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Airbus Xwind Landings/takeoff Control Inputs  
User currently offlinebueb0g From United Kingdom, joined Jul 2010, 643 posts, RR: 0
Posted (1 year 7 months 2 days 15 hours ago) and read 2907 times:

With FBW Airbii (or for that matter any FBW airliner that uses roll rate commands, like the 787), does the PF have to command into wind aileron, or can the stick be kept centred and the FBW computers will hold any potential rolling action? I ask this because I thought the control law is essentially direct on the ground for obvious reasons but at the same time I've never seen a bus pilot put into wind stick.

Thanks all

[Edited 2013-02-21 11:15:15]


Roger roger, what's our vector, victor?
11 replies: All unread, jump to last
 
User currently onlinePihero From France, joined Jan 2005, 4450 posts, RR: 76
Reply 1, posted (1 year 7 months 2 days 14 hours ago) and read 2882 times:
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As you correctly write, on the ground, there is a relationship between sidestick lateral input and aileron deflection. Problem is if you go further than a half roll input, the spoilers start to deploy, which is not very good as you then increase the drag on the wind side, causing the airplane to turn into it ( some call it "weathercocking effect" ). There's no difference with an airplane with classical flight controls.
The difference arises when, around 100 ft, the law becomes normal flight law and a lateral stick input becomes roll rate demand.
The solution is to be careful with your inputs and strive to maintain wings level at all times.
For landings, the technique is crabbing, but on some high xwind conditionds, you'd be wise to use a mixture of crab and forward slip methods.

Quoting bueb0g (Thread starter):
I've never seen a bus pilot put into wind stick

Up until 20 kt xwind, it is not really necessary



Contrail designer
User currently offlinebueb0g From United Kingdom, joined Jul 2010, 643 posts, RR: 0
Reply 2, posted (1 year 7 months 2 days 13 hours ago) and read 2823 times:

Quoting Pihero (Reply 1):
Problem is if you go further than a half roll input, the spoilers start to deploy, which is not very good as you then increase the drag on the wind side, causing the airplane to turn into it ( some call it "weathercocking effect" ). There's no difference with an airplane with classical flight controls.

Thanks for that!

Quoting Pihero (Reply 1):
The difference arises when, around 100 ft, the law becomes normal flight law and a lateral stick input becomes roll rate demand.
The solution is to be careful with your inputs and strive to maintain wings level at all times.

I've heard that can also catch people out on landing coming through 100ft again (although I guess this is mostly conjecture from people who haven't flown an Airbus)



Roger roger, what's our vector, victor?
User currently onlinePihero From France, joined Jan 2005, 4450 posts, RR: 76
Reply 3, posted (1 year 7 months 2 days 9 hours ago) and read 2758 times:
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Quoting bueb0g (Reply 2):
I've heard that can also catch people out on landing coming through 100ft again (although I guess this is mostly conjecture from people who haven't flown an Airbus)

A lot has been said - mostly wrongly - about the 'Bus flight controls. Fact is, these Airbus solutions are very similar - if not identical to what is on a FBW Boeing.

In our everyday life, on a routine flight, you'd see :
- A "Ground" mode in which you have a direct relationship between stick deflection and elevator movement. (That surface deflection is reduced as the aircraft accelerates )
- A flight mode which is the "Normal Law " that's beginning to be understood whereby a stick demands a g load (the more you pull, the greater the acceleration).
- A "Flare" mode which is, basically, a pitch attitude demand from the stick... This mode is the least known from non-'Bus people, although I say it again, FBW Boeings display similar characteristics.

On landing, in addition to the Flare mode, a nose-down bias is introduced to the airplane , thus forcing the pilot to pull on the stick, reproducing the behaviour of a classic airplane.
The mode changes, from "Ground" to "Flight" on takeoff and from "Flight" to "Flare + bias" to "Ground" are very progressive and in fact totally transparent to the pilot.

NB : Some say the bias introduction isq an effective part of the flare mode. I prefer un-linking them

[Edited 2013-02-21 17:00:51]


Contrail designer
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 4, posted (1 year 7 months 2 days 7 hours ago) and read 2697 times:
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Quoting Pihero (Reply 3):
On landing, in addition to the Flare mode, a nose-down bias is introduced to the airplane , thus forcing the pilot to pull on the stick, reproducing the behaviour of a classic airplane.

Is that specifically to reproduce the same piloting feel of non-FBW airplanes? Or is there another reason for it?

Quoting Pihero (Reply 3):
The mode changes, from "Ground" to "Flight" on takeoff and from "Flight" to "Flare + bias" to "Ground" are very progressive and in fact totally transparent to the pilot.

Just to check, when you say "transparent" do you mean apparent or not apparent to the pilot? "Transparent" in that usage typically means obvious or apparent, but I'm not sure that's what you're saying.  



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User currently onlinePihero From France, joined Jan 2005, 4450 posts, RR: 76
Reply 5, posted (1 year 7 months 2 days ago) and read 2574 times:
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Quoting vikkyvik (Reply 4):
Is that specifically to reproduce the same piloting feel of non-FBW airplanes? Or is there another reason for it?

The main reason is that you can't ever ever land on "Normal pitch law" : you can't flare as a sidestick pitch demand will send you back to the skies ( remember : pitch demand = g load )... unless, of course, you tried something very exotic  
And, yes, the manoeuvre has exactly the same feel as a non-FBW airplane.

Quoting vikkyvik (Reply 4):
Just to check, when you say "transparent" do you mean apparent or not apparent to the pilot?

It's totally not apparent ( in this case, "transparent in French is, like for the invisible man something you don't see, because it's there but you see through it... and sometimes, I revert to French meanings which don't always match the English ones... sorry. )

[Edited 2013-02-22 02:18:00]

[Edited 2013-02-22 02:27:32]


Contrail designer
User currently offlineLAXPAX From United States of America, joined Oct 2005, 80 posts, RR: 0
Reply 6, posted (1 year 7 months 1 day 10 hours ago) and read 2344 times:

Thank you and the OP for the informative posts. And your use of "transparent" was correct in this context. This is analogous to software design, where they may say that a particular process should be "transparent to the user", i.e. occurring automatically.


"Remember, no matter where you go... there you are." -- Buckaroo Banzai
User currently onlinePihero From France, joined Jan 2005, 4450 posts, RR: 76
Reply 7, posted (1 year 7 months 22 hours ago) and read 2197 times:
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Thanks.
Speaking a second language, one falls regularly in the "meaning" trap, which I have accepted, though I try to use the correct word.



Contrail designer
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 8, posted (1 year 7 months 11 hours ago) and read 2070 times:
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Quoting LAXPAX (Reply 6):
And your use of "transparent" was correct in this context. This is analogous to software design, where they may say that a particular process should be "transparent to the user", i.e. occurring automatically.

Fair enough!

Quoting Pihero (Reply 5):
The main reason is that you can't ever ever land on "Normal pitch law" : you can't flare as a sidestick pitch demand will send you back to the skies ( remember : pitch demand = g load )... unless, of course, you tried something very exotic

That part I understand - I was specifically wondering about the nose-down bias, and the reasoning behind that.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently onlinePihero From France, joined Jan 2005, 4450 posts, RR: 76
Reply 9, posted (1 year 7 months 11 hours ago) and read 2053 times:
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Quoting vikkyvik (Reply 8):
I was specifically wondering about the nose-down bias, and the reasoning behind that.

Ah ! Ok.
Imagine you're about to land
- On a non-FBW airplane, you flare, meaning that you'd try to avoid touching down at too high a rate of descent, so you pull on the control column, increasing the nose-up attitude and the AoA with a reducing airspeed. The effort on the column is noticeable as you've stopped trimming and the airplane feels heavy. You keep doing that until you've achieved the greaser you were attemping. Bravo !

- On a 'Bus, the same movement and the same feeling are induced by the system, which had memorised your latest attitude before the flare ( it's in fact a bit more complicated, so I'm just giving you the gist ) and introduces a nose-down moment ( the "bias" ) which you'd instinctively counteract and resist with an effort in pulling the stick back, bringing back an increasing nose-up attitude that will end up with another greaser. So, the "Flare mode" which is about pitch attitude control + the bias are there to allow you to have a similar feeling as on a classic aircraft.



Contrail designer
User currently offlinevikkyvik From United States of America, joined Jul 2003, 10035 posts, RR: 26
Reply 10, posted (1 year 7 months 5 hours ago) and read 1981 times:
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Quoting Pihero (Reply 9):

Gotcha, thank you very much. It makes sense, but for some reason, I just find it odd that they do things like that to mimic the response in other aircraft.



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlinebueb0g From United Kingdom, joined Jul 2010, 643 posts, RR: 0
Reply 11, posted (1 year 6 months 4 weeks 1 day 15 hours ago) and read 1866 times:

Quoting vikkyvik (Reply 10):
I just find it odd that they do things like that to mimic the response in other aircraft.

Well, the reasoning is you want pilots who have learned to fly or have experience on conventional aircraft (ie all of them), to see the Airbus's handling as intuitive and natural. It's the same idea behind Boeing's design on the 777 and the 787 (although Boeing took it much further) - both fully FBW, but from the pilot's point of view, they "mimic" conventional aircraft (control yoke, moving throttles, manual trim (even though you're not actually trimming, it's all fakery) and speed stability). It's all designed so pilots, even those who have never flown a FBW aircraft before, will feel comfortable flying it and thus reduce manual handling errors.



Roger roger, what's our vector, victor?
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