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 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°.
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 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 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: 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 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 we could more-accurately say that the SLPS works in a small range of blade angles varying from, maybe, 11° to 13°.
Why is this important? You would expect the SLPS annunciator to cycle on and off as the solenoid did its cycling. Starting with serial number LJ-572, when an electro-optic SLPS trigger replaced the mechanical switch, this cycling of the annunciator indeed occurs. Earlier airplanes, however, include a latching circuit and once the SLPS annunciator illuminates it remains on even as the valve loses and then receives power again. This is not good!
Why? First, there must be a method that allows the pilot to finally extinguish the annunciator after the SLPS is no longer working and yet the annunciator is still locked in its illuminated condition. Second, it makes the recognition of a SLPS malfunction more difficult to detect.
Let’s talk about this SLPS malfunction. If you fly these airplanes long enough, I predict that you WILL experience the malfunction at least once! Here’s what happens: The SLPS electrical circuit activates (shorts out) when it shouldn’t, when the blade angle is in cruise well above the 12° triggering angle. Because this is a switch malfunction, there is an excellent chance that the malfunction will remain until an A & P does some work.
With the SLPS solenoid now powered, the oil to the propeller is blocked but the transfer gland leakage slowly but surely sends the propeller blade angle to larger and larger values. Given enough time (maybe 30 seconds or so) the prop will be totally feathered. Of course, unless the power lever was retarded, there is an excellent chance that redline torque will be exceeded.
The indications of this malfunction are (1) the SLPS annunciator illuminates, and (2) RPM starts slowly decreasing and torque starts slowing increasing. What should the pilot do in this emergency? First, reduce power as necessary to keep torque within limits. Second, remove power from the shorted switch by pulling the system circuit breaker (CB).
Now here’s where it gets interesting! There is only one CB that protects the circuit for both side’s SLPS system. That’s the good news: That you don’t have to find a left or right CB. The bad news is twofold. First, the CB is not labeled “SLPS” but instead is “Prop Gov Test.” You see, Beech uses this same CB to protect both the SLPS wiring as well as the circuit that allows the overspeed governor’s RPM to be reduced enough so that it may be tested. I wish the SLPS label were there by the CB also, but it’s not. The second piece of bad news is much worse: This CB is in the very worst possible position for the pilot to find it and pull it! Where? It’s the one farthest to the left on the second row down on the co-pilot’s right subpanel. In this position, it is totally hidden from the pilot’s view by the co-pilot’s control wheel shaft. Yes, it is easily pulled by stretching across the cockpit, reaching under the co-pilot’s wheel, and feeling for the leftmost CB. I strongly encourage any readers who are operating a King Air with this system to install a red plastic “pull cap” on that sucker to make it easier to find and pull!
A few paragraphs earlier I wrote: “Starting with serial number LJ-572, when an electro-optic SLPS trigger replaced the mechanical switch, this cycling of the annunciator indeed occurs. Earlier airplanes, however, include a latching circuit and once the SLPS annunciator illuminates it remains on even as the valve loses and then receives power again. This is not good!”
The reason it is not good is because it allows the annunciator to sometimes illuminate when nothing is wrong. Unless the illumination is accompanied by the RPM decrease and torque increase, it merely is the result of some temporary “glitch” that activated the circuit erroneously. Moisture, perhaps? There is no need to find and pull the CB unless you experience the RPM decrease and torque increase.
If the SLPS were not removed when reverse thrust is desired, then we could never achieve reversing. As the power levers are moved through the Beta and Reverse ranges, moving the PLPS to smaller and then negative blade angles, the SLPS would prevent the blades from going flatter than 12°. The remedy to this predicament is to eliminate the SLPS whenever the power levers are lifted. Lifting is of course required to get behind Idle and into Beta and Reverse. Lift either power lever and both SLPSs disappear.
Here is a very common nuisance: The SLPS annunciators, one or both, illuminate and stay illuminated when the power levers are returned to 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 had the not-uncommon situation of some malfunction taking place in our SLPS system that locked off the oil and caused the propeller to slowly start feathering. Well-trained instructor that I was, I pulled the CB and all returned to normal as we headed back to Beech field to have the malfunction addressed and remedied.
The airplane got pulled into the delivery center hangar and one of the senior Beechcraft A&Ps was assigned to cure the problem. This great gentleman – Red Martin – had probably been a Beechcrafter since Staggerwings were the product. He and I had developed a liking for each other. As he worked on the short-circuited SLPS solenoid, he opined to me that it was stupid why this solenoid was still installed. “Why?” I asked, in my ignorance.
“Because if anything causes the PLPS to fail, the prop will immediately feather” was Mr. Martin’s response.
After the conclusion of that flight training session, I rushed back to the Training Center to verify that what Red had stated was correct … and it was! I became an evangelist! I went to the other instructors, to Don Cary, my boss at the Beechcraft Training Center, and with his blessing, I contacted the head of the engine engineering department. To my amazement, this gentleman said “Tom, you’re right, we know this. But the Federal Aviation Administration (FAA) won’t allow use to remove the SLPS system.”
My blood began to boil! It had to be mere ignorance on the FAA’s part that keeping this unneeded and problem-prone system was necessary. I asked the head of engine engineering about this and his response was “We’ve explained it until we’re blue in the face, but they just don’t buy it.”
“Wait!” I said, “you guys are the engineers. We are the training experts. Let us have a crack at convincing the FAA.”
And so, a month or so later, I was in front of a “class” of maybe five FAA engineering employees and described to them the working of the beta valve and why the SLPS was unneeded and undesirable. Well, bless their souls! Within a few weeks the SLPS requirement on beta valve type governors was removed and Beech issued a Service Bulletin to direct its removal.
Hence, all 100s, E90s, and 200s that had this unnecessary system installed as factory standard, were allowed to remove it entirely. That explains why these airplanes have switches that are spring-loaded to the center position but have no label nor action when pressed down. Only, the top, OSG Test position, is still hooked up.
Before we end our story, there is one more fact to mention. The A100 model that replaced the straight 100 model in 1971, was the very first King Air to have four-blade propellers as factory-standard equipment. Because of the potential for “reactionless vibration” when idling at too low a speed on the ground, Beech solved this challenge by (1) increasing Idle N1 speed from about 50% to about 60%, and (2) flattening the LPS setting to provide less rotational resistance and thus an elevated propeller speed. But now – wow! – that flatter propeller provided too much drag in the landing flare. The fix? Install anther Low Pitch Stop at a larger blade angle but have it only operate in flight, not on the ground. This was the origination of the Flight Low Pitch Stop (FLPS) and the Ground Low Pitch Stop (GLPS). And to create the FLPS? Why the bad old, unreliable, electric lock pitch solenoid was reintroduced! This system was installed on both the A100 and the earlier F90s but, thank goodness, it was replaced with a simpler and less malfunction-prone system on later F90s and all members of the 300-series.
I hope you now see why I said that you pilots flying later King Airs without the lock-pitch propeller solenoids are so lucky!