When the Airbus A320’s flight control system is operating in mechanical law (the simplest and lowest-tech of the A320’s flight control laws, where deflections of the pilots’ joysticks are transmitted directly to the control surfaces via ye olde hydraulickes, with absolutely no envelope protections at all; mechanical law is the A320’s only completely non-fly-by-wire flight control law, and is generally only used in the event of an utter and total failure of the aircraft’s flight control computers), the aircraft’s ailerons and spoilerons are inoperative, and lateral control is accomplished solely by means of the rudder.
However, the rudder is a horrible lateral control surface; its intended purpose is for directional control, controlling yaw, not roll. Although an aircraft’s rudder can, in a pinch, be used to roll the aircraft in whichever direction the rudder is deflected, it does this indirectly, by inducing sideslip. The rudder, being quite close to the aircraft’s centerline, produces only a small direct rolling moment - which, to make matters worse, for aircraft with the rudder mounted above the aircraft’s center of mass (e.g., essentially all commercial airliners, the A320 included), is in the wrong direction. Thus, if one wants to turn right, and applies right rudder to that effect, the aircraft will initially roll slightly to the left, then stop, then, as the nose yaws to the right and the sideslip angle grows, finally roll suddenly and violently to the right. Once you’re actually in a turn, the situation is improved not at all, as now you have to religiously avoid applying any further rudder in the direction of the turn, lest you start skidding (turning with an excess of yaw, resulting in the aircraft’s nose pointing to the inside of the turn), which is an excellent, time-tested way of entering an unrecoverable spin and making your insurance company give your family lots of money.
If one had to choose just one set of control surfaces to use when turning, one would think that the optimal choice would be the lateral controls (ailerons and spoilerons), not the directional controls (rudder); that way, the required turning technique would be essentially unchanged from during normal flight, as modern aircraft do not use the rudder at all under normal circumstances (advances in aileron design1 having freed newer designs from the severe adverse yaw that bedeviled the Wrights and was the rudder’s original raison d’etre).2 Indeed, the A320’s stateside counterpart, the Boeing 737, does exactly this; when operating in manual reversion (the closest 737 equivalent to the A320’s mechanical law, given that the A320 inexplicably has no manual-reversion capability despite being small enough that it should have been a breeze to include), the 737’s rudder is (nominally) inoperative,3 and the aircraft is turned using only the ailerons and spoilerons.
Given the immense superiority of the ailerons and spoilerons over the rudder for lateral control, why did the A320’s designers go with the latter?
1: Consisting of various ways of artificially increasing the drag experienced by the falling wing in order to balance out the increase in induced drag on the rising wing resulting from its downturned aileron. The major way of doing this is simply by adding spoilerons to the system, which has the double benefit of eliminating adverse yaw (or even turning it into proverse yaw - that is, yaw in the direction of the turn, rather than out of it) and considerably increasing roll authority (especially at low speeds - and, hence, high angles of attack - where the maximum downwards aileron deflection is severely restricted to avoid stalling the rising wing, which would produce a rolling moment in the wrong direction, somewhat hampering one’s efforts to turn the plane in the direction intended); other methods include deflecting the upturned aileron on the falling wing much further from its faired position than the downturned aileron on the rising wing (thus balancing out the increase in induced drag on the downturned aileron with an equal or greater increase in pressure drag - and, at the transonic cruising speeds of modern jetliners, potentially wave drag as well - on the upturned aileron) and building the ailerons such that, when deflected upwards, part of the hinge end of the aileron protrudes below the wing (where it produces extra drag on the now-falling wing to balance out the extra drag on the now-rising wing).
2: In modern aircraft, the rudder is only used when it is necessary, for some reason, to generate a large sideslip angle, with these cases falling into three general categories:
The rudder’s design case (the scenario that places the greatest demand on the rudder, and, thus, determines the minimum control authority - and, thus, indirectly, the size - required of the rudder) is an engine failure at low speed, with the absolute worst case being a sudden, total failure of the furthest-outboard engine during takeoff, immediately after V1. At these low speeds, the rudder has to be large - especially on wing-engined aircraft - to counter the large yawing moment from a failed engine, which is why the rudder has enough control authority to overpower the lateral controls at low speed, which is why all aircraft have a crossover airspeed.
The most common use of the rudder, in contrast, is during crosswind landings, where the aircraft, in order to keep its nose pointed into the relative wind during the descent and flare, has to approach the runway with a large crab angle (i.e., with the aircraft’s longitudinal axis forming a large angle with that of the runway, rather than the two being parallel, or, ideally, coinciding), and then, upon mainwheel touchdown, must immediately (within a second or two) yaw to the runway heading (which requires a very high yaw rate, especially with a high crosswind component and resultant very large crab angle, and, thus - as the tiller can’t be used for steering until the nosewheel touches down, and, at any rate, would be largely ineffective, except at destroying the nosewheel tyres, at the high speeds of a touching-down jetliner - a sudden, large rudder input) in order to avoid running off the side of the runway.
The third main rudder-use scenario is if the aircraft has to turn suddenly when moving at speed on the ground (for instance, to avoid colliding with a baggagecart/deer/other plane/very lost driver/inattentive groundcrewperson/tiger griffin/tumbleweed that’s somehow found its way onto the runway you’re using to take off and/or land). In this case, the aircraft cannot roll, because the ground is in the way, so the rudder (along with differential braking, and, at lower speeds, some tiller input) is used to turn the aircraft.
3: In actuality, a small amount of rudder deflection is available in manual-reversion flight if enough force is applied to the rudder pedals, due to the specifics of the design of the 737’s rudder control system.
