Page 24 - June 2022

ASK THE EXPERT
Oh No! Not the Secondary Low Pitch Stop!
by Tom Clements
You younger men and women flying King Airs are lucky: You never had to learn the Secondary Low Pitch Stop (SLPS) system. This hard-to-understand and accident-prone system is a very important necessity on King Airs powered by the PT6A-20 engine, only. That covers all the A90, B90 and early C90 models. But – and here’s a very interesting tidbit of information that this article will explain – the SLPS system was also installed on all straight 100s as well as many E90s and early 200s ... even though it never should have been! Let’s go back in time ...
The Primary Propeller Governor on the PT6A-20 – which first appeared on the A90 model – was the first to be associated with a reversing propeller. The propeller blades now moved in a larger range than when non- reversing propellers were the norm. Blade angle – the angle between the chord of the propeller blade and the plane of propeller rotation – went from a number close to 90°, the feathered position, down to something close to 10° in the non-reversing props. The extreme limits of blade angle travel are determined by metal hitting metal. Unless something major breaks inside the propeller hub (which is almost unheard of!) blade angles can never exceed these “hard” limits.
When power or airspeed is reduced, the propeller wants to slow down. A constant speed governor, as you all know, reacts by decreasing blade angle. The “flatter” propeller makes it now have less rotational resistance and thus the speed (RPM) is maintained. A combination of both low power and low airspeed causes the propeller blades to flatten as far as they can go. This situation is shown to the pilot by the propeller slowing down, no longer maintaining constant speed. It typically does not happen in flight except on short final for landing ... or when power-off stall practice is conducted.
With reversing props, the blade angle range is increased from about 90° down to a negative number, close to -10°, where the air was pushed forward instead of propelled
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aft. As before, the extreme limits of blade angle travel are determined by metal hitting metal. But when low airspeed and low power occur together – such as on short final – the tremendous increase in drag as the propeller starts pushing air forward very likely would result in loss of elevator control and a hard, short touchdown with a damaged airplane almost assured.
It follows, therefore, that reversing propellers must have a Low Pitch Stop (LPS) that exists on the positive side of flat pitch, just like in the non-reversing props. If this new LPS were not movable, however, what good would it be? We’d be stuck with a non-reversing propeller again, just like before.
Hence, the LPS on a reversing propeller must be movable, must be able to be controlled from the cockpit and operate anywhere in the range of approximately +10° to -10°. How this all happens is part of the reason your initial King Air ground training devoted four hours or more to the propeller system. For now, I am merely going to state that the LPS is repositioned when the power lever is lifted and moved from Idle (+10°) back to Maximum Reverse (-10°). Since it is oil pressure created by a pump inside the governor that is driving the propeller blade flatter, any mechanism that shuts off this supply of oil creates an LPS. To summarize in somewhat technical terms, the LPS is a mechanically activated oil shut-off valve and is movable from approximately +10° to -10°.
JUNE 2022