Page 30 - June 2022

Idle from Beta or Reverse. Here’s what is occurring: The electrical circuit for the SLPS was removed when the power levers were lifted. But if the power levers are moved rather rapidly to Idle when exiting Reverse, the electrical circuit can be reactivated before the blade angle becomes greater than 12°. This of course activates the SLPS and causes the annunciator to appear. The transfer gland leakage allows the blades to keep moving up to the PLPS now at 15° (Idle) but that darn latching circuit keeps the annunciators on. The annunciators also illuminate during the run-up test of the SLPS system and latch in the “on” position. How do we get rid of these lying lights? Tap the SLPS test switch down.
The primary purpose of the SLPS test switch is to keep the SLPS circuit alive after the power levers have been lifted. In that manner, we can move the PLPS back to flat pitch and verify that the SLPS activates as it should, stopping the blades at about 12° as they attempt to go flat. The secondary purpose of the switch is to allow clearing of the annunciator after it has activated and locked on.
A piece of trivia: Back in the days of PT6A-20-powered King Airs, the designers decided to have separate test switches for the left and right propellers. These switches are located on the pilot’s right subpanel and they are spring-loaded to the center, off position. When held up,
they activate the reset mechanism for the OSG to bring its speed down to a value that may be reached without exceeding normal takeoff RPM. When held down to the SLPS test position, as stated in the previous paragraph, they keep the SLPS circuit alive after the power levers have been lifted. The left switch, obviously, activates the left OSG’s reset solenoid and the right switch does the same for the right side. But here’s the trivia ... the bottom position keeps the SLPS circuit alive on BOTH sides. Recall that there is only one CB that supplies the power to both SLPSs and the test switch also keeps power available for BOTH sides. By the way, later King Air models use a single switch to activate both the left and the right OSG’s reset solenoids so Beech could easily have decided to have a single switch in the earlier King Airs as well. Tapping either switch down will clear a latched-on SLPS annunciator regardless of which side it is. Likewise, either switch in the down position will keep the SLPS circuits active for both sides.
The PT6A-28 – and all subsequent King Air engines that appeared after 1969 – have a totally new PLPS system that involves the beta valve. If any part of the reversing mechanism to this valve fails, the propeller will immediately feather. Even as a King Air instructor at Beech, I did not know this until a significant revelation occurred in 1973. I was with a student in an E90 and we
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28 • KING AIR MAGAZINE
JUNE 2022