This article’s title pertains to a somewhat common request that I have received over the years of my King Air training career: Help me find a fix for the autopilot. Even when no question has been asked, I find that quite often when I ride as an instructor or passenger in a King Air, I find that the wings are not level when the autopilot is handling routine “straight and level” flight.
I am happy whenever I get this question because the answer is so simple. In fact, a lot of my readers – the ones who already know what the answer is – may as well go ahead to the magazine’s next article, because you’ll find nothing new here. But stick around if you’re waiting to learn the magical fix.
Ready? Want to know where the problem lies and what is the magical fix? Here it is: Turn the rudder trim wheel toward the lower wing.
OK, I’ll see you next month.
What?! You’re still here? Well OK, I’ll spend a little more time explaining what this is all about.
Only one autopilot (AP) that has been certified in King Airs has rudder trim capability … the rare King KFC-400 system. If you are flying a King Air with that system, the autopilot will position the rudder trim as it sees fit. Yes, you can manually turn the rudder trim wheel to a new position, but when you take your hand away the wheel will move back to where the AP wants it. If you find your KFC-400 flying with one wing low, an avionics technician with experience in that system will be required. (This does not apply to the much-more-common KFC-300 system.)
All other autopilot systems available in King Airs have four, not five, autopilot servomotors, more commonly called servos. One servo controls roll by moving the ailerons, one controls pitch by moving the elevators, one controls yaw by moving the rudder, and one adjusts the elevator trim to lighten the load on the elevator servo. Only the KFC-400 has the fifth one that adjusts the rudder trim.
A single-axis autopilot controls only roll. A two-axis autopilot controls both roll and pitch. A three-axis autopilot (yes, you guessed it!) controls all three: roll, pitch and yaw. To the best of my knowledge all King Air autopilots are and always have been the three-axis type.
As you know, the only trim control that routinely gets much use is pitch trim. Change airspeed? Trim. Change configuration? Trim. Aileron trim hardly ever gets touched unless a large fuel imbalance exists. Rudder trim? Many pilots must believe it is akin to aileron trim … hardly ever used (except for single-engine work). Yes, a normal flight can be successfully completed with never a tweak of the rudder trim wheel. Is that the way to go? No! Did you hear me? NO!
The rudder (yaw) servo is there for only one purpose … to dampen yaw. It helps in keeping the nose from swinging side-to-side. Cruising in perfectly smooth air with no change in power the rudder servo would never be needed and would never activate. But since air is rarely that smooth, imagine keeping your feet on the floor while flying manually. What? You say you’ve ridden with pilots who do that? Yes, I have too … and it drives me nuts! Now every little bump usually leads to some nose-swinging. Even in perfectly smooth air, lack of rudder awareness and proper usage leads to what this article is all about … correcting a wing-low situation.
Imagine this scenario, which is a very good one for instructors to teach/demonstrate to their flying students. In level flight, tell the student to keep both feet on the floor, away from the rudder pedals. Now assign a heading – let’s use an example of 270 degrees – and observe the student doing the proper job, on the control wheel only, to maintain altitude and heading. Now suppose the instructor slowly put some force on the right rudder pedal. The airplane will respond by swinging the nose slightly to the right, making the heading change to, say, 275 degrees. The student, following the assignment that was given, will turn the wheel counterclockwise, dropping the left wing, and turning back to 270. Now, to keep holding that assigned heading while the instructor is still pressing the right pedal, the wings cannot be brought back to level. Instead, a slight left bank must be retained.
Has the light bulb illuminated? Has your brain grasped why the autopilot is not flying wings level? It is doing so in order to fly the heading it wants while compensating for an incorrectly-adjusted rudder. Keep in mind that although the autopilot may be in GPSS, Nav or Approach mode, not Heading mode, it still must find and hold an exact heading to track the course. Sure, that heading may be changing often as conditions – especially crosswind components – change, but at any given time the autopilot has a heading target when holding a heading or tracking a course.
It is common to see a pilot attempt to raise the wing that the autopilot is keeping low by turning the aileron trim wheel. If the bank angle changes due to this action then no longer will the target heading be held, so the AP will not let that wing come up. Oh sure, there is a reaction time so the wing will probably rise before it returns to the position it needs. Eventually, enough aileron trim will likely overpower the strength of the roll servo and the wing will indeed come up … and keep coming! Thus, it is incorrect and fruitless to adjust aileron trim to correct the wing-low condition. Instead, just adjust rudder trim to add rudder force on the low-wing side.
Back to our training scenario: If we are now banking two degrees left-wing-down to compensate for the right rudder force our instructor sneaked in, we could take the rudder trim wheel – assuming our trainer has one – and turn it left, toward the low wing. Eventually, the rudder trim would create just the right amount of left-rudder force to balance the right-rudder force that the instructor was applying and we would have the wings level again as we held the assigned 270 heading.
“Patience is a virtue.” I am sure you’ve heard that adage. If we move the rudder trim wheel in our King Air rapidly, the nose will of course momentarily swing in the direction of the rudder force that has now been applied. The Yaw Damper, obediently doing its job, will apply opposite rudder to dampen that yaw. Go slow with the trim and then stop to give the airplane and autopilot time to stabilize. Truly, in a King Air, the initial motion of the rudder trim wheel toward the low wing may be no more than one-fourth of one index division on the trim indicator. Expressed another way, the geared knurled knob that you are moving with your hand probably rotated only 30 degrees or less. Wait 30 seconds or so and inspect the wings. Better, but not level? Turn the trim wheel another small amount and wait again. Eventually you’ll have it nailed in level flight. Doing this perfectly throughout an entire flight (without an engine failure!) I will state that the rudder trim index is never more than one unit from center. Be patient; go slow.
Rarely do two different airplanes fly identically and hence what is right for one may not apply to others, even others of the exact same model. But let me tell you, readers, every King Air needs rudder trim adjustment throughout a flight! Why it drives me nuts when I ride with pilots who treat rudder trim and aileron trim the same – rarely if ever touching either one – is because the result is uncoordinated flight. Please work at being more aware of coordination. “But I am aware! I keep the ball pretty darn well centered!”
Yes, I am sure the ball is “pretty darn well centered” most of the time, as it should be. Give yourself a pat on the back; you’re doing good thus far. However, the amount the ball is out of the center – or the slip-skid bar is not aligned with the bank index – in level cruise flight is so small as to be almost unnoticeable even though the wings are obviously not level. Holding a constant heading with level wings and equal engine power, by definition, means that the airplane is perfectly coordinated. It is my belief that comparing wing-tip-to-horizon alignment – in almost all cases – is a more accurate way of gauging the true state of coordination than the ball.
You will come to find that adding some right rudder trim will always be required as you climb after takeoff, unless you dialed in a little while setting trim wheel positions before takeoff. A small left rudder trim tweak will invariably be required as you level off and accelerate into cruise. If speed picks up in the descent, you’ll need a dab of more left rudder trim. Extension of flaps and gear? Because other things are changing so quickly now – airspeed, power, probably altitude – there usually won’t be enough time to make the wing-low evaluation of trim. But be alert to the ball. Trim as needed.
As you all know, trim merely lets us reduce control forces. Everything I am saying about rudder trim could be accomplished by pushing the correct rudder pedal. Of course, there are times when forceful and timely rudder pedal input is necessary … like when an engine experiences a loss of power! When hand-flying without the Yaw Damp engaged, yes indeed, it may be easier to just press a little harder on one pedal than spending time adjusting the trim. However, since we almost always have Yaw Damp engaged – and, 90 percent of the time, the autopilot as well – the rudder pedals resist our attempts to move them due to the Yaw Damper doing its thing. Rather than overpower the rudder servo or get a leg cramp because of the continuous force being applied, I suggest adjustment of rudder trim is much preferred to the application of actual pedal force when correcting the wing-low problem.
If this sounds familiar to you, you are not mistaken. I included a reference to the need for rudder trim adjustment in a previous article discussing Yaw Dampers. However, I believe this is an important topic that deserves another look. I still see a lot of King Air pilots accepting one-wing-low as normal. Folks, we can be better than that!
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