Now, it's time to explore the control surfaces on the wings. For this chapter, we have selected the two roll control surfaces: the ailerons and the flaperons. Flaperons serve two purposes on the aircraft. During a roll command, the flaperons act as inboard ailerons, and during flap extension, they droop to assist the main flaps. In this chapter, we will understand the roll function. Three inputs can operate the surfaces: control wheels, the autopilot command, and the aileron trim function.

Let's start with the control wheels. Control wheels are used to give a left or right roll command. When rotated, the wheels move the control cables. The cable movement rotates the cable drums. The drums rotate the left and right shaft assemblies. The two shaft assemblies are connected, so the rotation of the wheel on one side transfers the movement to the other wheel. This ensures both pilots have an indication of aircraft roll command. Feel and centering mechanisms provide the spring force to the control wheels. The further it is rotated, the harder it gets, and once the wheels are released, the spring returns them to the neutral position.

Let's look at the aileron and flaperon deflection due to control wheel rotation. The wheel position is picked up by the position transducers and sent to the actuator control electronics (ACE). The ACE relays the signal to the primary flight computer (PFC) to calculate the movement. The PFC instructs the direction and deflection positions for all four surfaces. The ACE will now operate eight power control units (PCUs), two for each surface. PCUs are electrically controlled hydraulic actuators and use different hydraulic systems of the aircraft for redundancy. For a left roll command, the flaperon and the aileron on the left wing are deflected upwards. The command signal for the right wing surfaces is inverted, so the flaperon and the aileron on the right wing are deflected downwards. Position transducers on the PCUs indicate the surface position to the computer, allowing precise control of the control surfaces.

Moving on to the autopilot roll control. The autopilot computer gives a roll command to the primary flight computer (PFC). The PFC instructs the ACE to move the control surfaces to meet the autopilot demand. At the same time, the PFC instructs the autopilot computer to backdrive the control wheels. The autopilot computer now engages the backdrive actuators, runs the motors, and rotates the control wheel to match the control surface deflection. This gives an indication to the pilots of an autopilot roll change. Once the command is over, the actuators are disengaged, and the control wheels return to neutral.

Finally, the aileron trim function. Aileron trim measurement is different from pitch and rudder trim. During a pitch trim command, a single control surface deflects, which is the horizontal stabilizer. Therefore, it is easy for the indicator to represent one unit as one degree of horizontal stabilizer movement. Likewise, during rudder trim, the rudder is the single surface that deflects, and one trim unit indicates one degree of rudder movement. Trimming the aircraft in the roll axis will result in the movement of four control surfaces: the two flaperons and the two ailerons. They also deflect in varying degrees and opposite directions. Therefore, for simplification, the aileron trim is in proportion to the control wheel movement and not the control surfaces. One unit of aileron trim indicates five degrees of control wheel movement.

So let's trim for 20 degrees left control wheel position. When the trim switch is used, it sends a signal to the primary flight computer to rotate the control wheels. The PFC acknowledges and operates the aileron trim actuator. The control wheels are rotated until the switch is released. The wheel rotation results in flaperon and aileron deflection, just like a manual input. The trim actuator will now hold the control wheels in the trimmed state until a new trim input is given by using the switch again.