A320 Lateral Control NORMAL LAW
 

Hello,
Could someone please explain the lateral control under normal law which assist the pilot during decrab, as per the FCOM DSC 27-20-10-30(Lateral Control) it states "During Decarb phase, the aircraft will limit induced roll after pedal input, in order to ease flight crew control"
Say, I have a strong crosswind from the right on approach till ground level. As I decrab (Left rudder) will the FCTL SYSTEM :
(1) Immediately command spoiler extension on the right wing to prevent the induced roll ?
(2) As per the wording i.e. "after pedal input" Does it mean at the instance of the application of the opposite ruder? even before the YAW-COUPLING is sensed?
(3) Say the FCTL LAW deploys the Spoilers on the right wing to prevent induced roll to the LEFT (Right crosswind), and at the same time the crew demands a roll rate to the right to come back to the centerline, will the roll order be exacerbated or the crew roll rate demand will take precedence?

Thanks

Vilas or someone with more technical expertise can provide a better answer, but the simple “hands on” way to understand it is that laterally roll control remains substantially the same all the way to the ground. With no stick input the system will continue to maintain a zero roll RATE using whatever control deflections it needs. Induced roll caused by yaw during decrab is no different, and if your rudder inputs are smooth can sometimes lead to the airplane perfectly wings level or a very slight bank into the wind right at touchdown with little to no lateral stick correction at all. Initiate the flare still in the crab, and ease the nose around straight as you set the airplane down thru the last bit of the flare. Nice and easy, even in a big wind. A couple of light taps on the stick will be enough, if you need any lateral pressure at all. Big gusts may need some more positive correction but it will still just be quick pressure, back to center.

The worst gusty crosswind landings are usually a result of a rapid “stomp” on a rudder pedal, done too high in the flare. Rapid yaw = rapid induced roll = abrupt (over)correction by normal law. This usually surprises the pilot and causes him to “wag” the stick instinctively in the opposite direction, which begins a “bob-and-weave” that feels like Dutch roll, followed by an awful pancake of a landing. Heavy gusts make it even more entertaining.

You can tell this is coming if your partner is PF landing somewhere with a crosswind gusting into the 30s or 40s with a lot of turbulence and shear. If they say something like “I hate landing in crosswinds in this airplane because you never really know what it is going to do”, you are guaranteed a show.

Thanks for the reply, so in order for the aircraft to inject a bank into the wind during a decrab i.e. prevent the induced roll, will it extend the spoilers on upwind wing or use the ailerons ?

I had replied something long time ago I don't remember now. Best way to find out is to do an auto land in simulator with strong crosswind selecting flt Ctl page on SD. I think it's the aileron that is used to keep wings level.

Here we go again...

https://cimg0.ibsrv.net/gimg/pprune....217701ff1e.jpg

Thanks for the "here we go again" anyway so it means that when PEDAL INPUT is sensed with wings level, the lateral normal law initiates/injects a bank angle opposite the the direction of YAW i.e. preventing induced roll? and any idea if this bank angle is initiated by the ailerons or spoilers?

I believe roll controls are usually done with the ailerons and the spoilers are brought in above a certain point. It should be transparent to the user and shouldn't matter where the roll command comes from.

Right rudder, yawing to the right, bank to the right, negative sideslip. Steady state (with no input on the stick) you'll end in a steady bank and steady sideslip. Which when you release the pedal becomes zero bank and zero side slip again (assuming you were flying wings level). This only makes sense if you accept that the normal law roll is implemented with an inner bank angle target loop, which some people don't and that's why I wrote "here we go again", not the first time discussing that...

Both spoilers and ailerons are used. If spoilers are also deflected depends on the magnitude of the "global deflection order" generated by the control loop of the law and then fed through the kinematics... They are a function of s/f so this will change...