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I was recently reading the Flight Controls chapter in the FCOM and I read that spoilers 2-5 extend a maximum of 35 degrees on the A320 however on the A321 spoilers 2,4 & 5 extend a max of 35 degrees but spoiler 3 extends only a max of 7 degrees (during a roll).

What is the reason behind this?

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  • $\begingroup$ A321-100 or A321-200? $\endgroup$
    – Jan Hudec
    Feb 5 at 8:09
  • $\begingroup$ @JanHudec my company operates the A321-271NX and A321-251NX. $\endgroup$
    – Jai
    Feb 5 at 11:18
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    $\begingroup$ That applies during rolling, not the in-flight SB / ground spoiler function. I clarified that part in the question body. The reason is probably aerodynamic and has to do with the A321's different flaps, or wing loading; hopefully this clarification can help the aerodynamicists here to deduce the reason. $\endgroup$
    – ymb1
    Feb 5 at 12:00
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One possible reason is to reduce the roll sensitivity in icing, which affects only the A321. This was discovered after multiple control issues and further investigation by Airbus:

Main findings were:

  • Similar ice accretion on both A/C.
  • Increased spoilers efficiency on both A/C.
  • Increased roll sensitivity only on A321.

[...]

  • Analytic roll inputs showed unusual A321 roll response of the iced A/C (overshoot, degraded turn coordination…). (A320/319 are OK on this aspect).
  • This degradation was mainly observed in Conf Full, to a much lower extent in Conf 3.

Source: Airbus via smartcockpit.com

I skimmed the presentation but there doesn't seem to be a clearly stated underlying reason, though the 321's wing is slightly bigger.

If this is not it, then at least this shows a difference in roll performance between the 320 and 321. After all the 321 is bigger with higher MTOW, so the lift is also bigger when heavier, and the wing is only slightly bigger (i.e. higher wing loading).

Another possibility is the longer fuselage, which gives the rudder a bigger moment arm. Since spoilers in a bank counter the adverse yaw, having a bigger rudder moment arm would call for less spoilers.

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  • $\begingroup$ appreciate your in depth input. Any way to prove that this is the reason and not something else. As so far the reasoning which I have got from majority people is that it's a design feature apparently which I don't really believe. $\endgroup$
    – Jai
    Sep 9 at 1:40
  • $\begingroup$ @Jai: Since it does not impact the operation, Airbus is not obligated to share how they came to that decision. Only the designers at Airbus know and they've signed agreements not to share their work. As I've written, I'm in no way saying my answer is definitive. I just thought it helpful to highlight how the tiny wing differences have an impact in roll. $\endgroup$
    – ymb1
    Sep 9 at 2:02
  • $\begingroup$ alright makes sense if the designers have signed NDA's any other agreements. But thanks for sharing an input that does shed some light on it. $\endgroup$
    – Jai
    Sep 9 at 9:34
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    $\begingroup$ @Jai: No doubt it's interesting. I added another possibility. $\endgroup$
    – ymb1
    Sep 9 at 14:27

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