Walk down a ramp where numerous King Airs are parked and look closely at their propeller blades. I wager that you will see quite a variety of conditions. Some will look almost new, whereas others will be badly sandblasted. It may be that the reason for the difference is reasonable and unavoidable. Namely, one airplane operates only on long, paved, well-maintained runways, and the other aircraft spends much of its life operating from a short, dirt strip on the owner’s ranch. But it also may be that the one with the sad-looking props, even though it spends much of its time on good runways, is also suffering from one of two things, or a combination of both: Poor pilot technique and/or power levers that are poorly adjusted in the Beta and Reverse ranges. The goal of this article is to review proper operating techniques with you, as well as, providing a procedure for knowing if your Beta/Reverse rigging is as it should be.
Let’s start with the rigging discussion and first review the three-blade model 200 graph below.
This particular graph presents numbers applicable to a three-blade model 200, and except for the numbers, it applies to all PT6-powered King Airs. The later models have the Ground Fine stop between Beta and Reverse and do not have the red stripes. The (+) and (-) symbols represent the areas where positive or negative thrust occurs, statically on the ramp.
As the graph shows, the position of the power lever controls two different things: Compressor Speed (N1 or Ng) and the position of the propeller’s Low Pitch Stop (LPS). (This stops also goes by the name of “Flight Idle Stop” in some references, including portions of the Maintenance Manuals. I have always believed that “Low Pitch Stop” is a more obvious term that better describes exactly what is being repositioned.)
Notice the flat portion of the upper line, the Beta area. This flat portion, in which N1 should not change, is appropriately known as the “Dead Band” since movement of the power lever within this range causes no response – dead reaction – from the Fuel Control Unit (FCU). By definition, the Beta Range is where the propeller’s LPS is being repositioned to flatter blade angles while N1 is not changing.
Behind Beta is the Reverse Range. By definition, not only is the LPS continuing to be repositioned to lesser blade angles – it is, in fact, going to negative angles, meaning that the propeller is pushing air forward instead of aft – but also N1 is proportionally increasing, getting greater the more aft the power lever is moved. Typically, Maximum Reverse, all the way aft, should yield an N1 speed of about 85 percent. Remember that the relative speed of the compressor is not the same as the engine’s relative power output. In other words, 85 percent N1 does not yield 85 percent power. On the contrary, 85 percent N1 is probably a bit less than 50 percent power!
That King Air on the ramp with the badly sandblasted props? I’ll bet its dead band is too small, too narrow. The engines are increasing N1 speed before the propeller blades reach flat pitch.
This not uncommon problem means that the airplane is difficult to slow down while taxiing. Before the blades can reach flat pitch, when the propeller is then acting as a large disk giving neither positive nor negative thrust, power is already being added. In other words, an N1 increase is being encountered before we have reached the bottom of the Beta range. When power is added while the blades are still providing a positive bite of air, we start to go faster, not slower!
What many misguided pilots do in this situation is to pull the power levers back more until finally the taxi speed slows down. What has taken place is that at last the residual thrust has been eliminated by forcing the LPS to flat or even negative pitch but at the expense of a higher-than-needed and higher-than-desired propeller speed (Np), since the increased N1 is creating more exhaust gases that are driving the propeller with more power. This higher prop speed, usually associated with a slightly negative blade angle, causes lots of blade erosion.
I have received this question many times during my King Air training events: “Why don’t we get similar blade erosion when the blade angle is at, say +10°, then when it is at -10°? Even with High Idle selected, we can taxi all day with the power levers at Idle and not erode the props, yet we chew up the blades at -10° and 70 percent N1. This doesn’t seem to make sense.”
The reason why a blade angle of -10° leads to more erosion than an angle of +10°: Realize that there is a pronounced twist in each propeller blade, such that the inboard areas are taking a significantly larger bite than the outboard areas. So, when +10° is happening at the 30-inch station – the normal location out from the hub where angles are measured – the blade tip near the ground may be almost flat. That flat tip creates very little airflow disturbance so the sand and grit and gravel and dirt on the surface are disturbed little. But when the angle is -10° at the 30-inch station, the tip may be at -20° creating a great little sucking vortex that vacuums the debris off the ground with unfortunate efficiency!
Vice versa, suppose the dead band is too large, like the graph below.
Now it is easy to kill residual thrust without an increase in N1 speed (and I surely like that!), but it is now common to find that propeller speed decreases so much before N1 increases, that Reverse is sluggish and often asymmetric. Also, especially on the Honeywell (née Bendix) FCUs, Maximum Reverse is usually not near the proper 85 percent value. Starting to rotate the FCU’s speed setting shaft too late may not allow it to rotate far enough for the proper amount of Reverse power.
My preference would definitely be to have a bit too much dead band than not enough. So long as Maximum Reverse delivers reasonable stopping power, the wider dead band ensures being able to kill residual thrust for taxi.
By the way, how many readers are pulling the propeller levers all the way back into Feather while taxiing? With some situational awareness, this is a great technique! Not only can we achieve a propeller feathering check, but also with the blades slapping the air “sideways” as they rotate we have zero taxi thrust. Plus, it is quiet!
So, what is this “situational awareness” I mentioned? First, although the propellers feather quite rapidly – just a few seconds – they take as much as 30 seconds to unfeather. So, if you will need positive taxi thrust to make it up that hill ahead or to maneuver with some tight turns on the ramp, it is not the time to feather. Second, we must remember that it is only safe to feather when the power levers are at Idle, not back in Beta or Reverse. Third, if we roll to a stop and leave the props in feather, there is a chance that our hot exhaust gases will not be blown safely away, but may negatively impact the nacelle and nose paint, oil temperature, as well as cause overheating of the nose-mounted avionics boxes. Remember to push those prop levers forward when stopped.
This in-and-out of feathering while taxiing is especially useful is we have found that our dead band is too small – N1 is picking up too early – yet the mechanic has not yet had time to adjust it properly. It is easy to taxi without residual thrust, no matter how messed up our rigging is, by using the feathering technique.
Also, remember this useful “trick.” When starting to taxi, if the airplane does not begin to roll when the brakes are released, try a quick in-and-out feathering instead of an application of power. Isn’t that cool?!
The momentary bigger bite of air is just what was needed to make the plane begin to roll, yet with zero chance of blade erosion.
Another time that it is easy to erode the prop blades is during high-power run-ups. For example, the Overspeed Governor test requires a lot of power. Please make every effort to find and use a rather clean, paved area of the tarmac when conducting your checks.
Similarly, consider the condition of the runway as you initiate the takeoff roll. If it is unpaved or the pavement is in poor shape, now is the time to make a rolling takeoff with power application coming in proportionally as the airspeed increases. Of course, when the runway is of minimum length, we won’t have the luxury of slow power application. But when there is excess runway, it is a technique that has merit.
How about landing? How do we avoid blade erosion now when we need and want to use Reverse? Easy answer: Go in quickly and deeply, then get out.
For a landing where aggressive Reverse will be used, it is common to run the propeller levers full forward well before touchdown so that we waste no time moving them after touchdown. All we have to do is lift and pull the power levers aft. Here is a time that aggressive, fast action is indeed called for and won’t harm a thing. Remember when I stated that Maximum Reverse is less than 50 percent power? Hence, there is no way that torque, ITT, not N1, is of any concern to you, the pilot, when those power levers are buried all the way back. “Slam” is a word used rarely when talking about flight and engine controls but, truly, here is the time to slam those power levers into Maximum Reverse without delay. Also realize that the power levers move in an arc, not in a straight line. To position them at Max Reverse requires more of a downward push during the last bit of travel, than an aft pull.
There are three important reasons for obtaining Max Reverse immediately. First, the sooner we can establish full reverse thrust, the shorter our landing distance will be. Second, the drag that Reverse provides is dependent upon airspeed squared. That is, at 80 knots, the drag is four times as effective as at 40 knots. Third, we only want to utilize Reverse when we are moving forward fast enough to leave the sucked-up dirt and debris behind us.
It is maddeningly common for me to observe a pilot who uses very little Beta or Reverse after touchdown but then, when he sees the turnoff coming into view, he at last starts pulling Reverse thrust. No, no, no! Now, not only is Reverse not very effective due to the slow airspeed, but also blade erosion is almost guaranteed if the surface is less than perfect!
Sure, if you are quite familiar with the airport layout and know that the turnoff is far ahead, the use of Beta only after touchdown – and maybe not even much of that – is just fine. But when the turnoff is a bit “unknown,”
it is much better to be aggressive first, then play with the Beta range only when at 40 KIAS or below. Remember that the POH states that Reverse should not be used below 40 KIAS. I suggest that you begin slowly moving the power levers forward from the all-the-way back position when you see 60 KIAS, and make sure that they are at Ground Fine or at the bottom of Beta by the time you see 40. Don’t make the common mistake of thinking you need to be over the Idle Gate by 40 KIAS. No, staying in Beta is the proper procedure, but just make sure you are out of Reverse, back into the dead band, Beta area.
To conclude, let’s see how we can evaluate our Beta and Reverse rigging, from a pilot’s standpoint. The first thing to do is to make sure that your Low Pitch Stop (LPS) begins its travel back into Beta at the proper blade angle. Since it is almost impossible to find a mechanic who will use a protractor on a blade while it is spinning, angle is verified not by an actual angle measurement, but rather by a “Flight Idle Torque” setting. A graph exists in Chapter 76 of the Maintenance Manual that shows what this torque should be, and at what RPM, for any given altitude and OAT. Realize that the value is not the same for most retrofit props, as it is for the standard propeller options. I’ll make it easy for you. The chart below provides most of the values for different King Air models and different propellers, at Sea Level on a Standard (15°C) day:
While still in the run-up area, select High Idle on the condition levers and bring the power levers to Idle. Next, move either power lever back over the Idle gate – even over the Ground Fine gate, if need be – while watching the propeller speed. As the blade flattens, giving less rotational resistance, the RPM should rise. As the blade angle goes negative, the extra rotational resistance will cause the RPM to fall. Experiment until you find exactly the flattest pitch position and make a mark on the power quadrant where the aft edge of the power lever shaft is now located. (Putting some masking tape next to the slot makes this task easier and less messy.) a clean run-up area, aim into the wind, make sure the propeller levers are fully forward, then add power until you reach the specified propeller speed. Record both left and right torque values, as well as OAT and Pressure Altitude (29.92 in Hg), and pass them on to your maintenance folks. (If the wind is really howling that day, take both an upwind and downwind reading so that they may be averaged out.)
Now do the same with the other power lever: Find where the RPM is the highest and mark it appropriately. Are both sides close together? I hope so, but the marks will tell the story to your mechanic. Next, while the power levers are still at the flat pitch position, retard the condition levers back to Low Idle. If both left and right N1 speeds do indeed fall back to Low Idle, that’s great! It confirms that your dead band is large enough to kill your residual taxi thrust without adding power.
However, if one or both N1 speeds hang up at something between Low and High Idle, then your dead band is too narrow and your props are candidates for sandblasting … not good. I have even discovered rigging so out-of-spec that the N1 started to exceed High Idle before we found that flat pitch, peak RPM position. Yuck! This dead band is much too small!
We are not quite done yet. Presuming your N1 speeds did indeed drop to Low Idle when the condition levers were pulled back to the hooks – as they should – now take each power lever individually and pull it back further from your Flat Pitch mark until you see an N1 increase. We hope it happens almost immediately, before we move even 1/8 of an inch. If we need to move significantly more than that, then the dead band is so large that a big decrease in propeller speed will be seen, leading to sluggish reaction when Maximum Reverse is reached.
There is one last check to make and record: What is the stabilized N1, Np, and Torque in Max Reverse? Make sure the run-up surface is very, very clean before selecting full Reverse while stopped. Do this with one engine at a time, since there is a possibility of rocking back onto the ventral fin if both propellers are in Maximum Reverse at the same time. If no such ultra clean run-up pad exists, then record the values after you have selected Max Reverse while rolling down the runway soon after touchdown, before beginning to ease out of Max Reverse when 60 KIAS shows up. We hope the N1s come out near 85 percent and that Np is within 100 to 200 RPM of takeoff redline.
It takes an experienced and dedicated PT6 mechanic to make the proper rigging adjustments in a timely and accurate manner. If you have access to such an individual, I am happy for you. If you don’t, then it will be a time-consuming and frustrating endeavor. The description of the work in the Maintenance Manual leaves much to be desired. Having access to an old-timer with lots of experience is invaluable!
Poor Beta and Reverse range rigging is common to find and, in truth, has little impact on safety. But when the rigging is correct, the pilot’s job is easier and more enjoyable, and the propeller blades will fare much better!
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