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Airbus Flight Controls Question:

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Airbus Flight Controls Question:

Old 19th Feb 2024, 02:09
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Airbus Flight Controls Question:

Looking for some insight into A320 Flight Control Operation specifically during Normal Law, Autopilot Off, gusty wind conditions say on Final.

In "Understanding Air France 447" Bill Palmer writes "One common instance is during approach with Turbulence. Most pilots of conventionally controlled airplanes are used to instantly responding with a wind gust that tips a wing with a corresponding lateral control input. With the side stick in neutral, an Airbus will attempt to maintain a zero roll rate on its own, and will automatically input a roll command in response to the gust. Pilots who react to each bump and gust end up creating their own turbulence by wagging the sidestick back and forth faster than the airplane can respond."

Then I found something online on Airbusdriver.net. (can't post a URL here until I have 8 posts and this is my first I think).

One of the commenters says that the Airbus will look at the last bank angle and try to return to that point.

So this is really digging into the weeds and going down the rabbit hole, and I've noticed myself not thinking about any of this on a gusty final. But it still has me wondering when I'm not in the plane.

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?

The same thing could be asked with vertical forces. You hit a downdraft on final...see yourself going below glideslope. Since you didn't command a less that one G force, (since you are in Normal Law-Load Factor demand) with a neutral sidestick, the Flight Controls would seem to attempt to maintain one G flight by pitching up. Again, you see your glideslope going up and you react by pitching up. Are you augmenting the flight controls? Or does the system directly give you what your commanding on the Sidestick?

Thanks!



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Old 19th Feb 2024, 04:55
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Originally Posted by Coupe84
Looking for some insight into A320 Flight Control Operation specifically during Normal Law, Autopilot Off, gusty wind conditions say on Final.

In "Understanding Air France 447" Bill Palmer writes "One common instance is during approach with Turbulence. Most pilots of conventionally controlled airplanes are used to instantly responding with a wind gust that tips a wing with a corresponding lateral control input. With the side stick in neutral, an Airbus will attempt to maintain a zero roll rate on its own, and will automatically input a roll command in response to the gust. Pilots who react to each bump and gust end up creating their own turbulence by wagging the sidestick back and forth faster than the airplane can respond."

Then I found something online on Airbusdriver.net. (can't post a URL here until I have 8 posts and this is my first I think).

One of the commenters says that the Airbus will look at the last bank angle and try to return to that point.

So this is really digging into the weeds and going down the rabbit hole, and I've noticed myself not thinking about any of this on a gusty final. But it still has me wondering when I'm not in the plane.

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?

The same thing could be asked with vertical forces. You hit a downdraft on final...see yourself going below glideslope. Since you didn't command a less that one G force, (since you are in Normal Law-Load Factor demand) with a neutral sidestick, the Flight Controls would seem to attempt to maintain one G flight by pitching up. Again, you see your glideslope going up and you react by pitching up. Are you augmenting the flight controls? Or does the system directly give you what your commanding on the Sidestick?

Thanks!
AB FBW side stick out of neutral in roll commands a rate of roll not bank which is indirect result to fulfill the commanded rate of roll. Similarly in pitch stick in neutral commands 1g. FCC even without pilot input will use necessary flight controls to achieve that. So any pilot input on stick during that stage will augment it leading to pilot induced oscillations. In turbulence corrective pilot inputs should only be made after giving the system a chance to do what it's supposed to do. On approach etc what happens in turbulence is aircraft may automatically stabilize in 1g or zero bank etc but gets displaced from desired course or glide path which needs additional manoeuvring to get it on it's previous trajectory. In manual flight it won't happen automatically.
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Old 19th Feb 2024, 06:46
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Airbus used to call this “global corrections”. So on approach, you don’t respond to every lump and bump as you would a conventional aircraft, but you must make “global corrections” to return the aircraft to the correct trajectory.
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Old 19th Feb 2024, 08:50
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Just to add to the previous two answers to the OP question:

Your side-stick inputs do not cancel the FBW system. The FBW looks at the ADIRS, your side-stick inputs, the current attitude and feedbacks from accelerometers etc to compute a flight control surface response to all those inputs. With no control inputs, the FBW will (try to) maintain the last commanded attitude from either you or the autopilot. (Within limits).

One way of thinking about Airbus FBW is to imagine that the autopilot disconnect switch is in a different part of the control chain.

A traditional chain with autopilot engaged is :

Navigation guidance - Autopilot - # - Control surfaces.

In a conventional jet like that; the autopilot disconnect switch, '#' is between the autopilot and the control surfaces, so when you disconnect, your inputs go directly to the control surfaces.

With Airbus FBW the chain has an extra element :

Navigation guidance - Autopilot - # - FBW - Control surfaces.

When you disconnect in an Airbus FBW, your inputs join at '#' and go into the FBW, not to the control surfaces*.

*This is a simplification, (and refers to Normal Law), but it should help to appreciate why you can get into PIOs in Airbus FBW. Especially if you have not been trained to use the side-stick properly, (and no TRE ever explained the side-stick properly to me - I had to work it out for myself).

Anyway, just be aware that with the autopilot disconnected, the FBW is still in the circuit and will still be working the control surfaces to maintain the last commanded attitude. So don't stir the stick, just move the side-stick until you have the attitude you want and as soon as it is there, return the side-stick to neutral:

Correct - neutral. Correct - neutral. Correct - neutral etc.

Your inputs are "added" to the FBW commands, so In strong turbulence you might need to hold full deflection for a moment, to 'pick up a wing' for example. But as soon as the wing starts picking up, return the side-stick to neutral.

Airbus FBW helps you and assists with the flying task

Last edited by Uplinker; 19th Feb 2024 at 09:02.
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Old 20th Feb 2024, 07:26
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One of the commenters says that the Airbus will look at the last bank angle and try to return to that point.
Have a look at this thread: A320 aileron authority in Conf 3 vs Conf Full
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Old 20th Feb 2024, 09:04
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My advice, after 20+ years flying the A320, is don't over think it.

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Old 29th Feb 2024, 01:35
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Originally Posted by Max Angle
My advice, after 20+ years flying the A320, is don't over think it.
Thank you Max Angle. That's the advice I am giving myself. I'm totally doing that when not in the airplane. When on final and in the flare? No.
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Old 29th Feb 2024, 01:37
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Thank you Sim25. I'll take a look.
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Old 29th Feb 2024, 01:42
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Originally Posted by Uplinker
Just to add to the previous two answers to the OP question:

Your side-stick inputs do not cancel the FBW system. The FBW looks at the ADIRS, your side-stick inputs, the current attitude and feedbacks from accelerometers etc to compute a flight control surface response to all those inputs. With no control inputs, the FBW will (try to) maintain the last commanded attitude from either you or the autopilot. (Within limits).

One way of thinking about Airbus FBW is to imagine that the autopilot disconnect switch is in a different part of the control chain.

A traditional chain with autopilot engaged is :

Navigation guidance - Autopilot - # - Control surfaces.

In a conventional jet like that; the autopilot disconnect switch, '#' is between the autopilot and the control surfaces, so when you disconnect, your inputs go directly to the control surfaces.

With Airbus FBW the chain has an extra element :

Navigation guidance - Autopilot - # - FBW - Control surfaces.

When you disconnect in an Airbus FBW, your inputs join at '#' and go into the FBW, not to the control surfaces*.

*This is a simplification, (and refers to Normal Law), but it should help to appreciate why you can get into PIOs in Airbus FBW. Especially if you have not been trained to use the side-stick properly, (and no TRE ever explained the side-stick properly to me - I had to work it out for myself).

Anyway, just be aware that with the autopilot disconnected, the FBW is still in the circuit and will still be working the control surfaces to maintain the last commanded attitude. So don't stir the stick, just move the side-stick until you have the attitude you want and as soon as it is there, return the side-stick to neutral:

Correct - neutral. Correct - neutral. Correct - neutral etc.

Your inputs are "added" to the FBW commands, so In strong turbulence you might need to hold full deflection for a moment, to 'pick up a wing' for example. But as soon as the wing starts picking up, return the side-stick to neutral.

Airbus FBW helps you and assists with the flying task
Thanks Uplinker. You are right. No one ever explained this to me and the FCOM seems to have no insights into this.


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