A320 fly-by-wire

[BeechBoy]

I have a friend who flies an A320 and he was telling me about an annoying quirk it has. When you're landing, if a gust of wind causes one of the wings to lift momentarily the computer "sees" the wing rise and it also knows that the pilot didn't do it because it didn't sense any stick input. It will "help" you out and provide control inputs to bring the wing down even though the autopilot is disengaged. If the pilot tries to apply a correction of his own it causes overcorrection and he said it was quite annoying.

There's an unwritten rule that there should only be one pilot flying the airplane. If the situation he described to me is accurate, it seems that Airbus has it so there's sometimes two pilots flying the plane.

Is this how the A320 actually behaves? Do any Airbus drivers have any inputs as to whether they like or dislike this feature?

[seagull]

All the FBW commercial transports essentially use C* control law (The 777 inanely has C*U, which essentially means you have to trim the darn thing manually - sheesh!!). C* law is a rate command, meaning your input is not moving the control surface a given amount, but rather commanding a roll or pitch rate (or lack of one). Given that, obviously the aircraft will move the control surfaces to maintain the rate input. This is not unique to Airbus, contrary to the implication in your post.

On landing, at a certain point, the control law shifts to direct law, where you are controlling the aircraft more conventionally (without getting into mundane details).

[BeechBoy]

Originally Posted by seagull

Given that, obviously the aircraft will move the control surfaces to maintain the rate input. This is not unique to Airbus, contrary to the implication in your post.

If the yoke/stick is in the neutral position doesn't that mean that a "zero" rate is being commanded? If that's the case why is the FBW commanding something other than a zero rate to bring the wings back to level?

(Forgive my ignorance. The plane I fly uses cable actuated flight controls so I have no experience with FBW).

[Minuteman]

Originally Posted by BeechBoy

If the yoke/stick is in the neutral position doesn't that mean that a "zero" rate is being commanded? If that's the case why is the FBW commanding something other than a zero rate to bring the wings back to level?

The aircraft it trying to maintain given load factors (or, instead, rates?). In your scenario, the fly-by-wire isn't trying to return to wings level, it is trying to counter the uncommanded rate the aircraft experiences because of the wind gust.

[BobDDuck]

With the stick in the neutral position the aircraft is going to hold what ever was the last commanded input. For example if the aircraft is flying wings level and the pilot (or autopilot) pushes the stick to the right for 3 seconds and then back to center the aircraft will roll 5 degrees right (I'm just making up numbers) and stay there until another input is made. In order to go wings level the stick would have to be moved to the left for 3 seconds to roll out those 5 degrees.

In the case you are talking about with the aircraft level and a gust picking up the wing, the system would sense that the stick never changed position and hence the wing should still be in it's previously commanded position (level) and will take measures to put it there.

[Nick]

Originally Posted by BeechBoy

I have a friend who flies an A320 and he was telling me about an annoying quirk it has. When you're landing, if a gust of wind causes one of the wings to lift momentarily the computer "sees" the wing rise and it also knows that the pilot didn't do it because it didn't sense any stick input. It will "help" you out and provide control inputs to bring the wing down even though the autopilot is disengaged. If the pilot tries to apply a correction of his own it causes overcorrection and he said it was quite annoying.

There's an unwritten rule that there should only be one pilot flying the airplane. If the situation he described to me is accurate, it seems that Airbus has it so there's sometimes two pilots flying the plane.

Is this how the A320 actually behaves? Do any Airbus drivers have any inputs as to whether they like or dislike this feature?

Could you re-phrase what you mean by "Airbus has it so there's sometimes two pilots flying the plane" ? I am not sure I understand what you are referring to, if you mean one pilot plus the flight law that the pilot is flying with, or both actual pilots.

[Nick]

Originally Posted by seagull

On landing, at a certain point, the control law shifts to direct law, where you are controlling the aircraft more conventionally (without getting into mundane details).

And specifically if I remember correctly the Airbus goes from flight mode to flare mode of normal law at 50' RA during landing.

It takes a "snapshot" of the pitch at that moment.

[BeechBoy]

Originally Posted by Nick

Could you re-phrase what you mean by "Airbus has it so there's sometimes two pilots flying the plane" ? I am not sure I understand what you are referring to, if you mean one pilot plus the flight law that the pilot is flying with, or both actual pilots.

What I meant is that even though the autopilot is off (and therefore, the human should have control of the airplane) the fly-by-wire system will occasionally provide control inputs. I'm used to systems where, when the autopilot if clicked off, I and I alone am flying the airplane.

I don't mean to be overly critical of the system. I've never flown it - maybe it's not such a big deal but it seems strange to have a system designed this way.

[seagull]

Originally Posted by BeechBoy

What I meant is that even though the autopilot is off (and therefore, the human should have control of the airplane) the fly-by-wire system will occasionally provide control inputs. I'm used to systems where, when the autopilot if clicked off, I and I alone am flying the airplane.

I don't mean to be overly critical of the system. I've never flown it - maybe it's not such a big deal but it seems strange to have a system designed this way.

I am hard pressed to think of a modern large transport that does NOT have some control inputs coming in from a computer when you're hand flying it. Just yaw dampers alone are doing that! The 777 and 787 are going to do much the same as the Bus, as you are really just telling the computer what you want to do. CWS of some sort is pretty standard these days on a lot of transports as well. Stability augmentation is also fairly standard.

On top of that, the artificial feel is a major difference, where a computer is making your controls heavier as you go faster, not aerodynamics. That is true of virtually every transport jet flying, at least the medium ones on up.

[falconvalley]

Is this what you guys are talking about??

http://airlinepilotcentral.com/web_v...070118184.html

[seagull]

Originally Posted by falconvalley

Is this what you guys are talking about??

http://airlinepilotcentral.com/web_v...070118184.html

No, that was a known problem of lateral PIO in direct law that the A320 series has/had. B757 has a similar issue.