLPS failure, then we could fly a faster and flatter final approach with some power on until touchdown. If we’re lucky enough, then perhaps we wouldn’t come off the governors and the blades would not go into maximum reverse blade angle until we were on the runway.)
Since in nearly 100% of cases we would never know that our PLPS was missing, there is an urgent and huge need for a Secondary LPS ... and the PT6A-20s have that additional system.
The SLPS is based on the action of a device called a “lock pitch solenoid.” As blade angle flattens to something less than the setting of the PLPS, an electrical signal is sent to this normally open (N.O.) solenoid to drive it closed. When it goes shut, the oil from the governor to the prop is terminated and hence blade angle cannot continue going any flatter. The traditionally listed numbers have the PLPS working at a 15° blade angle and the SLPS at 12°. Since these are quite close, it would be unlikely that the pilot would notice that the PLPS had
JUNE 2022