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.
