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?

  • 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. – falstro Dec 18 '13 at 23:19
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    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. – DeltaLima Dec 19 '13 at 0:23
  • 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. – Qantas 94 Heavy Dec 19 '13 at 1:19
  • 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. – Philippe Leybaert Dec 19 '13 at 1:26
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    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. – Qantas 94 Heavy Dec 19 '13 at 3:06
up vote 12 down vote accepted

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.

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.

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.

I know it is been a while since this was asked, just want to contribute a little:

As you are a private pilot you probably have flown some little airplanes like a Cessna or a Piper and so on. As you know, the speed range of those planes goes from about 44 kt to 160 kt (Vso and Vne for a PA 28). So, in general your speed range is about 120 kt. A airbus has a never exceed speed of 0.9 Mach, which is (at FL350, ISA conditions) about 520 kt, and a stall speed of 112 kt. There is a 400 kt difference there! As you have surely noticed when flying, ruder corrections in cruise (fast) flights need to be much less extensive then on approach (slow flight). This is because there is less air flowing over the wing in a certain amount of time, therefore less lift is created. And if you lower an aileron to create more lift on one side, the lift created by that aileron is less then when you are flying fast. Therefore the force acting on the plane which would roll it is not as big, more extensive controls are needed!

To get back to the Airbus: The ailerons are designed to be used in fast flight, not so much for slow flight. As the wingspan on a Airbus is about 4 times the one of a PA28, the arm that a aileron has is higher on the airbus. Thus it does not have to create as much lift force to have the same effect(relatively!). Therefore it is not designed to create that much lift when it is being put down. Now as you get into slow flight, the lift created is less then in fast flight, and as moment = arm * force, and the force is reduced much, the moment is reduces, thus the effectiveness of the aileron is decreased.

Now the spoilers don't go down, they go up only when the aileron on the same wing would go up. Looking at a wing you will notice that the cord line changes when moving away from the airplane: The closer you get to the fuselage, the higher the angle is (You can see that on a prop especially!!). This again has to do with the arm, the wing can not carry as much load on the far end. Therefore in slow flight, the spoilers are lifted. As you see in the video by DeltaLima, you can actually see through the wing when the flaps are lowered. With spoilers down, the air can not rejoin before the flaps, hast to travel a longer distance over the flaps and therefore creates more lift. In other words: The angle between cord line and relative wind (angle of attack) is raised. When the spoilers go up, part of the air can actually travel through the gap, decreasing the angle of attack, therefore decreasing the lift on that side of the wing and therefore helping out the not so effective aileron on the wing.

I hope this makes stuff a little more clear, and you didn't get bored reading through!

  • Please rethink what you have written: Lift is proportional to weight, so it will be the same regardless of speed. Big airplanes have more aileron moment, but also higher inertias: They need more moment. At high speed the wing will twist a lot with aileron deflection; ailerons are more effective at low speed. More distance traveled does not indicate more lift; the flap gap helps to make the flap more effective. Limited aileron deflections at low speed should help to avoid stalls on the outer wing. THAT is why spoilers are used for roll control. – Peter Kämpf Dec 21 '14 at 11:44
  • I don't agree on all of that! Lift is NOT proportional to weight. Lift required is proportional to the weight to keep an aircraft flying! The higher the speed, the lower the angle of attack needs to be to create the same amount of lift! Big airplanes do have a lot more aileron moment, and need more, which is why I put "(relatively)". You will also notice that the ailerons on small planes are bigger then the ones on large planes (relatively!). The more air flows over a wing, the more effective changes in the cord line are! Too much of a flap gap will reverse the effect! I agree on the stall! – Maverick283 Dec 21 '14 at 12:45
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    The A320 has Vmo = 350 kias and Vne = 381 kias. It may be 520 true at altitude, but true speeds are totally irrelevant when talking about lifts and control forces that are all dependent on dynamic pressure. – Jan Hudec Dec 21 '14 at 22:17

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