(This is an A380 on a right turn. The inner and center ailerons (big arrow) has a higher deflection compared to outer ailerons (small arrow). It is hard to see the delay in center aileron deflection because the delay is just 0.35 seconds)
The reason for three ailerons is to increase redundancy. The most outboard aileron is controlled by two servo actuators supplied by two engine driven hydraulic systems (green and yellow hydraulics). The center aileron is powered by a servo actuator which is supplied by the green hydraulic system and the inner aileron’s actuator is powered by the yellow system. Both the inner and center ailerons also have an electrohydrostatic actuator which can function in a situation where there is a total loss of hydraulic pressure.
The aileron design of the A380 has an interesting story behind it. Back in 2005 in a particularly windy day, the A380 was undergoing a test flight over Toulouse. The test pilots found out that a lot of rolling action was being required to control the aircraft in turbulence and gust generated by the strong winds. The lateral forces caused by the quick rolling actions were not only uncomfortable for the pilots, but it was way worse in the cabin. The problem was considered unsatisfactory and some tweaks were required.
The solution Airbus came up with was to modify the control laws of the roll computer. The new modification allowed the individual control of each of the ailerons (outer, center and inner). This law made it possible to tweak ailerons as desired to control the aircraft roll. With the new computer, they did test flights to find out what would give the pilots the best roll handling of the aircraft especially in gusty conditions. The whole idea was to reduce the aileron movement to make the aircraft less sensitive in the roll axis. To achieve that, they made the inner aileron deflect twice that of the outer aileron. The center aileron deflection was delayed to about a half second. This means that if the aircraft is making a right turn, you will see the right outer aileron and inner aileron deflect together with the latter deflecting twice that of the former. The center aileron will deflect half a second later with the same deflection as the inner aileron. As you can see, the idea is to reduce the rolling moment. The fact, that the outer aileron deflect less that the inner aileron considerably reduced the rolling moment.
The aileron action is more visible during take offs, landings and in gusty conditions. During take off and landing more roll inputs are applied by the pilot to control the aircraft trajectory. So, the ailerons are active during these phases of the flight. It is hard to see the actual deflection angles that easily because pilots on landing and take offs will be putting in quick right and left rolling actions to keep the aircraft stable on the flight path.
Author – Anas Maaz (Airline Pilot)
