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Rudder Trim v Wings Level

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Rudder Trim v Wings Level

Old 29th Sep 2005, 21:53
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Rudder Trim v Wings Level

Can anyone explain why the crew trim the rudder instead of the aileron trim to level the wings , is this a standard procedure and aerodynamically how does this work ? I saw a burst in the FIM for aileron trim problems and it talked about how much rudder trim was required to keep the wings level
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Old 30th Sep 2005, 02:16
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If you needed aileron trim it would be because the aircraft had a residual (non zero) rolling moment - which means that the aircraft would actually be continuously rolling at a rate where the roill damping balanced out the rolling moment asymmetry (ignoring cross coupling terms, such as roll due to yaw etc)

Therefore if you have a stable bank angle, the problem isn't due to a rolling moment offset; rather it's indicative of a non-zero sideforce, which is being balanced by banking slightly to use the gravity vector in the aircraft Y-axis to balance the sideforce.

Therefore, to eliminate the bank you need to zero out the sideforce; that means inducing (or possibly removing) sideslip, and the way to do that is by rudder trim.
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Old 30th Sep 2005, 08:19
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Lets try using the example of a common scenario.

Imagine an aircraft has just lifted off the runway following a normal takeoff. The pilot flying notes that a light constant right roll input pressure is required to prevent the aircraft from rolling to the left. Normally not a condition of great urgency, but no pilot is satisfied with the feel of an out of trim airplane. Bad form because it requires more effort in order to precisely and smoothly control the flight attitude. Very early on, the pilot will check the position of the inclinometer ball. In many cases, only verifying something allready suspected and reacted to with rudder pressure. Suppose the ball is displaced to the right, as it might be in this case. Right rudder pressure would be applied to center the ball. Right rudder trim is then gradually applied and rudder pressure reduced simultaniously until rudder pressure is no longer required to center the ball. If done smoothly, nobody will feel it being done.The need to hold right roll input would likely be reduced or eliminated. Now, with a centered ball, aileron trim may be used to relieve any remaining need to hold roll pressure on the controls. This is all done in a matter of a few seconds by experienced pilots out of long established habit. The force imbalance condition is now corrected enough to fly comfortably. Every day stuff.

Why should a "wing heavy" condition be corrected in this manner and not the other way 'round? To prevent "cross trimming", a condition where the airplane is flying with a yaw tendency acting in one direction where the resultant roll tendency in the same direction is countered by trim generated roll input in the opposite direction. The wings could be trimmed nearly level using this method but the airplane would then be flying with it's longitudinal axis not alligned with the flightpath. The additional drag penalty from the sideslip angle plus the constantly displaced flight controls is not desirable. In at least one type though, this condition seems to dampen dutch roll tendency in place of an inoperative yaw damper!

If aileron trim was stuck in a position that induced a rolling tendancy, rudder trim could be used to hold rudder input and counter the rolling tendency. The aircraft would then be in a "cross trimmed" condition, but would tend to maintain a constant slight bank and a heading slightly offset from the flightpath as above. It could be controlled and landed safely with inoperative aileron trim.

Through heavy use and multiple maintenance adjustments, some jets "of age" that I have flown just don't fly straight! Rudder trim right for climb and left for descent just like a prop single. We like to say they are "bent". Probably just poorly rigged.

Best regards,

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Old 2nd Oct 2005, 03:09
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Can anyone explain why the crew trim the rudder instead of the aileron trim to level the wings…

Your question can be rephrased in such that the answer becomes clearer.

Why aren’t the wings level? Because when the wings are level the aircraft won’t hold a steady heading.

Rudder trim is therefore required to reduce the yaw rate to zero for the aircraft to hold a steady heading with wings level.

However, it is easier for the pilot to trim the rudder on a steady heading until the wings are level, because it is easier for the pilot to detect small bank angles with zero yaw rate, than it is to detect small yaw rates with zero bank angle.
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Old 5th Oct 2005, 09:19
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But the reality is, you do both. First of all get the plane skid/slip free (rudder trim) and then get the ailerons trimmed. If the plane is really bent, you have to re-do it every time you change speed (and in a turbo-prop, every time you change power as well). Unfortunately, it appears very difficult to get a straight aircraft. But considering how big they are, it is amazing that they are built to the tolerances that they actually are.
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Old 5th Oct 2005, 23:16
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Mr Boeing always used to advocate putting in enough rudder to level the control wheel..On the 727 every indice of, out of trim,aileron cost one 96lbs of fuel hour
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Old 6th Oct 2005, 04:27
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I'm surprised that no-one has raised the point of first checking for any fuel imbalance. If encountering a 'cross-trim' situation, my first action is to balance the wing fuel before taking the next step.

The second step is to follow oldebloke's recommendations (as per Boeing) of putting in enough rudder to level the control wheel, and to then apply trim to unload the rudder pressure.

The reasoning behind this is that we must assume that the aircraft is correctly rigged (no way to know if it isn't), and with equal wing fuel, the only real disturbance to trimmed flight will be assymetric thrust. Even if the engine indicating parameters (N1, EPR etc.) are exactly matched, the ACTUAL thrust output will vary from engine to engine depending upon their condition, leading to some 'built-in' yaw.

So, follow oldebloke / Boeing, apply rudder to centre the control wheel, use rudder trim to trim out the rudder force, and take your hands and feet off. If there is now any residual roll tendancy, apply Roll trim as required.

Works for oldebloke, works for Boeing, works for me


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Old 6th Oct 2005, 05:30
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100% agree with old smokey. Thats exactly what I was taught on the 707 many years ago.

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Old 6th Oct 2005, 06:52
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You might want to read about the Boeing 747-400 which almost had a catastrophe climbing out of San Feancisco ( SFO) several years ago, due to a pilot's preoccupation with the ailerons instead of the rudder, as an engine suffered a compressor stall or surge (whatever).

Engine vibration made it difficult to read the flight instruments. The second crew was sitting at the rear of the c0ckp1t.
It is an interesting story.
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Old 6th Oct 2005, 07:20
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Do you have a link to the report - or a report number?
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Old 6th Oct 2005, 08:09
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Fuel imbalance is certainly one possible (and common) cause of a "wing heavy" condition to consider even though fuel balance is normally checked and corrected if neccesary, before takeoff. However, fuel balance is checked by referring to the fuel quantity indicators, which in some older aircraft at least, may be of questionable accuracy due to a number of age and maintenance related factors such as bio-contamination of the probes, corroded electrical connections and calibration drift in the fuel quantity signal conditioners. I have seen this quite a few times in my former life as a mechanic while troubleshooting fuel quantity indication problems. Balancing the guages may or may not balance the fuel load in each wing. One could transfer fuel until a level control wheel, level wings and centered ball can be achieved with all control forces trimmed out and accept the fuel quantity split as indication error. Zero sideslip angle and no deflected controls. This assumes that the inclinometers were properly centered while the aircraft was laterally levelled in the hangar.

Just as likely is the possibility that slight rigging assymitry of the flaps, slats or ailerons is the culprit. I have seen a new airplane delivered in rather poor rig right from the factory. (the delivery crew squawked it) Older airplanes have had flight control components adjusted, repaired, replaced or removed and re-installed any number of times. Each airplane is unique to some extent. These kind of rig-related errors are normally evidenced by an increased rolling moment with increased airspeed. Continnual corrections are required as airsppeed changes. Fuel imbalance induced rolling moment is more noticable (and critical) at lower speeds because of the greater control deflections needed to counter it.

Asymmetric thrust is (as Smokey mentioned) another possible cause of yaw-induced rolling moment. Easily compensated for with rudder and rudder trim.

My point in presenting all these observations is to illustrate the fact that we cannot make any assumption that the control rigging, fuel quantity indication or inclinometer ball are properly adjusted when correcting for a "wing heavy" condition. There are just too many variables beyond our control to make any unverified assumptions about these systems.

What we can do is stop any yaw with rudder, stop any roll with aileron and then follow any known flight manual procedures or recommendations.(as was suggested by Oldebloke) If a Boeing, then their procedure should be followed. Lacking of any official guidance, I would do the following if a "heavy wing"was noticed just after liftoff:

1) Stop roll with coordinated aileron/rudder if yaw is absent. Add more rudder input if aileron does not have immediate effect or yaw rate is detected. Correct yaw with rudder. Center the ball. (recommended for the airplane I currently fly) The gear should be in transit by now.

2) Once control is assured, trim away control forces, rudder first. Have the PNF trim if both hands are busy. Get away from the ground and get to a comfortable climb airspeed. After the airplane is trimmed and a safe altitude (400' min for my plane) and speed are reached, the cause of the problem may be investigated by the PNF and remedial action taken in coordination with the PF. Treat this problem like any other abnormal in that flying the airplane is the first priority. I don't prefer to fly around out of trim while investigating the problem and probably have a DP to fly. I'd rather fly it at least roughly trimmed. If fuel imbalance is found to be the culprit, I can re-trim as the fuel is transferred. I would wait until after the workload eased off to do this.

So the only differences of opinion I seem to have with Old Smokey's assessment is that I would trim BEFORE investigating so I can fly the airplane more precisely. And in the airplane I fly, they call for centering the ball rather than the control wheel. We do not have spoilers tied into the aileron control system. (one possible reason for Boeing's recommendation?) If the heavy wing is discovered in some other phase of flight, then I would guard the controls and maybe roll in some trim while George continued to fly and the other pilot looks for the cause. I recall that B-747 split S job over the Pacific! Normally, I would expect that we would monitor our fuel state more closely than that, but I wasn't there.

Thanks for your indulgence and I apologize for being unable to make my case in fewer words! I welcome any corrections or difference of opinion as I consider all exchange on this forum to be a learning experience.

Best regards,

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Old 6th Oct 2005, 11:27
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You JET guys! No 1 reason for rudder trim is that you're flying with props and the speed/power has changed. LOL.
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Old 6th Oct 2005, 15:09
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When working as an engineer on B737-200 in the Gulf I once had an aircraft that failed the Flight Test by dropping the right wing well before the stall. It was straight off a D check and all the controls had been rigged to the book. We adjusted the flap drive shaft by half a turn to lower the right flaps compared to the left. It was indiscernable by looking at them, but cured the problem. I was amazed how a tiny variation of the flap rigging could make such a difference.
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Old 6th Oct 2005, 16:24
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Osaka Joe: How right you are! No place for sleepy feet.

Swedish Steve: Yeah, those Boeings have a whole bunch of flap drive couplings. You just know that during a check that includes inspection/lube of the flap drives, Murphy will make an appearance! Post maintenance test flights are sometimes quite interesting.

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Old 11th Oct 2005, 21:15
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Ignition Override wrote:
You might want to read about the Boeing 747-400 which almost had a catastrophe climbing out of San Feancisco ( SFO) several years ago, due to a pilot's preoccupation with the ailerons instead of the rudder, as an engine suffered a compressor stall or surge (whatever).
Engine vibration made it difficult to read the flight instruments. The second crew was sitting at the rear of the c0ckp1t.
It is an interesting story.
As I remember this particular instance, the engine problem was an inboard engine (thankfully) and the pilot flying (F/O) was flying just exactly the way he had been taught; “…keep the wings level with aileron. If the wheel is not centered (i.e., level), add rudder on the down-side of the wheel, keeping the wings level with aileron control, until the wheel is centered.” The written training procedures for an engine failure on takeoff was something like, “…address uncoordinated flight with the use of wheel and rudder.” The verbal advice was, “because the rudder is a very powerful flight control, we don’t want our pilots kicking the rudder around indiscriminately.”

In the instant case, the airplane narrowly missed hitting the hills just off the departure end of the runway because the pilot was flying a cross-controlled airplane and adding to the drag while one engine was failing to provide proper thrust. Had it been an outboard engine causing the problems, the yaw would have been greater – requiring a greater lateral control input – adding significantly to the cross-control situation and a higher level of drag. Would this have caused a crash? I guess there is no way to tell – except that some say the airplane missed “mother earth” by a scant 50 feet as it was.

On the other side of the scale … there are folks who maintain that the Airbus accident (on departure out of New York) was caused by the pilot flying making very rapid high magnitude rudder inputs – first in one direction, then in the opposite direction. Ultimately, the tail failed. I guess the jury may still be “out” on whether this is an accurate description of what occurred, but the issue of rudder vs. aileron apparently continues.

My question is “what is wrong with keeping the ball in the center?”
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