Last month’s article described the King Air’s reversing propellers and made the statement: A pilot cannot force the propeller to reverse; he can only allow it to do so. Reversing is accomplished by repositioning the propeller’s Low Pitch Stop (LPS). Therefore, unless the propeller’s blade angle is being determined by the LPS, then the blades will not follow the LPS to smaller and even negative angles.
Only when a combination of low power and low airspeed (less windmilling air blast) force the propeller governor to flatten the propeller’s bite of air enough that the LPS is finally reached, does the LPS play its role of preventing further flattening. Now the RPM of this temporarily fixed-pitch propeller will follow the airspeed: Speed up and the RPM increases; slow down and, of course, it decreases. The conclusion is that our “finding” the LPS occurs sooner – at a higher airspeed – when we ask for the maximum propeller speed that the governor can provide. That explains why King Airs contain an annunciator light – RVS NOT RDY – that will not extinguish when the landing gear handle is down until we push both propeller levers fully forward to set the propeller governor to its maximum speed.
The Fuel Topping Governor plays an important role here.
In the early King Air 90-series, powered with PT6A-6 or PT6A-20 engines, the three propeller governors – Primary, Overspeed (often called Secondary) and Fuel Topping (sometimes called the Power Turbine Governor) – were separate devices mounted on the three pads on the Reduction Gearbox housing at the 12, 9 and 3 o’clock positions respectively, as viewed from the rear of the engine. With the advent of the PT6A-28 and after (literally all PT6 King air engines except for the -6 and -20), a major simplification and improvement of the propeller reversing linkage occurred and a part of this saw the Primary and Fuel Topping governors being combined into one device, sitting on the 12 o’clock pad, which correctly – although rarely – is called the Constant Speed Unit (CSU).
If this unit falls off the engine because, say, no one remembered to install nuts on the mounting studs, you would lose both Primary and Fuel Topping Governor functions. However, it would not be significant because this unit also includes the pump that boosts engine oil pressure up to the approximately 300-400 psi level needed for the propeller pitch change mechanism to operate. Without that oil pressure, the counterweights and feathering springs would drive the blades to the feathered position immediately. Of course, were that to happen, it would be impossible to overspeed the propeller due to its huge bite of air, so no governors would be required!
Even though the Primary and Fuel Topping Governors now share the same housing, the same speeder spring, and the same spinning flyweights, the PPG (Primary Prop Governor) portion gets the job done by controlling oil flow into and out of the propeller shaft whereas the FTG (Fuel Topping Governor) portion gets the job done by venting or not venting Py air from the Fuel Control Unit (FCU). When that air is vented and the FCU loses air pressure, fuel flow gets reduced, usually going all the way back to Minimum Fuel Flow, typically 80-100 pph, a bit less than the Low Idle fuel flow at Sea Level. Thus, I have always taught my students that although a single failure could wipe out both governors, there is no guarantee that it will affect both since one involves oil and the other involves air.
In my 49 years of King Air flying and instructing I have yet to hear of a PPG failing such that either the OSG nor the FTG have ever had to react. Don’t lose any sleep worrying about this “what if.”
Realize that the FTG’s operating speed changes from about 6% above the PPG’s setting to about 5% below when the power lever is moved to the Maximum Reverse position. This means that, in the case of a B200 with the propeller levers fully forward (setting the PPG at 2,000 RPM) that the FTG would be “waiting” at 2,120 RPM (2,000 x 1.06) but that it would move down to 1,900 RPM (2000 x 0.95) when Max Reverse is selected. What is the propeller speed limitation in Reverse from the POH? Yes, 1,900 RPM! Hmmm, do we see a tie-in? Yes indeed.
Has it been explained to you what would happen if propeller speed could ever reach the PPG’s setting while the blade angle was in Reverse? Think this through: Whenever the PPG senses an overspeed condition, it vents oil from the propeller dome back into the engine’s nose case, allowing the springs and counterweights to make the blade angle increase, adding rotational resistance and bringing the speed back to where it should be. But if the blade angle starts out on the wrong side of flat pitch – at, say, minus 10 degrees – when the governor activates, the release of oil will again send the propeller to a bigger angle but (here it comes!) this bigger angle will be a lesser bite, less rotational resistance as the propeller moves toward flat pitch, so it would not stabilize its speed but instead overspeed even more in an unstable manner until finally a positive bite of the correct amount were established!
Three things should prevent the propeller speed from ever reaching the PPG’s speed setting when Max Reverse is selected: (1) The pilot remembers to push the propeller levers fully forward, setting the PPG at its highest setting of 2,000 RPM. It is obviously harder to reach 2,000 RPM than anything lower. The “Reverse Not Ready” annunciator mentioned above exists to remind us to push the propeller levers forward. (2) The mechanic – the engine and propeller installer – has done his job correctly and adjusted things so that the Ng reached in Max Reverse is only 82-88%, not the normal 100%+ when the same power levers go fully forward. And lastly, (3) The “icing on the cake,” as it were, is that even if the pilot makes a mistake and leaves the PPG set for too low of a speed and even if the mechanic had the Ng juiced up to, say, 95% in Max Reverse, the good old FTG should step in when the RPM gets within about 5% of “all hell breaking loose,” and come to the rescue by preventing fuel flow from continuing to go any higher. You would not get much reverse action, but that would surely be better than one prop instantaneously slamming from reverse thrust to positive thrust on the after-landing rollout, eh?
Let me tell you of an experience I personally had in which indeed “all hell broke loose” while using Maximum Reverse. I was asked to conduct a pre-purchase flight evaluation on an F90-1 model that was being considered by a potential buyer. I was told that this one-owner airplane had been both flown and maintained by only one pilot/mechanic from when it left the Beechcraft factory until now. “Isn’t that great?!” the prospective buyer asked. My reply? “Well, it may be. But also recall the adage ‘familiarity breeds contempt.’ There can be advantages of having a second or third set of eyes and hands involved at some point.”
Overall, the airplane checked out well. The paperwork was thorough and the systems worked correctly. The engines met power specifications and the pressurization worked as it should. While doing a flight evaluation, I always use maximum reverse on the landing to see if both engines come up to proper specifications. As I rapidly moved both power levers to the max reverse position after touchdown, my first thought was “Wow! This is really putting out the stopping power!” But then, the force that had been pushing me against the shoulder straps suddenly turned into a massive push and I was now slammed against the seatback. I have never flown an airplane with JATO – Jet Assisted Takeoff – but I imagine it would have felt similar. All of that reverse thrust instantaneously became forward thrust. I thank my lucky stars that both engines did the same thing. Had one remained in reverse and this had happened on only the other side, I am quite certain we would have experienced an excursion onto the grass.
Somehow this pilot/mechanic had misadjusted the Beta/Reverse rigging such that not only was too much increase in Ng being experienced but also the Fuel Topping Governor was not being reset enough to prevent the propeller speed from indeed reaching the primary governor’s maximum RPM setting … 1,900 in the case of the F90-1. Thus, we experienced the phenomenon discussed above: The unstable jump from a negative to a positive blade angle as the primary governor reacted to the momentary overspeed situation by releasing oil from the prop back into the engine. The springs and counterweights sent the blade angle to a higher value. But remember, starting from a negative angle, this action causes less, not more, resistance to rotation! The governor cannot work properly until the blade angle jumps forward through flat pitch and finds itself with a positive, not negative, value.
As stated above, I am lucky that both sides were mis-rigged the same. Otherwise, I am sure I would have left the runway.
It is my belief that the primary reason for having the Fuel Topping Governor is the necessary protection that it stands by to provide whenever Maximum Reverse is selected. We could probably fly thousands and thousands of King Air hours without an FTG and never notice the difference … if the rigging is halfway correct. However, it is surely comforting to know that we have our little FTG friend standing by to save us from harm in Max Reverse if we and/or the mechanic did not do our jobs correctly. That the FTG can provide one more overspeed protection device in normal operation? I personally think that’s just where we “store” it until we select Maximum Reverse the next time!
As you have perhaps been taught, something that prevents blade angle from being able to change will render both the primary and overspeed propeller governors inoperative since they both accomplish their jobs by varying blade angle. Yes, that’s true. That is where the FTG would be helpful in flight. But you know what? In my 49 years being around King Airs I have yet to hear of that “unable to change blade angle” scenario ever happening. Comforting, eh?