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To eliminate the need for pilots (human or automatic) to maintain continuous force on their aircrafts’ yoke/joystick and rudder pedals while flying, aircraft generally have some sort of trim mechanism for each of their three rotational axes. The underlying operating mechanisms vary,1 but, from the pilots’ perspectives, they adjust the trim so that it takes up the forces that the pilots would otherwise need to apply to the aircraft to maintain its desired attitude. (For instance, if a pilot has to hold continuous nose-up pressure to hold attitude while slowing to land, they can dial in nose-up trim until the aircraft’s pitch remains steady even if they let go of the yoke or joystick; if they have to continuously hold down the right rudder pedal to counteract the yawing moment from a left-engine failure, they can dial in right rudder trim until they no longer have to stand on the pedal; and, if they have to turn the yoke [or tilt the joystick] and hold it in this position to compensate for a fuel imbalance or flap/slat/spoiler/whatevs asymmetry, they can dial in left/right-wing-down aileron trim until the aircraft no longer rolls one way or the other with the yoke or joystick released.)

Aircraft generally have, for each rotational axis, one or more sets of wheels or switches to allow a human pilot(s) to manually adjust the trim in the desired axis; additionally, most autopilot-equipped aircraft possess the ability to automatically trim the aircraft in at least the pitch and roll axes (and frequently the yaw axis as well), a feature known, unimaginatively, as autotrim. For pitch trim (the one a human pilot will have to adjust most frequently, by quite a wide margin, as well as, for almost-all larger-than-small aircraft, the only axis in which the control authority of the trim system exceeds that of the aircraft’s primary flight controls [and, thus, the only axis about which an unchecked trim runaway is capable of generating rotational moments that cannot be countered simply by manipulating the primary flight controls]), there are generally two or three redundant sets of trim controls (two electrical trim-adjustment mechanisms - one on the yoke or joystick, for easy, immediate access while flying the plane, and one on the instrument panel or center console - plus usually a pair of backup manual trim wheels next to the pilots’ inboard thighs). Roll and yaw trim, in contrast, are used much less often and have less potential for catastrophe in the event of a trim runaway that can’t be stopped manually, so they can usually get by with just one manual-operation mechanism each, usually a thumbwheel or pair of more-left/more-right switches; even so, however, the vast majority of aircraft do have at least some means for a human pilot to adjust rudder or aileron trim.

Enter the A320, which apparently has no provision for allowing a human pilot to retrim the ailerons:

Cockpit Controls

Each pilot has a side-stick controller with which to exercise manual control of pitch and roll. These are on their respective lateral consoles. The two side-stick controllers are not coupled mechanically, and they send separate sets of electronic signals to the flight control computers. Two pairs of pedals, which are rigidly interconnected, give the pilots mechanical control of the rudder.

The pilots use mechanically interconnected hand wheels on each side of the centre pedestal to control the trimmable horizontal stabilizer.

The pilots use a single control on the centre pedestal to set the rudder trim. There is no manual switch for trimming the ailerons. [National Transportation Safety Committee Aircraft Accident Investigation Report KNKT.14.12.29.04, page 30 (page 32 of the PDF file of the report). Header text bolded in original; other emphasis mine.]

This seems like a pretty dodgy design decision - in the event of an aileron-autotrim runaway, or of an aileron-autotrim failure combined with anything that would impart a large rolling moment to the aircraft (for instance, a fuel leak draining fuel from one wing, or an asymmetric-flap-or-spoiler situation), a human pilot would have to haul the joystick over to oppose the resultant roll, and hold it there, without any ability to trim out the rolling moment and recenter the joystick, for as long as the rolling-moment-inducing situation lasted (all with one hand, mind you, since Airbus place their joysticks way off to the side where a pilot can only use their outboard hand to manipulate them), while also making any necessary pitch inputs at the same time with the same joystick, likely resulting in heavy cross-coupling between pitch and roll, what with the pilot trying, all with one hand, to maintain a steady lateral roll input while simultaneously pushing and pulling the joystick forwards and back to make pitch inputs. Why doesn’t the A320 allow its human pilots to trim its ailerons?


1: For roll and yaw trim, plus pitch trim on most small aircraft, this usually takes the form of some sort of mechanism that adjusts the neutral position of the control surface(s) in question, typically either (for aircraft with fully-mechanical flight controls) by adjusting control-surface-mounted trim tabs (which, when deflected, create an aerodynamic force that causes the control surface in question, instead of returning to its faired position when the cockpit controls are released, to seek out a position a greater or lesser distance away from the faired position),2,3 or (for aircraft with hydraulic or electrohydraulic flight controls) by biasing the aircraft’s artificial-feels mechanism to make the relevant control-surface actuators apply a continuous force of the desired magnitude and direction. On most midsize-and-larger aircraft, pitch trim works by adjusting the angle of incidence of the aircraft’s horizontal stabiliser; the much-greater area of the horizontal stabiliser compared to the elevators greatly increases the aircraft’s maximum pitch-trim authority (allowing the aircraft to be trimmed for stick-free flight throughout much greater ranges of airspeeds and center-of-mass positions, but making a pitch-trim runaway potentially far more serious - a severe pitch-trim runaway in an aircraft with a trimmable horizontal stabiliser can easily generate a pitching moment in excess of the elevators’ maximum ability to counter) and requires much less deflection for a given trim input than the elevators would (reducing drag), while, as the elevators are mounted to the horizontal stabiliser, and move with it, elevator and pitch-trim inputs stack on top of each other, instead of pitch-trim inputs being taken out of the available elevator authority (as is the case for aircraft using the elevators for pitch trim). A few aircraft use fuel transfer or pumpable water ballast to move the aircraft’s center of mass forwards or aft for pitch trim, removing the need to move the elevators or stabilisers at all, but making large trim changes using this method requires the carriage of a correspondingly-large mass and weight of trim fuel or water ballast (although it can still be quite useful in reducing trim drag midflight, while the aircraft still has quite a lot of fuel left).

2: On many aircraft, these tabs are also used to move the control surfaces in response to flight-control inputs; when operating in this function, they are known as servo tabs. Most larger aircraft with manually-operated or manually-operable flight controls have servo tabs (even ones that don’t use tabs for trim), as they greatly reduce the required control forces in manual flight (compared to if the pilot(s) had to haul around the big primary control surfaces directly).

3: Some aircraft have trim tabs that are not mounted to the aircraft’s primary flight-control surfaces, but, rather, directly to the aircraft’s immovable structure; instead of trimming the aircraft by adjusting the positions of the primary control surfaces, these act as an additional slow-moving elevator/aileron/rudder/whatnot whose deflection can be varied to adjust the total rolling/pitching/yawing torque on the aircraft.

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    $\begingroup$ Pretty sure the FBW system takes care of it in the background. The biggest function of aileron trim is fixing minor rolls because the rigging isn't right on or a wing plank has a bit of twist. A given airplane will generally need the same aileron trim input all the time. $\endgroup$
    – John K
    Jan 14 at 2:09
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The normal flight law behaves as if it auto-trims the aileron, but there is no actual trim involved. The computer simply derives the desired deflection of ailerons using the pilot input and feedback from the inertial reference system. So

  • In normal law (which requires working inertial reference) neutral side-stick means maintain bank angle, so effectively auto-trim.
  • In alternate and direct law (after various failures; mostly multiple failures required) neutral side-stick means neutral ailerons. They don't have anything to add bias to them, only the hydraulic actuators, so the risk that significant deflection is needed to maintain wings level is extremely improbable. Some periodic adjustment might be needed though. It only happens upon multiple failures and you are probably trying to land ASAP anyway.
  • In mechanical backup the ailerons are not active at all (not sure whether they stay neutral or even float free) and yaw-to-roll coupling is used. Mechanical backup is only used after many things have failed. There was one accident where it was needed, and the aircraft was controllable enough that while it did get damaged on landing, nobody was hurt.

It is different from elevator, where there is a trimmable horizontal stabilizer that does maintain position without any command, so the trim wheels for that are essential. And from rudder, which has mechanical link (for sake of the mechanical backup), so needs a mechanical trim as well.

The yaw-to-roll coupling also means that some tendency to bank can be trimmed away by adding a bit of side-slip with rudder trim.

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  • $\begingroup$ In alternate law lateral control is direct for the A320. You need constant adjustments to keep the aircraft wings level. $\endgroup$
    – Anas Maaz
    Jan 14 at 18:08
  • $\begingroup$ @AnasMaaz you are right; I misremembered that part. It's a bit strange though; the inertial reference, specifically vertical acceleration, is still used for the pitch control, so it seems strange to stop using roll rate for roll control. $\endgroup$
    – Jan Hudec
    Jan 14 at 21:13
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There is no need for aileron trim because the flight control system keeps the bank angle constant if there is no lateral input on the side stick. The pilots use the side stick to roll the aircraft to the desired bank angle and then hold the stick neutral. The FCS will then maintain that bank angle by giving the right amount of aileron input. The exception is that if an extreme bank angle is commanded (> 33°), the FCS will reduce that bank angle to 33° and then maintain it.

From the same accident report:

Lateral Control

When the PF performs a lateral input on the sidestick, a roll rate is ordered and naturally obtained. Therefore, at a bank angle of less than 33°, with no input on the sidestick, a zero roll rate is ordered, and the current bank angle is maintained. Consequently, the aircraft is laterally stable, and no aileron trim is required.

Source: National Transportation Safety Committee Aircraft Accident Investigation Report KNKT.14.12.29.04

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Airplanes are designed with less lateral stability when compared to longitudinal stability and directional stability. This means, they are very nimble and easy to control in the roll axis. If you have flown any aircraft you will realise that it is way easier to roll an aircraft than pitch it up and down. A longitudinally unstable aircraft can easily go out of control. Imagine a disturbance pitching up an aircraft. If it has less longitudinal stability it will continue to pitch up and may cause a stall. The pilot will struggle to control the pitch up as the reduced stability will make it harder to keep it from pitching up. An aircraft which has less lateral stability is easier to manage. It does not have consequences such as increased load factor on the air frame and it cannot stall an aircraft either. Because longitudinal stability is high you need trimming, to keep the aircraft's own stability from preventing a pilot desired maneuver. Roll stability is comparatively low, so a pilot can control it with little to no trim.

In large aircraft, aileron trim is not used in normal flying 99% of the times. I flew the Dash 8s and it had an aileron trim and even when we hand flew the aircraft, I have not seen anyone messing up with the aileron trim. If the control cables are set correctly, the aircraft is extremely easy on the hands. Even in engine failure situations, you do not require the used of aileron trim. The Dash 8s have a strong tendency to roll towards the failed engine. You could use the aileron trim, to trim this out but it is easier to counteract this without any trim at all, as power changes during the flight and particularly in approach requires too much re-trimming.

You also asked about a trim runaway. In a trim runway, you want to disable the trim it self. Again, in the Dash 8 if we have an aileron or a rudder trim runway, the pilot flying will use the yoke to wings level the aircraft and the pilot monitoring have to use the trim switch to center the trim and at the same time find the circuit breaker for the trim and pull it. As the aircraft has an electrically operated trimmer, this will immediately shut it off. There is a special QRH procedure for it.

Now, let us talk about Airbus. Modern Airbus aircraft in normal law have automatic trim in pitch and roll axis. In A320, if the aircraft control laws degrade to alternate law, the roll control becomes a direct stick to control surface relationship. As with other aircraft, the roll control in direct is very easy to control in the A320. You have mentioned about a fuel leak causing an imbalance in the tanks which can tire a pilot. It is most certainly possible for an imbalance to occur. But such an imbalance would not result in extreme rolling moments which are harder to manage. Neither will flap or a spoiler asymmetry. And keep in mind that use of aileron trim will always cause the control column has to be physically deflected to maintain the aircraft wings level. This is not what exactly we want. You do not want to land an aircraft with a deflected yoke or stick. If you need to continuously deflect the control column, be it a yoke or a side stick. The better way to do this will be to introduce a little side slip by the use of rudder trim. There is a procedure which requires this in A320. When the aircraft suffers from a Green + Yellow hydraulic system failure, the right aileron momentarily is not powered because a) Green hydraulic power is gone and b) The ELAC 2, the computer which powers one of the right aileron actuators is shed. This will move the right aileron into its zero hinge moment position. In this position, there is an aileron up float which can cause a rolling moment. Airbus recommends to use rudder trim in this situation so that you do not need to apply a constant side stick deflection to keep the aircraft wings level. The application of the left rudder will increase the speed of air flow over the right wing, which will increase the lift over the wing and the aircraft will go wings level.

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For a fuel imbalance situation, again the aircraft is flight tested to fly with one full wing tank and one empty wing tank to the landing.

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