Open aircraft.cfg of A321 and modify the parameter of fly_by_wire from 1 to 0 under the [airplane_geometry] section
This will disable the fly-by-wire function simulated on the aircraft and allows the Auto-Pilot Wing Leveler (Ctrl-V) to work.
Should someone wants to keep the A321’s fly-by-wire function, one could make use of the FSX control FLY_BY_WIRE_SEC_TOGGLE to toggle the function whenever needed.
For example, one can assign the control to a button of Saitek’s Flight Instrument Panel (FIP) as follows:
<Button Id=”6″ Name=”FBW” Event=”FLY_BY_WIRE_SEC_TOGGLE”/>
By pressing the button, one can then disable (or enable) the fly-by-wire function to allow the Wing Leveler to work (or stop).
It is necessary to FIRSTLY press this assigned button to disable the fly-by-wire function before entering Auto Pilot mode. Otherwise, the Wing Leveler still won’t work.
Fly-By-Wire Technology Background
Airbus A321 is the first jetliner that employed the digital fly-by-wire control system when entering service in 1988. The company states that FBW technology is one of Airbus’s principle competitive advantages as it brings weight savings, lower maintenance costs and greater flying precision to the company airliners.
Fly-by-Wire is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted into electronic signals transmitted by wires (hence the fly-by-wire term), and flight control computers determine how to move the actuators at each control surface to provide the ordered response, according to Wikipedia.