How exactly does the apparent speed-stability of the Airbus A220 work?
Joined: Sep 2011
Posts: 350
Likes: 147
From: FL390
I’m pretty sure the Boeing controls, like the autothrottle, are back-driven, not spring loaded. You can pull up a live page that shows the force being applied in each axis.
On the Airbus, with the sidestick in the neutral position, there will be no force on the controls irrespective of speed, attitude, etc.? On the Boeing FBW, if you are out of trim with the controls centred, you will feel a force. It’s also how the envelope protection works in manual flight: you will feel an increasing resistance to your commands (as you would go outside normal limits) but can override this with greater force.
On the Airbus, with the sidestick in the neutral position, there will be no force on the controls irrespective of speed, attitude, etc.? On the Boeing FBW, if you are out of trim with the controls centred, you will feel a force. It’s also how the envelope protection works in manual flight: you will feel an increasing resistance to your commands (as you would go outside normal limits) but can override this with greater force.
Joined: Dec 2003
Posts: 2,338
Likes: 821
From: Tring, UK
The 777 and 787 use springs and cam-roller mechanisms to provide pitch feel. At higher airspeeds, the feel and centering mechanism increases the forces by increasing the preload on the springs. The backdrive actuators are active only when the autopilot is engaged or when flight envelope protections are active. The springs are never overridden.
It's possible to measure the column force regardless of how the feel and centering is accomplished. The force transducers are in series between the controls and the feel and centering mechanisms. Control forces are required FDR parameters since 2009 (IIRC). That means that 767s and 737s have to measure it as well. Even aircraft from the 60s have force transducers; they're needed for the control wheel steering mode of the autopilot.
It's possible to measure the column force regardless of how the feel and centering is accomplished. The force transducers are in series between the controls and the feel and centering mechanisms. Control forces are required FDR parameters since 2009 (IIRC). That means that 767s and 737s have to measure it as well. Even aircraft from the 60s have force transducers; they're needed for the control wheel steering mode of the autopilot.
Joined: Jun 2025
Posts: 7
Likes: 2
From: Zurich
I'll give it a go.
The side stick has springs, the force required to move the stick from centre increases linearly with angle of movement so with a little null of about 2 deg the force you need to apply increases from nothing up to 18lb at about 15deg from centre. This is known as the soft stop and is considered the normal maximum command. However, if you are brave and increase the force the stick will move a bit further. max physical deflection of the stick is 20 deg and that last 5 deg of movement required force from just under 30lb up to 35lb to hold it at the mechanical stop, this is known as the hard stop and is there so you can pull a bit harder and go over your 2.5g limit if you really have to. If no one told you that extra 5 deg of movement was there you probably would never know about it because the jump from 18lb to 30lb in force is completely unnatural and that is what is required to move it from 15 to 15.1deg. From there on it's linear again.
OK so what does moving the stick in pitch do? It commands a pitch rate. The rate is pretty much the same at all weights, CG, speed and configuration. (Pretty much but not exactly). So if you're in flight and you pull the stick back 5 deg you feel 8lb of spring force and the aircraft pitches up at x degrees per second. When you are at the pitch attitude of you choosing just release the stick. No force and it springs back to neutral. This is now commanding ZERO PITCH RATE. Sorry for the caps.
Simple so far. You pitched up and you start to climb because of all that basic principles of flight stuff.
Now if you did nothing else the aircraft will decelerate. This is where the speed stability part comes in.
As the speed starts to deviate from your previous trimmed speed this creates the same type of pitch rate command as you moving the stick. The further away from the trimmed speed (lets think of that as just like the neutral position of the side stick) the higher the commanded pitch rate. So as you decelerate the FBW starts commanding a nose down rate and the more you deviate the higher the rate. Exactly like moving the stick from the neutral position.
So with no further input from you and of course no change of thrust while any of this is happening the aircraft pitches nose down, at a slow rate initially but the rate increases as you get further from your trimmed speed (with a reasonable max limit) until you start accelerating again.
If you are acceleration you must be descending and the pitch rate reduces until you get back to your trimmed speed. Rate = 0. However you are descending and acceleration so the opposite happens. You go above your trimmed speed and the nose starts to come up. You can expect about 3 or 4 overshoots in the familiar phugoid oscillation.
The aircraft will end up flying roughly level at your trimmed speed so long as your altitude(and therefore thrust to drag) hasn't changed too much.
This is mimicking exactly what a positively stable aircraft would do if it deviates. But it's done through elevator inputs based on speed deviation rather than all those complicated AOA changes on your chuck glider.
Now if when you let go the stick earlier you didn't want the nose to go down you could always start pulling gently back on the sidestick again. So long as you apply an equal and opposite rate command to what the FBW is doing in trying to get back to trimmed speed the two will cancel out and the rate will be zero. But you will of course keep decelerating. As you do the FBW will be increasing it's command and you will have to increase yours. That means pulling further against an increasing spring force.
Put your other hand to use and increase the thrust and the aircraft will stop decelerating. The force required will stop increasing and now you can maintain your current force and the speed will not change. Hey presto you are flying out of trim and it feels like every other conventional aircraft. You have to apply a constant force equal and opposite to the difference between your actual speed and the trimmed speed that the FBW is desperately trying to get back to!
My arm is getting tired of holding this force so the little trim switch under my thumb comes into play. I start adding nose up trim. With every blip of that switch the trimmed speed (and it's graphical representation on the air speed tape) get closer to my actual speed. The FBW reduces it's rate command and I need to do the same by inching the stick back towards neutral.
This is hellish hard to do smoothly and the aircraft will pitch up and down as you work your way back into trim. Eventually the little bug will be on your current speed FBW will no longer be trying to put the nose down and the sidestick will be in neutral. You can now let go and grab the sick bag.
We fly pitch on the trim contrary to straight and level 101 to avoid this oscillation when retrimming.
Oh and while all this was happening the Stab was minding it's own business. It is only concerned with keeping the elevator somewhere near central. It looks at deflection and time and if it get's annoyed it quickly moves the stab to put the elevator in the middle of it's travel again.
Questions?
The side stick has springs, the force required to move the stick from centre increases linearly with angle of movement so with a little null of about 2 deg the force you need to apply increases from nothing up to 18lb at about 15deg from centre. This is known as the soft stop and is considered the normal maximum command. However, if you are brave and increase the force the stick will move a bit further. max physical deflection of the stick is 20 deg and that last 5 deg of movement required force from just under 30lb up to 35lb to hold it at the mechanical stop, this is known as the hard stop and is there so you can pull a bit harder and go over your 2.5g limit if you really have to. If no one told you that extra 5 deg of movement was there you probably would never know about it because the jump from 18lb to 30lb in force is completely unnatural and that is what is required to move it from 15 to 15.1deg. From there on it's linear again.
OK so what does moving the stick in pitch do? It commands a pitch rate. The rate is pretty much the same at all weights, CG, speed and configuration. (Pretty much but not exactly). So if you're in flight and you pull the stick back 5 deg you feel 8lb of spring force and the aircraft pitches up at x degrees per second. When you are at the pitch attitude of you choosing just release the stick. No force and it springs back to neutral. This is now commanding ZERO PITCH RATE. Sorry for the caps.
Simple so far. You pitched up and you start to climb because of all that basic principles of flight stuff.
Now if you did nothing else the aircraft will decelerate. This is where the speed stability part comes in.
As the speed starts to deviate from your previous trimmed speed this creates the same type of pitch rate command as you moving the stick. The further away from the trimmed speed (lets think of that as just like the neutral position of the side stick) the higher the commanded pitch rate. So as you decelerate the FBW starts commanding a nose down rate and the more you deviate the higher the rate. Exactly like moving the stick from the neutral position.
So with no further input from you and of course no change of thrust while any of this is happening the aircraft pitches nose down, at a slow rate initially but the rate increases as you get further from your trimmed speed (with a reasonable max limit) until you start accelerating again.
If you are acceleration you must be descending and the pitch rate reduces until you get back to your trimmed speed. Rate = 0. However you are descending and acceleration so the opposite happens. You go above your trimmed speed and the nose starts to come up. You can expect about 3 or 4 overshoots in the familiar phugoid oscillation.
The aircraft will end up flying roughly level at your trimmed speed so long as your altitude(and therefore thrust to drag) hasn't changed too much.
This is mimicking exactly what a positively stable aircraft would do if it deviates. But it's done through elevator inputs based on speed deviation rather than all those complicated AOA changes on your chuck glider.
Now if when you let go the stick earlier you didn't want the nose to go down you could always start pulling gently back on the sidestick again. So long as you apply an equal and opposite rate command to what the FBW is doing in trying to get back to trimmed speed the two will cancel out and the rate will be zero. But you will of course keep decelerating. As you do the FBW will be increasing it's command and you will have to increase yours. That means pulling further against an increasing spring force.
Put your other hand to use and increase the thrust and the aircraft will stop decelerating. The force required will stop increasing and now you can maintain your current force and the speed will not change. Hey presto you are flying out of trim and it feels like every other conventional aircraft. You have to apply a constant force equal and opposite to the difference between your actual speed and the trimmed speed that the FBW is desperately trying to get back to!
My arm is getting tired of holding this force so the little trim switch under my thumb comes into play. I start adding nose up trim. With every blip of that switch the trimmed speed (and it's graphical representation on the air speed tape) get closer to my actual speed. The FBW reduces it's rate command and I need to do the same by inching the stick back towards neutral.
This is hellish hard to do smoothly and the aircraft will pitch up and down as you work your way back into trim. Eventually the little bug will be on your current speed FBW will no longer be trying to put the nose down and the sidestick will be in neutral. You can now let go and grab the sick bag.
We fly pitch on the trim contrary to straight and level 101 to avoid this oscillation when retrimming.
Oh and while all this was happening the Stab was minding it's own business. It is only concerned with keeping the elevator somewhere near central. It looks at deflection and time and if it get's annoyed it quickly moves the stab to put the elevator in the middle of it's travel again.
Questions?
