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As a private pilot I have zero knowledge about how big jets are being flown so I was a little surprised to see that smaller Airbuses (A32x) only use spoilers for roll control on final approach.

During the initial phase of the approach ailerons are being used but at a certain point they stay in the neutral position and roll is controlled by the spoilers.

When I mentioned this to the pilot when exiting the aircraft he was a little annoyed by the question and he said I must have seen it wrong. I've seen this numerous times since then so I'm pretty sure I wasn't mistaken.

Is this actually how the spoilers are used on approach, and if so, why?

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    $\begingroup$ The B738 uses spoilers primarily for roll control as well on final. Although I've seen a bit of aileron movement, most of the corrections are using spoilers. Also clueless as to why, wing-tip-stall risk perhaps? I understand swept-wing aircraft stall tip first. $\endgroup$ – falstro Dec 18 '13 at 23:19
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    $\begingroup$ Airbus has various versions of the aileron and spoiler control system on the A320 family. But all these control surfaces are used together to control roll, yaw and alleviate gust loading on the wing. On the short field kit, the ailerons will move upwards on both sides to assist in steeper descent. It is very common to use spoilers for roll control during low speeds, most big jets use it. $\endgroup$ – DeltaLima Dec 19 '13 at 0:23
  • $\begingroup$ Spoilers and ailerons are both used on the approach -- the computers on the Airbus dictate how much control deflection is required, but I don't know the criteria for the ratio between aileron and spoiler deflection. $\endgroup$ – Qantas 94 Heavy Dec 19 '13 at 1:19
  • $\begingroup$ Every time I observed the control surfaces during final approach, the ailerons didn't move at all. The spoilers on the other hand moved constantly. Even during a very gusty approach at LHR the ailerons remained fixed in the neutral position. $\endgroup$ – Philippe Leybaert Dec 19 '13 at 1:26
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    $\begingroup$ You might want to take a look at this YouTube video (just found one by the first few results when searching). From what I can see, there's definitely aileron movement on final. $\endgroup$ – Qantas 94 Heavy Dec 19 '13 at 3:06
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Downward aileron travel must be restricted when flaps are deflected. Since flaps change the local incidence on the flapped part of the wing, the outer wing will experience an increase in its local angle of attack. The increased suction over the inner wing will accelerate not only the air flowing over the inner wing, but also that air which will flow over the outer wing as well. Left to itself, the outer wing would stall if powerful Fowler flaps are deployed on the inner wing.

Adding slats will push the stall angle of attack up, and now the outer wing is back in business. However, if the aileron is deflected downwards, the stall angle of attack is reduced again. The outer wing could stall simply due to the aileron deflection!

I do not need to point out that an asymmetric stall on approach is extremely undesirable.

Now roll control could still be achieved by only moving one aileron upwards. But this would decrease the induced drag at the wingtip at low speed. Remember, at low speed the induced drag is dominant (it is proportional to the inverse of the flight speed squared), so the drag change due to an aileron deflection will cause an inverse yawing moment at a time when the pilot wants to keep the aircraft lined up with the runway.

Using the spoilers instead will incur a higher lift loss for the same rolling moment, but will also increase local drag, which creates a helpful yawing moment into the opposite direction of the one created by upward aileron deflection. Also, the change in local lift from spoiler deflection is much greater once the flaps are fully deployed. This means that spoilers are much more effective for roll control in the landing configuration than in the take-off or cruise configurations.

THAT's why spoilers are preferred for roll control on approach. I am surprised, however, that you observed no aileron motion at all. Normally, the ailerons are still used, but with a much restricted travel range, as @DeltaLima mentions in his answer.

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This HD video of an A320 landing in Chicago shows that the ailerons move all the time during the approach, although the excitations are very limited from the moment the final flaps are selected. It seems there is some kind of limiting functions activated.

Airbus has various versions of the aileron and spoiler control system on the A320 family. It is important to note that all these control surfaces are used together to control roll, yaw and alleviate gust loading on the wing. On the short field kit, the ailerons will move upwards on both sides to assist in steeper descent.

It is very common to use spoilers for augmented roll control during low speeds, most big jets use it. It is sometimes observed that spoilers deploy partially during takeoff under strong crosswind conditions. Here is an example of a B767 that has spoilers deflected during most of the take-off roll.

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