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Airbus laws: direct law

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Airbus laws: direct law

Old 2nd Feb 2015, 21:26
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Airbus laws: direct law

Hi,

When you are on ground, your F-CTLS' are in direct law.
When you are in flare, your F-CTLS' are in direct law.

But do you have yaw coordination and yaw damper in direct law? Roll oders?

F-COM says: " Minimize the lateral inputs on ground and during the rotation, to avoid spoiler extension"

So would it mean that you have roll oders on ground?

Thanks
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Old 3rd Feb 2015, 06:22
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On ground you are in ground mode direct law. Side stick commands aileron and roll Spoiler surface deflexion in ground mode. In flight mode it commands rate of roll. In Direct law yaw is mechanical. You do not have any automatic yaw compensations. you need to apply the rudder in a turn. In approach at 50ft you get in flare mode. It is a flight mode with at difference, it is only direct stick to elevator relationship but not direct law. Sometimes referred as flare law. Side stick still commands rate of roll.
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Old 3rd Feb 2015, 09:20
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Airbus say the reason to avoid spoiler extension on the take-off is because that would increase drag, (which would be asymmetric and therefore also induce yaw). As far as I know, this is why they do not recommend using into wind aileron.
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Old 3rd Feb 2015, 15:31
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Thanks a lot:

So in ground mode - direct link you don't have anything.

In flare: you have a direct side stick to elevator command.
But you still have yaw coordination / yaw dampers.

1) Do you agree?

2) @Uplinker:
Hein?? Okay got your point but if you have roll oders it means that you are in flight mode.
But F-COM says that spoilers can extend on ground!!! But how is that possible? You don't have roll....

Thanks
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Old 3rd Feb 2015, 16:31
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You don't have roll rate demand, but that's not the same as "don't have roll".
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Old 3rd Feb 2015, 17:07
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Very intresting Amadis...

Yeah, think I got it: you don't have roll = roll is no more available. There is a problem (like a bad "communication" between ELAC and SEC's).

When roll demand is not there it just means that roll is available BUT the computers are programmed to not give it at X moment (like in direct law!)

Right?
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Old 3rd Feb 2015, 17:12
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Normally, you stay in Normal LAW.
You change between different MODES.

Ground MODE (of Normal LAW) is similar to direct LAW with the addition of reduced elevator deflection and trim resetting on landing i.e. it's not direct law.

Ground MODE blends into flight MODE (of Normal LAW) at different rates for pitch and lateral control. See the picture.

Flare MODE (of Normal LAW) is not the same as direct LAW as a nose down term is added.

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Old 3rd Feb 2015, 22:49
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Thanks a lot!

Ok, so let me try again:


Normal law - Ground mode:

- Direct link with ailerons/rudder/elevators.
- You don't have roll rate demand - yaw coordination/ yaw damper.
- trim resets after landing.
- Reduced elevator deflection.

Normal law - Flare mode:

- Pitch trim stops, direct stick to elevator command.
- Bank trim available.
- Roll demand available.
- Nose down term is added.

1) Is everything right?

2) Sorry, but I still don't understand how spoilers can extend on ground as you don't have roll rate demand....You are in direct link with ailerons/ elevators and mechanical link with rudder's.

3) You picture is very intresting...
What happens after lift-off? Pitch attitude < 8°? So the FBW will try to not go below 8° pitch angle after lift-off? What does 0,5 seconds and 5 seconds refer too?

Thanks a lot.
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Old 4th Feb 2015, 01:11
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Airbus Mode Transitions

AF330 - I believe that the notations on the diagram for altitude, attitude, and times refer to the triggers for mode transitions and the time periods for smoothly making the switch from one mode to the next. I am much more familiar with the Boeing logic, but many parts of this look similar. The pitch angles shown do not have anything to do with what the control system is targeting once in the mode, just when the transition is initiated.
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Old 4th Feb 2015, 02:12
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That's correct FCeng84, the flight controls begin to transition from Ground Mode to Flight Mode as soon as the pitch attitude is greater than 8 degrees after lift-off. The pitch controls transition to Flight Mode over a period of 5 seconds, while the lateral controls transition over a period of 0.5 seconds.

AF330:
2) Sorry, but I still don't understand how spoilers can extend on ground as you don't have roll rate demand....You are in direct link with ailerons/ elevators and mechanical link with rudder's.
The sidestick commands the ailerons AND the spoilers to move, both in the air and on the ground; however, the relationship between the sidestick position and control surface movement changes.

On the ground (ie Ground Mode), there is a 'direct' relationship between the sidestick deflection and the aileron/spoiler deflection, modified for speed. That is, a given amount of sidestick deflection will cause a given amount of aileron/spoiler deflection at a given speed. As the aircraft's speed increases during take-off, the amount of aileron/spoiler deflection for a given sidestick position will reduce. That way, the pilot is able to maintain wings level with a fixed sidestick position throughout the take-off roll, instead of having to reduce the input as the speed increases, as you would with a conventional aircraft. The Airbus training manuals recommend against using large control inputs to avoid excessive spoiler deployment.

Once airborne (ie Flight Mode), the sidestick position commands a roll rate that is proportional to the sidestick deflection (ie a roll demand).

Last edited by BuzzBox; 4th Feb 2015 at 04:14.
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Old 4th Feb 2015, 06:32
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AFF330. Arbus has 3 laws and one mech.back up. Ground and Flare mode,also landing mode are submode of Normal law.
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Old 4th Feb 2015, 12:04
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Thanks a lot,

Ok, so let me give it another shot:

So you said:

On the ground (ie Ground Mode), there is a 'direct' relationship between the sidestick deflection and the aileron/spoiler deflection, modified for speed. That is, a given amount of sidestick deflection will cause a given amount of aileron/spoiler deflection at a given speed. As the aircraft's speed increases during take-off, the amount of aileron/spoiler deflection for a given sidestick position will reduce. That way, the pilot is able to maintain wings level with a fixed sidestick position throughout the take-off roll, instead of having to reduce the input as the speed increases, as you would with a conventional aircraft. The Airbus training manuals recommend against using large control inputs to avoid excessive spoiler deployment.

Hmm...

Ok, I don't understand the difference between these:

Roll Spoiler surface deflexion and "Rate of roll"...

Basically, in direct law, you have direct stick to elevator or roll control surface relationship.
So the ELAC will send a signal to SEC to extend spoilers.
So, as you said, the side-sticks are controlling spoilers AND ailerons.

So the rate of roll is the bank angle we have to reach? We can use ailerons, rudders and spoilers to get that angle.
Spoiler surface deflexion? No idea...

So in direct law (or ground mode normal law), you have roll demand but no yaw coordination turns...?

Thanks to explain the difference!
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Old 4th Feb 2015, 12:23
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On the ground, how much you move your stick directly determines how far the surfaces move. The roll spoilers work in tandem with the ailerons to give roll. When doing your control checks you get full deflection of the control surfaces for full stick deflection, so that you can check full, free, and correct movement.

In the air if you command a rate of roll, or a bank angle, the surfaces will move under control of the ELACs and/or SECs to give you the rate of roll or bank angle that you are commanding. There may be extra control surface movements going on that you are unaware of to compensate for turbulence and atmospheric upsets to maintain what you want without you having to constantly correct.

The amount of spoiler movement for a given aileron movement is determined through extensive flight tests to give the smoothest and most effective control. The pilot does not really need to know what the exact relationship is between ailerons and spoilers as long as the aircraft reacts in the way s/he is commanding.

I would have to look up your yaw damper/turn co-ordination question. It will be in the FCOM.
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Old 4th Feb 2015, 12:40
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Thanks a lot....

Ok, so: On ground, how much you move your stick directly determines how far the surfaces move. The roll spoilers work in tandem with the ailerons to give roll. When doing your control checks you get full deflection of the control surfaces for full stick deflection, so that you can check full, free, and correct movement.

Hmm... so the side-stick is proportional to the surfaces.
So deflection is how far the surfaces will move. Full stick deflection means that you have turned the stick as much as possible.

1) I have seen that after engines ON, the spoilers extend and retract. Is it during check-list? So to get full stick deflection, you need spoilers + ailerons?

2) But it is still controlled by ELAC and FAC, isn't it? But I am not able to get the difference between roll in flight and roll on ground...

Roll in flight: Spoilers + Ailerons, controlled by ELAC/FAC.
You turn the stick, the computers will give you the amount you asked for. (of course, under protections).

Roll on ground: Spoilers + Ailerons
You turn the stick, the computers will give you the amount you asked for.

Maybe I am not able to understand "deflection"....

Let's say you turn your stick 3° on the right on ground.
What will happen? The ailerons will be at 3°? What about spoilers?
On what If I do this in flight mode?

Do you mean to say that sticks are NOT proportional to the surfaces in ground mode? But it would mean that the computers are not giving what the pilots are asking for! (of course, I am talking under protections)
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Old 4th Feb 2015, 12:53
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AF330
In direct law surface movement is proportional to SS movement. Let's say you hit the stick to one side you get full aileron and full spoilers deflected and what ever rate of roll results depends on air speed. In normal law rate of roll is proportional to SS movement, full SS to one side is 15 degrees rate of roll independent of speed. Same is about pitch. In direct law you pull the stick back you produce a load factor that depends on speed. In normal law Load factor is proportional to SS movement irrespective of speed.
On ground SS orders are not modified by ELAC and SEC. On ground Full one side is full aileron and spoilers up and other side aileron full down.
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Old 4th Feb 2015, 13:48
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Thanks a lot vilas! Got your point now!

1) Ok so: " As the aircraft's speed increases during take-off, the amount of aileron/spoiler deflection for a given sidestick position will reduce.*"
Yeah...I now understand. So it this also a protection available in ground mode?

2) So you have roll demand in ground mode too. But you say that full SS deflection will extend spoilers and turn at max the aileron.
So has each SS deflection an amount of ailerons and spoilers that should be used? But I had thought that we only used to use spoilers when ailerons were not able to make the turn! So full side-stick deflection = always ailerons + spoilers? Also in Flight mode?
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Old 4th Feb 2015, 14:05
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AF330
You haven't got the point. As long as you are in ground mode the SS will command proportional aileron deflection and beyond certain degree of SS movement will deploy roll spoilers. Once you transition to flight mode SS will command rate of roll that is not be proportional to aileron deflection but rate of roll. You do not have roll demand on ground.
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Old 4th Feb 2015, 17:40
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Maneuver Demand vs. Surface Position

Let me try another approach with slightly different terminology.

In the simplest terms, control systems can be classified into two groups:
1. Surface Position Control
2. Maneuver Demand Control

With the former there is a direct gearing from pilot controller input to surface position. This gearing may be non-linear. An example is roll control where usually small commands move just wing trailing edge surfaces while larger commands bring in spoilers as well.

With the later, the pilot controller input is treated as a request for a certain amount of airplane response (e.g., load factor, pitch rate, angle of attack, roll rate, yaw rate, sideslip angle). With this type of system the pilot's maneuver command is compared against measured airplane response and the surfaces are adjusted as needed to achieve that maneuver command. Usually such arrangements involve integral control such that the error between to maneuver commanded and the measured response is driven to zero long-term.

Both types of control systems can be implemented using FBW. It's a matter of how the control algorithm within the FBW equipment is defined.

When designing a maneuver command system, it is important to take into account what maneuvers the airplane is capable in each phase of flight. For instance, it makes no sense to have a pitch rate maneuver command system when the airplane is on the ground at low speed such that no amount of elevator or stabilizer will generate pitch rate.

It is for these reasons that airplanes employing maneuver command control modes when in-air must transition to surface command control modes on ground. That is what the chart provided above is all about.
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Old 4th Feb 2015, 19:36
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Thanks a lot!

Ok, so in the ground:
Everything is programmed. You are in direct link with surfaces. The turn will depend on the speed of the aircraft. Even if you turn the SS with Full deflection, everything depends on speed.Usually, small deflection = ailerons. Big/full SS deflection = spoiler + Aileron.
You don't have roll demand. It is simply programmed. But again, speed of the aircraft will make the turn.

So during check-lists, when you see the spoilers extending, they are simply testing full SS deflection.
In ground mode, during take-off roll, elevator deflexion and aileron/spoiler deflexion will reduce (so turning the stick while taxi and on RWY is not the same thing - The Airbhs knows that while TO, the plane will take speed so it will reduce deflexion on surfaces.)

You don't have yaw coordination in ground mode.

In flight mode/flare mode, the roll rate is calculated. If you turn the stick at a certain deflexion, the plane will get the asked bank angle by using aileron, spoilers and rudder at any speed. The computers will calculate the best surface to use to react as precise as possible to satisfy our demand. You have roll demand.

Am I right now?

Last edited by AF330; 4th Feb 2015 at 19:47.
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Old 4th Feb 2015, 20:48
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AF330 - a couple of clarifications

AF330 - seems you are close. A couple of points about your last paragraph on in-air operation:

1. Lateral stick commands roll rate. The appropriate amount of roll control surface deployment will be produced to give the commanded roll rate, not roll angle. With the stick centered, the control law commands zero roll rate - essentially bank angle hold.

2. You reference using the right surfaces to give precise control. It is more a matter of the amount of surface rather than changing the mix of surfaces. In the roll axis, if only a small amount of command is needed, that will be accomplished with ailerons alone. If more control power is needed the spoilers will also be deployed.

It is good to have these discussions as I think all pilots flying airplanes with maneuver demand control systems benefit from fully understanding the basics of how this is achieved. Remember the physics and keep control power limitations in mind!
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