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There is an aileron trimmer in some Boeings. When and why should it be used?

P.S. I'm not a pilot, can you answer with less smart words?

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Given your request to "answer with less smart words," it's hard to know what knowledge can be assumed, but I'll give it a try.

Let's say you're hand-flying the airplane straight and level, and you find that the right wing keeps wanting to drop, which you prevent by using left-aileron pressure on the control yoke. You can "trim out" the need for that left aileron pressure by rotating the aileron trim to the left (counter-clockwise).

Aileron trim is not often used for the simple reason that in normal aircraft operation, it's usually set where it should be from the previous flight. In other words, you don't fuss with it unless you notice a need to fuss with it. In 12 years on 727s and 747s, I think I used aileron trim less than a dozen times.

The reasons that I can think of offhand that would make you want to fuss with it include:

  • The trim wheel was inadvertently moved (or deliberately as a bit of a prank) by someone.
  • A fuel imbalance has occurred between the fuel in the left and right wings.
  • You're doing a 3-engine ferry on the 747 or continuing a flight after having lost an engine.

To the best of my knowledge, all Boeing aircraft have aileron trim capability, not just some. Indeed, I would be surprised to find any transport category aircraft that does not have aileron trim capability.

Essentially, aileron trim can be used to to keep and airplane from responding to a situation where one side of an airplane is significantly heavier than the other side.

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    $\begingroup$ Another main one is with most jets the wings are often not perfectly straight after coming out of the jig. Variations in riveting sequence can apply torsion forces that are relaxed when out of the jig and the wing box twists ever so slightly. This may show up in flight, and the manufacturer will try to rig ailerons and flaps to counteract it, within the rigging tolerances of the surfaces. Sometimes that is not quite enough and a tweak of aileron trim is required. The OEM will usually have a quality control based limit on the amount of trim required to stop a roll for a new delivery. $\endgroup$ – John K Dec 5 '18 at 21:54
  • $\begingroup$ Does a single engine failure usually require aileron trim? Seems like you'd have to counter the asymmetric thrust with rudder trim and counter the rudder roll with aileron. $\endgroup$ – TomMcW Dec 6 '18 at 19:09
  • $\begingroup$ @TomMcW I think your comment would make a good regular question. I was never particularly good at aerodynamic theory, and the almost 20 years that have passed since I retired and my increasing dementia have made me unable to answer that with confidence. What I seem to remember from multi-engine training and the two real-life engine failures I had where we returned for engine-out landings, was that we didn't fuss with aileron trim, but I'm fairly sure we did when doing two 3-engine ferries and a continued flight after an engine failure in the 747. $\endgroup$ – Terry Dec 6 '18 at 21:16
  • $\begingroup$ Pilots playing pranks on each other can turn out bad. I read about a flight that crashed because during flight, while the captain was in the bathroom, a pilot in the jumpeat engaged the rudder gust lock as a prank $\endgroup$ – TomMcW Dec 6 '18 at 21:28
  • $\begingroup$ @TomMcW I very much agree, but that's a no-brainer for me as I'm not a prankster. When I was flying the F-27, we had a big, 300 lb senior captain who, when deadheading, would walk up and down the aisle, forcing those flying to re-trim. He was also known for leaving a fake but very realistic looking pile of fecal matter on seats. $\endgroup$ – Terry Dec 6 '18 at 21:41
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In short almost NEVER.

I have flown the A320/A330/A340 and they don’t have an option to trim the ailerons.

I am currently flying the 777, and when you feel that the aircraft is not flying straight we generally tend to use the rudder trim over the aileron trim.

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Aileron trim is used in case of wings fuel assymetry, this is used until you correct the assymetry by providing the engines from a single wing.

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  • $\begingroup$ This doesn't really add anything to existing answers. $\endgroup$ – fooot♦ Jun 3 '19 at 19:52

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