A frustrated King Air instructor-pilot and friend asked:
Why do so many pilots think that rudder pedal and rudder trim usage are unnecessary after they turn on the yaw damper? I have observed pilots, even experienced ones that fly an entire King Air flight without ever making a rudder trim adjustment. That’s crazy! Don’t they feel that the airplane is not flying coordinated? Don’t they ever look at the ball? I think rudder usage is a rapidly dying art, dang it!
Would you please write an article that explains why a yaw damper does not eliminate rudder usage?
This is a great request, one, because I am always open to suggestions for worthwhile King Air article topics and two, because I observe the same problem myself. I have concluded that many pilots expect the yaw damp system to do more than it can.
Before continuing, let me remind you that Webster’s dictionary includes at least three definitions of damp: (1) a noun, meaning “a slight wetness”; (2) an adjective, meaning “somewhat moist or wet”; (3) a transitive verb, meaning “to check or reduce.” It’s that last one that applies to the device that checks or reduces rotation around the airplane’s vertical axis. Further, Webster does not even include the word dampner. Instead, dampener is a noun for something that makes things moist whereas damper can be “anything that deadens or depresses.” Many King Airs are equipped with yaw dampers, but I have yet to see a King Air option for a yaw dampner, something that … what? … sprays water on the tail?
While we are reviewing definitions, yaw means “to turn from the desired heading” and, specifically as applied to aviation, “rotation around the vertical axis.” Notice the word “rotation”? Yaw occurs when rotation is taking place, not before or after. In other words, press a rudder pedal while using the ailerons to keep the wings level. Of course, the nose moves, yaws, in the direction of the pedal you push. But if you were to keep pedal pressure applied and maintain wings level, there is no longer yaw. A slip? Sure. Uncoordinated flight? Of course. But the yaw stopped once the nose, or longitudinal axis, stabilized at whatever position it reached relative to the plane’s direction of travel.
Vice versa, release that pedal pressure while keeping the wings level and the airplane should yaw back toward trimmed, coordinated, flight in which the longitudinal axis is pointed exactly in the direction of travel.
It is obvious that yaw action is uncomfortable and hence is something we pilots need to do our best to reduce or eliminate so as to avoid inflicting this discomfort on our passengers. The old-time pilot who could provide a reasonably yaw-free flight through turbulence in a late-‘40s or early-‘50s Bonanza (I almost wrote “V-tail Bonanza” but back then there weren’t any other kinds) was rare to find and was putting his feet and legs through a good workout!
Thank goodness for the advent of electronic yaw dampers! With these, the autopilot’s rudder control – the autopilot’s third axis of operation, the one that takes a back seat to aileron (roll) and elevator (pitch) control – could be used for reducing yaw. Not all yaw damp systems are designed the same, but usually all have an accelerometer of some type that measures lateral acceleration in the tail. Some also receive input from a heading gyro. The autopilot computer, based on these inputs, then commands the rudder to move to resist the acceleration. Simply, and obviously, when the nose yaws to the right, the left rudder is applied and when the nose goes left, the right rudder input is made.
The yaw damper does not try to center the ball, to create coordinated flight. It just reduces yaw. Someday, when you’re on a deadhead leg at altitude, with the autopilot engaged, crank the rudder trim two, three or four units to one side and watch what happens. Since the yaw damper is resisting nose movement, it may take nearly a minute or more to observe the final outcome. Eventually you’ll see that the airplane is in a grossly uncoordinated condition, yet the yaw damper is still working just fine.
In my experience, some yaw dampers operate much better than others and when we fly an airplane with a really strong and accurate yaw damper, it certainly makes for tremendously improved ride comfort. Now it feels like the rudder pedals are immersed in heavy mud, nearly concrete. Yes, they can be moved with foot force, but it takes quite a push!
I think that the feeling of the pedals being nearly unresponsive plays the major role in why too many pilots forfeit rudder control totally to the yaw damp system. Guess what? I agree, and rarely do I push the pedals hard enough to overcome the yaw damp system’s pedal force! But I sure use a lot of rudder trim! Let me explain further.
Entire chapters in aeronautical texts have been devoted to the myriad of forces acting on an airplane in flight and most of my King Air readers have been exposed to these writings. I will not reconstruct the reasons that these texts provide, but I will emphasize the fact that more right rudder input is needed during higher-power, slower-speed, takeoff and climb operation than is required during high-speed cruise and descent operation (at least for aircraft, like all King Airs that have clockwise-rotating propellers). If appropriate rudder changes are not made, then the result is some level of uncoordinated flight. By “uncoordinated flight” I mean that the bank angle is not proper for the rate of turn being achieved, leading to the ball (the slip/skid indicator white rectangle, or bar, for you EFIS-watchers) not being centered.
(A side note: I flew my first few years with an uneducated butt…buttocks. Yes, I included the ball in my scan and felt that I did a decent job of flying in a comfortable and coordinated manner. It was not until taking some further lessons from an aerobatic instructor that I was taught that my posterior, sitting in the pilot’s seat, could be a rather good ball substitute. If my body’s weight was evenly distributed so that both butt cheeks carried an equal amount, I knew the ball would be centered. Anytime I felt one side heavier than the other, I was flying uncoordinated and needed to apply more rudder force on the heavier side. Has anyone mentioned this to you? Go up with a pilot friend or instructor and try it yourself. Close your eyes while the other pilot uses the rudder pedals or rudder trim to create some rather gross ball-out-of-center conditions. Now, with your eyes still closed, take the controls and apply rudder force on the side of your heavier cheek until you feel proper left-right weight balance. If you have rudder trim, use it, too. Now open your eyes. Pretty cool, eh? Whether your safety pilot left you in level flight or in a 45-degree bank, you’ll always be able to return to reasonably coordinated flight just by using your now-educated butt.
So let’s say that we’ve just passed through 400 feet HAA (Height Above Airport) following our takeoff in a C90GTx and have engaged the autopilot, which of course brought on the yaw damper, too. After the system follows the FMS command into a turn toward the first departure procedure waypoint and levels out on a steady heading, we notice that the right winglet is a little lower on the horizon than the left one and the slip/skid bar is deflected a little to the right. (The weight is just a tiny bit heavier on our right butt cheek as well.) We could, with some forceful, steady, effort on the right rudder pedal correct this situation, but instead we reach down to the rudder trim wheel and rotate it clockwise – toward the “ball,” toward the low wing – just a tad, maybe one-eighth of one trim unit. Now we continue with our flying duties of traffic scanning, engine gauge monitoring, executing the After Takeoff checklist, etc. A couple of minutes later we note that the ball is still a tiny bit to the right, so we add another eighth of a unit on the rudder trim wheel. Now we see that the wings are level, the slip/skid bar is right under the sky pointer at the top of the PFD, and all is well in our flying world.
As we level out at FL230 – ATC gave us an unrestricted climb today – we keep climb power while we accelerate into cruise flight. This gives us time to do some more cockpit monitoring and perhaps even consult the Cruise Power chart. It slowly dawns on us that now the left wing is lower than the right and that darn slip/skid bar is now quite a bit left of center. Our hand goes to the rudder trim wheel, again turning it toward the skid bar, toward the lower wing, but not too much … no more than an eighth or quarter of a trim unit. Let at least 30 seconds or so elapse before making another minor adjustment. Keep doing this until coordination is perfect.
The same minor rudder trimming will be needed during the descent, approach maneuvering and throughout the final descent to landing. Since it’s not as obvious that a very small amount of uncoordination is taking place when lots of banking and turning is going on for the approach and landing, you likely won’t have to worry as much about this now as you did during steady-state climb, cruise and descent. In fact, leaving the trim wheel alone – although not a perfect technique – will probably yield a ride that is close enough to perfection that the difference is immaterial.
“Gee whiz, Tom! Are you telling me that I need to make rudder trim adjustments virtually all the time whenever I make a configuration or speed or power change?! You’ve got to be kidding! If I had to do that I’d trade my airplane in for one that is better-rigged!”
Sorry, but yes, that’s exactly what I am telling you. You won’t find a better-rigged airplane in your search, since there’s nothing wrong with the one you’re already flying. It is merely observing the laws of aerodynamics.
There are lots and lots of pilots – not just King Air pilots, but that’s the group we are targeting here – who fly entire flights and never move the rudder trim wheel unless they are facing an engine-out situation. Are they bad pilots because of this? There are so many other factors that go into measuring whether a pilot is “good” or “bad” that I won’t offer a direct answer. However, this I know: Those who never adjust rudder trim have not yet developed a superior sense of coordinated flight and, at least in this small area, their flying skill can use improvement. It is my belief that they likely have set the trim to be about right for cruise conditions and overlook the slightly out-of-trim conditions they have during other flight regimes.
A very few King Air B200s and 300s were delivered with the King KFC-400 autopilot/flight director system. In this unique and rare system, there was one additional servo, one used for rudder trim. It is surprising to watch the trim wheel rotate without a human hand anywhere nearby! When this system is installed and engaged, the rudder trim always tries to create coordinated, ball-centered flight. If you move the wheel manually while the yaw damper is engaged, the system will soon rotate the wheel back to where it wants it, not where you left it. As you might expect, this can be quite a nice feature when it is adjusted perfectly and it really does create flight conditions with exact coordination. On the other hand, if it is misadjusted, it’s a nightmare. Get a knowledgeable avionics technician to work, pronto, if you can never seem to enjoy perfectly coordinated flight with this system. And don’t confuse the KFC-400 with its predecessor, the KFC-300 that has no rudder trim servo.
Let me close with three additional comments. First, I firmly believe that there are hundreds, if not thousands, of pilots who, when they observe their autopilot-equipped and engaged airplane flying left-wing-down in cruise, and who have unsuccessfully tried to correct the problem by use of aileron trim, having given up and concluded, “That’s just the way it is.” Little do they know that a shot of rudder trim toward the low wing will bring it up.
Realize that the yaw axis of the autopilot does not provide turning or steering of the airplane. It just reduces yaw, period. What turns the airplane is the horizontal component of lift, and we create that by banking the wings.
Play a “Let’s Pretend” game with me. Let’s pretend that you’re in the fixed-wing plane you used for your initial pilot training and the instructor tells you to put your feet on the floor and use only the ailerons to keep the nose pointed at some spot on the horizon. “Easy game!” you’re thinking. Now let’s pretend that the instructor slowly applies some left rudder. What you observe is the nose starting to move a little left of your aiming point, so you use the ailerons to bank right until the nose is back where it belongs. So now you are holding the correct, steady heading, but you’re in a slight right bank … which was caused by too much left rudder! As soon as the instructor lets up on that rudder pedal – bingo! – you’ll need to level the wings to keep the nose from slewing right of the aiming target.
The autopilot uses the ailerons to steer, just like you did in this pretend game. It can hold a constant heading with wings level only when the rudder is correctly positioned.
The second closing comment I wish to make – and it’s somewhat obvious based on what I’ve written thus far in this article – is how “naked” I feel when an airplane does not have rudder trim. I love Bonanzas, but the fact that Walter Beech didn’t make rudder trim standard or even available on many of them is a gross oversight, in my opinion. I get tired of (A) having to always climb with a lot of right rudder pedal force, or (B) flying with the ball out of center and a wing slightly down. Yuck!
Third and last, some airplanes more than others have significant stickiness or friction (“stiction?”) in their controls, including the rudder axis. If you make a few small rudder trim wheel adjustments but you see no results – the ball is not centering, the low wing is not rising – push the low side’s rudder pedal smoothly and firmly with your foot to overcome the stiction that is preventing the rudder from moving. There, that did it!
And you thought flying a King Air was easy?! Well, it is! Flying with perfect coordination, however, is one factor distinguishing true aviators from mere pilots.