Airbus Flight Controls Question:

[waito]

After users, including some pilots, received their answers some weeks ago, I like to use this thread to get my picture complete.

I'm a layman, no stick time on any real plane, just some Pro Sim hours on non FBW. So active pilots, best don't get distracted from my input if you're sensitive for distraction.

I'd be glad to first get confirmation if I understood the principle correct - of controlling the right parameters with Airbus Normal Law. First the roll/bank laws (Normal​​​​​ law, within limits, no a/p). So no considering the protections. Is my understanding correct:

With Sidestick the pilot controls exactly the roll rate. 0 position means 0 roll rate, fixed bank. Moving sidestick will command a roll rate, which results in banking. The more deflection the more roll rate, the faster bank angle does change. max is 15deg per second.

Assuming one disturbing turbulence during perfect final approach on LOC:

Wings level, roll rate 0, bank 0, HDG 360

A strong assymetric turbulence lifts the right wing. FBW senses deviation from roll rate 0 and stops the roll asap. It cannot do that in 0.000s, so there's a delta t to consider. Probably longer delta with stronger turbulence. Duration of turbulence shouldn't really matter.

result: roll rate 0 again, bank few degree to the left, HDG constantly decreases from 360°, progressive path deviation by a constant left turn.

FBW reaction completed - now it's the pilots job to return on path by "Global Correction":

sidestick to achieve Bank 0: not enough, HDG wrong, increasing deviation from path

sidestick to get back to HDG 360: not enough, constant deviation from flight path.

sidestick to return to path (LOC), HDG, wings level: Global correction completed.

essence: FBW helped to quickly arrest the roll, the major job to return to path must be done by pilot, but certainly dealing only with a rather minimal deviation and no rush in controlling the disturbance.

let's see the effect when the plane is hit in a standard left turn.

roll rate 0, bank constant 25°, HDG constant decrease, path is the temporary turn radius

A strong assy turbulence lifts the right wing. FBW senses deviation from roll rate 0 and stops the increasing roll asap. It cannot do that in 0.000s, so there's a delta t to consider.

result: roll rate 0 again, bank few degree more than 25°, HDG constantly decreases, now a small bit faster, path deviation by a constant left turn at slightly lower radius.

FBW reaction completed - now it's the pilots job to return on path by "Global Correction":

higher bank: correction to original bank 25, and this might be sufficient already.

HDG not yet reached the desired new value, it will be reached a second or so sooner.

new desired flight path not yet reached. intercept may begin a little bit earlier.

turn path: deviation is minimal and Global correction can be postponed to interception on new leg.

essence: FBW helped to quickly arrest the roll, and probably prevented to enter non standard banking. The Global correction is small by reducing bank and delay the rest to interception phase.

Finally the effect when the plane is hit in an initiation of a left turn.

pilot deflecting side stick to the left, roll rate constant, bank increasing to the left, HDG decrease builds up, path is dynamic.

A strong assy turbulence lifts the right wing. FBW senses deviation from actual (commanded) roll rate and returns to this rate. It cannot do that in 0.000s, so there's a delta t to consider.

result: roll rate as commanded again, bank increased quicker (bank angle decreased to negative left quicker) for a short time than desired, influence on HDG change and path very small and indirect.

FBW reaction completed - now it's the pilots job to return on path by "Global Correction", well:

correcting higher bank: not necessarily, when the turn was just initiated, it's still far from desired bank. it just banked faster with very small indirect influence on heading, path. The pilot may decide no correction necessary until intercepting the new leg.

essence: FBW helped to quickly arrest the roll rate increase, and probably prevented to enter non standard banking or reaching the desired bank angle in a blink.

The Global correction can be delayed to interception phase. Pilot fully benefits from FBW.

Let's compare a detail with non FBW planes: Here the strong turbulence could lead to reaching the desired target bank within a second, missing the FBW instant damping. Now how much will the pilot correct the disturbance? He should return to the former bank value and then continue to slowly build up bank. Is this realistic? Isn't there an attraction to think, "well here we are at the turn, let's continue with it!"? Which produces a larger deviation from course and a different intercept.

Essence from the essence: FBW relieves the pilot only of the instant arresting of undesired roll rate, which OTOH may be the major (or only) correction to perform in some scenarios.

Comments, corrections?