I've been fortunate enough to have had several flights in the A380 as a passenger, but this last one a few days ago was the first where I've been overlooking the wing. I was struck, during the approach and landing, by the fact that the aileron is split into 3-4 sections and - more to the point - they appear to act in opposition. Why would the system command simultaneous large up and down movements, as opposed to a single small movement?
This kind of movement is related to the Load Alleviation Function mainly. The Aileron Droop Function and Lift Dumping and Phased Lift Dumping Functions are involved as well.
Load Alleviation Function:
The aim of the Load Alleviation Function (LAF) is to alleviate the fatigue and static loads on the wings by reduction of the wing bending moment. The LAF is composed of: - The Passive Turbulence Alleviation (PTA) and, - The Active Turbulence Alleviation (ATA). The Passive Turbulence Alleviation alleviates the static loads in turbulence and during maneuvers. The activation is based on the vertical load factor given by normal law. Once the PTA is activated, it progressively decreases 5 seconds after the triggering condition disappeared. The computed orders are sent to the ailerons and spoilers 6 to 8. The pitch compensation linked to the deflections is sent to the inner elevators. The Active Turbulence Alleviation alleviates the fatigue and static loads. The activation is based on the measurement of the vertical load factor on the wings given by the accelerometer units installed on the outer pylons. The ATA uses the inner and mid ailerons. It can only be activated if: - The PTA is activated and, - At a speed above 240 knots. The ATA orders are added to the PTA orders, and can be added to the roll orders. There is no pitch compensation for the ATA. The LAF is activated above a given speed and vertical acceleration thresholds but is inhibited when slats and flaps are in full
Aileron Droop Function:
The aileron droop function is computed by the lateral law. When the pilot puts the slat/flap control lever in the position "1" or more, the Slat Flap control computers send to the PRIMs a "flap out" signal through ARINC 429. The PRIMs send deflection orders to the ailerons. The ailerons droop downward to increase the wing curve.
The ailerons continue to fulfill the roll function
Lift Dumping and Phased Lift Dumping Functions:
The lift dumping and phased lift dumping functions are used to: - stick the aircraft on ground by suppressing the lift, - reduce bound risks, - decelerate the aircraft and, - give acceptable level of passenger comfort at touch down. All available spoilers are used for these functions. Moreover, ailerons contribute to the lift dumping function. The ailerons move upward.
yes, the LAF or MLA (on A340/A330, but more or less a similar system) should reduce the lift of the wings (mainly outer section) to keep the g-loads an the in the design limits and reducing the load at the wing root section.
your absolutely correct, this systems helps to reduce structure weight and increase lifetime.
AFAIK it's installed in all "modern" Airbus and the L1011-500 was/is equipped with a very similar system (not really based on FBW, but both ailerons can deflect upwards). Maybe there some more types i don't know .
Thnaks, but I'm not sure that is what I was looking at. What has been described above is a relationship between the ailerons and the inner elevators (sic) to reduce wing bending moments. What I was describing was the outboard ailerons themselves being split into sections and then operating in opposition. So two neighbouring aileron surfaces would be operating in opposite senses simultaneously - why not simply move one surface by a smaller amount?
hmm, i've no A380 in my license, so i can't contribute with facts, sorry. There are some possibility's i think of, why the F/CTL-system act in this way, but that are just speculations of mine.
At least on the A330/A340 there is a function which supports the rudder during "low speed" situations and high "rudder demand" by splitting the ailerons and deploying Spoiler No6.
I noticed the movements of the ailerons as well on my recent flight on the A380. No-hoper, your post mentioned that the ATA activates when the PTA is active and above 240 knots. However, I noticed the split aileron movements just after lift-off to about 1000AGL. Any reason for this?
Sorry,don't know.Could be the PTA.My typcourse was nearly 2 month in the classroom,but sometimes i think it was not sufficient. The guy who developed the software for the 6 computers(splitted in A&B off course) might know the answer...
Likely the aileron deflection is a form of bias to prevent aileron limit cycle oscillation (LCO). By deflecting aileron segments on a wing oppositely there is no net roll input, but each aileron is pressed firmly against its actuator's rod end and thus cannot flutter. The flight control system can set the appropriate bias as necessary for the current flight conditions, thus minimizing drag. The A320 series, for example, apparently have a significant LCO induced maintence problem.