Airbus Flight Controls Question:
Joined: Apr 2022
Posts: 267
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From: France
The roll law is a roll rate law.
As per FCOM DSC-27-20-10-30 :
The roll rate requested by the pilot during flight is proportional to the sidestick deflection, with a maximum rate of 15 °/s when the sidestick is at the stop.
Joined: Feb 2023
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From: Aachen
Again the pilots input is integrated so the aircraft behaves the way it's described...
Joined: Feb 2023
Posts: 60
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From: Aachen
See https://arc.aiaa.org/doi/10.2514/6.1989-3594 for reference...
Joined: Apr 2022
Posts: 267
Likes: 21
From: France
See https://arc.aiaa.org/doi/10.2514/6.1989-3594 for reference...
If the pilot is asking for 0 roll rate, the target is zero roll rate. Not a constant bank angle.
At least, what I'm saying is consistent with the in-flight behavior of the plane. You still have to counter the gusts. Less than a conventional aircraft, but you still have to counter them.
Joined: Feb 2023
Posts: 60
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From: Aachen
Of course you have to make inputs in gusty conditions, you want the aircraft to stay on the dsired path (especially while maintaining the center line of the runway when landing). The loop isn't closed in this respect. If you enagage a nav lateral ap mode, it would, but not while hand flying.
Hope this clarifies it
Joined: Apr 2022
Posts: 267
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From: France
That's interesting. However I'm really not sure it would pass the test of reality. I'm really not sure the wings would stay level in gusty conditions. People fight with the stick in order to keep the wings level, not because they notice their flight path going sideways.
It's very surprising. We can't know for sure except if we're in the design office. I'll try to ask some friends in Toulouse.
It's very surprising. We can't know for sure except if we're in the design office. I'll try to ask some friends in Toulouse.
Joined: Apr 2022
Posts: 267
Likes: 21
From: France
I'm going to bet it's the guy whose name is written on top of it, who did a PhD in fly by wire for helicopters.
I'd love to read it, however I'm not gonna pay 25 bucks for this paper.
I'd be amazed if it was really unnecessary to use the sidestick to maintain a zero bank angle in gusty conditions.
Did you ever try that yourself in flight ?
I'd love to read it, however I'm not gonna pay 25 bucks for this paper.
I'd be amazed if it was really unnecessary to use the sidestick to maintain a zero bank angle in gusty conditions.
Did you ever try that yourself in flight ?
Joined: Jan 2004
Posts: 3,246
Likes: 202
From: Here and there
I manage flying without thinking about any computer equations.
I know the FBW, (and auto-thrust* and GS Mini), are there. I know what they do and how they help me, and I know how to operate the side-stick without creating PIOs. Then I just fly the 'plane by feel, observation and experience, and it all works very well.
*older A330 auto-thrust can be 'lazy' near the ground in certain conditions but you simply deal with it.
I know the FBW, (and auto-thrust* and GS Mini), are there. I know what they do and how they help me, and I know how to operate the side-stick without creating PIOs. Then I just fly the 'plane by feel, observation and experience, and it all works very well.
*older A330 auto-thrust can be 'lazy' near the ground in certain conditions but you simply deal with it.
Joined: Nov 1999
Aviation Qualifications: ATPL
Posts: 3,148
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From: UK
Fair comment. Don't get me wrong; so do I, but not down to such detail as in post #19 etc. On short finals on a gusty day; that is probably over-thinking it, somewhat !
(A lot of pilots do not understand GS Mini, but I made sure I do, and how it helps us, but I don't need to know any equations to understand it, just what it does. )
The OP asks:
I read that as not asking for a realy deep explanation of the computer programming; but more about how the FBW reacts and the relative authority of the FBW and the side-stick, and whether the side-stick inputs cancel the FBW inputs.
From memory, so please correct me; the FBW can command 25° roll rate, the side-stick 15°.
It's a fantastic system though; I absolutely love it
(A lot of pilots do not understand GS Mini, but I made sure I do, and how it helps us, but I don't need to know any equations to understand it, just what it does. )
The OP asks:
.......So your on final, Normal Law, AP off. Wing tips in a gust. Does the FBW flight control system stop the gust induced bank(because you didn't command it)? And does it actually attempt to return to its previous bank? Now you don't wait for all of this. You correct with sidestick. Are you augmenting the flight controls? Or does the plane sense your input and completely give you back full control? In other words...the plane with full sidestick is a 15 degree/sec roll rate. You correct with half sidestick, so what would be the roll rate command if the system is also responding to a gust?
I read that as not asking for a realy deep explanation of the computer programming; but more about how the FBW reacts and the relative authority of the FBW and the side-stick, and whether the side-stick inputs cancel the FBW inputs.
From memory, so please correct me; the FBW can command 25° roll rate, the side-stick 15°.
It's a fantastic system though; I absolutely love it
Joined: Feb 2024
Posts: 7
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From: Hong Kong
“If your stick is neutral, you're asking for 0 roll rate. So the aircraft will damp any roll rate. But the resulting roll won't be corrected.”
how can the system damp the roll rate and matain level,and the next sentence the resulting roll still exist???
how to understand this,in my mind it is no roll rate=no roll=0 bank
thxxxxx
how can the system damp the roll rate and matain level,and the next sentence the resulting roll still exist???
how to understand this,in my mind it is no roll rate=no roll=0 bank
thxxxxx
Joined: Sep 2016
Posts: 937
Likes: 67
From: USA
“If your stick is neutral, you're asking for 0 roll rate. So the aircraft will damp any roll rate. But the resulting roll won't be corrected.”
how can the system damp the roll rate and matain level,and the next sentence the resulting roll still exist???
how to understand this,in my mind it is no roll rate=no roll=0 bank
thxxxxx
how can the system damp the roll rate and matain level,and the next sentence the resulting roll still exist???
how to understand this,in my mind it is no roll rate=no roll=0 bank
thxxxxx
Ok last take on this: I cite (out of the paper I referenced): "The objectives are thus: To have an absolutely neutral spiral mode for a bank attitude lower than 33° ie: with the sidestick in neutral position, the lateral attitude will be frozen in spite of any lateral gusts." later it is discribed that "On the other hand, as far as gusts are concerned, the aircraft will respond with modes p1 and p2. If the aircraft is subjected to lateral gusts, as the stick remains in the zero position, the PHI_C target is therefore unchanged and the control loop will counter the gust to bring the aircraft back to its initial lateral attitude."
Joined: Feb 2024
Posts: 7
Likes: 0
From: Hong Kong
The gust overpowers the airplane's ability to damp to 0 roll rate, so some bank angle develops. After that, the bank stays there because the control law does not respond to bank angle within 33 degrees.
Seems like this is describing the part that brings the bank to 33 degrees if exceeded, and not the general case within 33 degrees.
Seems like this is describing the part that brings the bank to 33 degrees if exceeded, and not the general case within 33 degrees.
think i am confuse the roll between the roll rate,roll=bank (like 7degrees),and the roll rate is a rate（like 2degrees per sec）,so encounter the gust，roll/bank rate or bank angel appear,and computer will damp the rate to 0 and prevent more bank be make,thats it, the a/c still hold bank until move the side stick. that right?
thx!
Joined: Aug 2020
Posts: 485
Likes: 241
From: Far East
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?
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?
Last edited by waito; 9th July 2024 at 09:59.
Joined: May 2002
Posts: 379
Likes: 4
From: Sale
Hi,
That's a long response, but essentially correct.
In normal law (in flight mode) the roll mode is NORMAL not DIRECT.
It's more easily described as Neutrally Statically stable.
When turbulence tries to roll the aircraft, the aircraft will try to prevent this by maintaining the commanded roll rate (between 0 and 15 ⁰/sec).
So it makes no difference to the pilot whether you are flying level or rolling to a bank angle, you still have to make small "global" corrections.
That's why it's better to nudge the stick then wait.
Although if you watch the many YouTube videos, stirring the porridge is still commonplace.
Hope that helps.
That's a long response, but essentially correct.
In normal law (in flight mode) the roll mode is NORMAL not DIRECT.
It's more easily described as Neutrally Statically stable.
When turbulence tries to roll the aircraft, the aircraft will try to prevent this by maintaining the commanded roll rate (between 0 and 15 ⁰/sec).
So it makes no difference to the pilot whether you are flying level or rolling to a bank angle, you still have to make small "global" corrections.
That's why it's better to nudge the stick then wait.
Although if you watch the many YouTube videos, stirring the porridge is still commonplace.
Hope that helps.
Joined: Aug 2020
Posts: 485
Likes: 241
From: Far East
So it makes no difference to the pilot whether you are flying level or rolling to a bank angle, you still have to make small "global" corrections.
once we got it straight, I will switch to vertical control (pitch/load factor control) where I really struggle.
Joined: May 2002
Posts: 379
Likes: 4
From: Sale
I would simplify it further.
Don't make corrections for turbulence.
Just set the attitude that gives you the performance that you want and adjust as necessary.
Less is definitely more when flying the Airbus.
Don't make corrections for turbulence.
Just set the attitude that gives you the performance that you want and adjust as necessary.
Less is definitely more when flying the Airbus.
Joined: Nov 2010
Posts: 1,167
Likes: 180
From: USA
The way it works for me on the bus: correct for trends, not for upsets. Chances are, if the attitude is changing, it is because the airplane is correcting. So don't fight it. If you are hand flying with the FD OFF, look at the LOC/GS direction of movement, look at your course and VS. correct for trends on the LOC/GS, try to keep course and VS constant, and move the stick as little as possible, stop trying to fight attitude changes.
If you have the FD ON, keep the needles centered, or even better, turn the AP on, because you aren't really flying anyway.
Pretty much what FIS said
If you have the FD ON, keep the needles centered, or even better, turn the AP on, because you aren't really flying anyway.
Pretty much what FIS said
