Page 27 - June 2022

failed and the SLPS was the one preventing unwanted reverse. Therefore, two red, warning annunciators are installed (left and right) usually labeled “Secondary Low Pitch Stop,” that comes on whenever the SLPS solenoid receives power.
There’s more to our complicated story: The mechanically activated PLPS automatically compensates for the transfer gland’s leakage by not quite closing off all the flow of oil to the propeller. The pilot valve automatically finds the position that will let just enough extra oil flow toward the prop to equalize the amount that is lost through the leakage at the transfer gland.
The SLPS solenoid, on the other hand, has no “almost closed” position. It is either totally open, unpowered, or totally closed, powered. Hence, when the blade angle reaches 12° and the SLPS activates, the blade angle won’t remain at 12° but will start slowly going toward the feathered position as oil escapes from the prop back to the engine’s nose case through the leakage at the transfer gland. As soon as blade angle increases from 12° due to the leakage, the switch that had activated the SLPS solenoid is no longer activated, the solenoid relaxes open, new oil rushes in, and the blade angle is again decreased until the SLPS switch gets powered again. When I state that the SLPS works at 12°, I am simplifying the story. The SLPS solenoid’s constant cycling closed and open to compensate for the transfer gland’s leakage, means that
“ ... there is an urgent ... need for a Secondary LPS ... and the PT6A- 20s have that ... ”
JUNE 2022
KING AIR MAGAZINE • 25