Many pilots who come through training ask what the fuel topping governor really does and if it is really needed.
The King Air has three propeller governors: the primary propellor governor (PPG), the overspeed propeller governor (OSG) and the fuel topping governor (FTG). The FTG is in the same housing as the PPG. This combined unit is technically called the constant speed unit (CSU). The term constant speed unit is not used as much; most people use the terms primary governor and fuel topping governor separately. All three of these governors can control the speed of the propeller in certain situations.
We have all been taught that if the PPG fails and the speed of the propeller reaches the OSG speed, the OSG will limit the exceedance. If neither of the two governors (PPG or OSG) can slow the prop down by using oil pressure when the prop continues to exceed limits, the FTG will cause the fuel control unit to slow the engine down, which in turn slows down the prop.
Figure 1 (above) represents a typical King Air B200 propeller rpm range. The prop lever is shown at the bottom of the chart. By pushing the propeller lever full forward, we are asking the PPG to adjust the blade angle to achieve a prop rpm of 2,000 rpm. If the rpm exceeds 2,000 rpm the OSG will try to adjust the blade angle to maintain 2,080 rpm. If OSG cannot stop the rpm exceedance, then the FTG slows the engine down to maintain the prop rpm no faster than 2,120 rpm.
Let’s say the prop lever was pulled back to 1,700 rpm and the PPG fails to maintain the 1,700 rpm setting. Which governor will stop the exceedance first: the OSG or the FTG? Looking at Figure 1 (above) you can see the FTG is the answer. This is because the FTG limit is linked to the selected rpm set by the pilot using the prop lever. The FTG will limit the prop rpm to 6% above the selected 1,700 prop rpm setting. If you are cruising with the rpm set at 1,700 rpm and the propeller overspeeds, the FTG will stop the exceedance at 1,802 rpm. This is well below the fixed limit of the OSG at 2,080 rpm. The answer to the original question once again is the FTG will limit the rpm prior to the OSG.
What I have been describing is mostly academic due to the extreme reliability of the PPG and the OSG. I have never heard of either the PPG or OSG failing. One could think of the FTG as being “parked” at 6% above the PPG rpm setting during normal operations.
Reverse operations
What happens to power and prop rpm during reverse operations? How is the FTG involved? Looking at Figure 2 (next page), as the power lever is pulled back from high power setting to idle, the compressor speed (N1) slows down to an idle speed that is set by the condition lever. Let’s assume the condition lever is at low idle. When the power lever is reduced to idle, the N1 will be approximately 62%. If the condition lever was at high idle when the power lever was reduced to idle, the N1 would be approximately 71%. As we lift the power lever over the gate into beta range and continue aft, the N1 speed remains the same. However, the prop blade angle will now decrease toward zero. As we continue to move the power lever into the reverse, the blade angle goes negative and the power will increase. Why increase power? In reverse we want more power so we get more thrust pushing forward, causing the aircraft to slow down faster.
Did you see that squirrel? Now for a short side track. This paragraph will become relevant shortly so please bear with me. We are in our King Air B200 on the ramp, just after both engines are started. Our power levers are at idle, prop levers are full forward and the condition levers are at low idle. With the prop levers full forward, what prop rpm are we asking for? The answer is 2,000 rpm for the B200 with -42 engines, 4-bladed props. Are we getting what we are asking for? Nope. The rpm is probably just above 1,180 rpm. Why are we not getting 2,000 rpm? Most of you will say that we need to add power. You are correct. Why do we need to add power? What’s wrong with the PPG? We selected 2,000 rpm with the prop lever, so why doesn’t the PPG adjust the blade angle to make less rotational resistance causing the prop rpm to increase? Those of you who said the beta valve is blocking the oil to the hub have got the idea. The PPG is trying to send oil to the hub to lower the pitch so the rpm increases but the beta valve is stopping the oil pressure to keep the blade at its lowest safe pitch, otherwise known as the low pitch stop (LPS). Oil being blocked effectively makes this a fixed pitch prop for the time being. As we add power for takeoff, the prop rpm will increase even though the blade angle is not changing. We are spinning that fixed pitch prop faster with power. Once the prop rpm gets to 2,000 rpm, the PPG is now able to control the blade angle. As we add more power, the prop rpm wants to increase. However, to maintain 2,000 rpm while power is being increased, the PPG will allow oil to be pushed back into the engine casing. The prop blade moves toward feather. This will cause a bigger bite of air, resulting in larger rotational resistance and slower rpm. What’s the point? As we add power, the PPG will start controlling the prop at 2,000 rpm by increasing the blade angle to a more positive angle to maintain the 2,000 rpm. Remember this point for the next paragraph.
The squirrel is gone, time to focus back to Figure 2 (above). Looking at the reverse section, when the power lever is moved into reverse, the blade angle goes negative and the N1 increases. What would happen if the engine power spun up the prop speed in reverse to 2,000 rpm? What would the PPG do? Recalling the point we just talked about in the previous paragraph, the PPG would cause a more positive bite. Why? The only tool the PPG has to slow the prop down is to take a bigger bite. The blade angle would come out of full reverse, from about -9 degrees, speeding up greatly while passing through the smaller negative blade angles (less rotational resistance) toward zero, then going positive. The thrust would go from large negative to a large positive. Hang on for a wild ride! What a mess! We have overspeeding props, and I don’t even want to guess what happened to directional control.
How did the designers of the King Air solve for this potential problem? Figure 2 (above) shows us that when in full reverse, the N1 is limited to a maximum of approximately 88%. This 88% N1 equates to a prop rpm of about 1,900, which is below the maximum PPG speed of 2,000 rpm. If the prop rpm never reaches the PPG setting, the PPG will never try to increase the blade angle. Problem solved! How do we limit the N1 speed? This is the FTG’s purpose – to limit the N1 to a speed that results in a prop rpm that is 5% below whatever the PPG prop rpm setting is during reverse. This will prevent the PPG from taking over and dumping oil out of the hub causing a bigger bite. Refer again to Figure 1 (previous page). The FTG is the key to making reverse happen safely. Sounds like that fuel topping governor is pretty handy.
But wait, there’s more! Let’s say we land with the prop levers not full forward. The prop rpm is set for 1,700 and we use full reverse. What happens? The FTG will restrict the N1 to a speed that results in a prop rpm of 1,615. This is 5% below 1,700 rpm. Are we getting our maximum reverse? No. The engine and the prop are not as fast as they could have been if the prop was full forward. Less power … less performance.
What does the “Reverse Not Ready” light on the annunciator panel mean to you as a pilot? We know the light will illuminate if the prop levers are not forward and the gear is down. To me, it means I will not be able to get maximum performance if I need it for reverse. Hmm, making sure the props are forward for max performance landings sounds important.
I know many of you are wondering if you’ll damage the prop linkages if you go into reverse when the props are not forward. That will depend on your speed, but yes, there is a potential for damage.
I move my flap lever from the up position to the approach position. Nothing happens. Ugh … a no flap landing is in my future. No problem, just follow the checklist. OK, it looks like on my B200 flaps up landing Vref is 132 KIAS. I’ll pick an approach speed of 140 KIAS and plan Vref of 132 KIAS at 50 feet AGL. As soon as I land, I throw the power levers into reverse. Hmm, something doesn’t feel right. At 132 KIAS, the prop is windmilling so fast that the PPG has to increase the blade angle to keep the prop rpm at 2,000 rpm. The prop angle is above the LPS. The PPG is still controlling the prop. In other words, it is still “on the governor.” The prop needs to be sitting on the low pitch stop, “off the governor,” for the prop angle to continue to decrease when pulling the power levers back through beta range and into reverse. If the prop is still “on the governor” when pulling the power lever back, all you will do is bend/damage the linkage.
The bottom line is you will need to be below about 110 KIAS before the blade will start to rest on the LPS. If your prop levers are not full forward – let’s say they are set for 1,700 rpm – then you will need to slow to around 95 kts to get “off the governor” to be able pull the power levers into reverse without damaging linkage. When you plan to use reverse for landing, having the prop levers full forward will make you more certain you will be able to get into beta range then reverse without damage.
Phew, that’s a lot to consider. I hope you have learned the PPG and OSG are extremely dependable. Due to the low risk of both these governors failing, it is not necessary to have a third governor (FTG) for an overspeed protection. However, it is very necessary for our friend the fuel topping governor to be there for us in reverse. Knowing that the fuel topping governor is there for us in the background is a great feeling.