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I read this in the normal procedures section of the A320 FCOM. Why is it so?

When the speed brakes are deployed, extending the flaps beyond FLAPS 1 may induce a slight roll movement, and in calm conditions a small lateral control asymmetry may remain until disturbed by a control input or by an atmospheric disturbance.

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  • $\begingroup$ This affects almost all jet-liners with spoilers and flaps, it's best to make it generic and tag it with aerodynamics. It's not unique to the A320. $\endgroup$
    – user14897
    Commented Jan 23, 2018 at 16:26
  • $\begingroup$ @ymb1 could you explain this phenomena? $\endgroup$
    – Jai
    Commented Jan 23, 2018 at 17:08
  • $\begingroup$ Not really no, but I can look into it. $\endgroup$
    – user14897
    Commented Jan 23, 2018 at 17:24
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    $\begingroup$ @ymb1 it appears specific to the A320. At least in the B777 FCOM, I have: "There is no limitation for extension of speedbrakes in a landing configuration". $\endgroup$
    – kevin
    Commented Jan 23, 2018 at 19:32
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    $\begingroup$ @kevin - that's good info, maybe they solved it with extra FBW lines, them mentioning it in the first place hints at a common issue, agree? $\endgroup$
    – user14897
    Commented Jan 23, 2018 at 20:52

1 Answer 1

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enter image description here
https://www.eucass.eu/doi/EUCASS2017-318.pdf

Above shows a configuration where the flaps and spoilers are deployed. The blue line shows the vertical velocity of the wing's wake, which is indicative of the lift being generated. Since the effect of the flaps lessens outboard, that causes the downforce shown above near the end of the wing due to the spoilers.

Since spoilers are thin and are allowed to move/flex slightly under aero loads, and since this downforce is far out with a greater moment arm, any slight [asymmetric] disturbance to any outboard spoiler panel will result in a roll, when compared to:

  • Smaller flaps deflection: since the inboard part of the wing creates less lift (less flaps), the slope (lift to downforce) will be shallower, which lessens the impact of an outboard disturbance.
  • Speed-brakes are used on their own: there will be a uniform downforce distribution over the wing without the abrupt shift from lift to downforce shown above.

Note that the manual says may and slight. Do not expect a huge roll, just anticipate if it happens, the FBW is not programmed to correct it unless an input is made.

I found the same recommendation in the FCTM of the 747-400 and 737 (all variants). It's flaps 5 and 15 respectively. They don't state the reason, but buffeting was mentioned.

The use of speedbrakes with flaps extended should be avoided, if possible. With flaps 15 or greater, the speedbrakes should be retracted (737 FCTM).

@kevin also pointed out in the comments that the 777 explicitly states there is no issue with using the S/B in landing configuration, which to me sounds like extra care was taken with the FBW or spoiler design.

Another possible reason is to lessen the load on the actuators of the spoiler panels, with the flaps deployed that's a lot of suction behind them, but that doesn't answer the rolling part.

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