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

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.

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.

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

You don't have roll rate demand, but that's not the same as "don't have roll".

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?

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.

http://i1119.photobucket.com/albums/...ckay/Blend.jpg

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.

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.

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.

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).

AFF330. Arbus has 3 laws and one mech.back up. Ground and Flare mode,also landing mode are submode of Normal law.

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!

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.

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)

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.

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?

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.

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.

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?

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!

As FCeng84 commented, in flight mode 'roll demand' commands a rate of roll, not an angle of bank. A large sidestick input commands a large rate of roll, while a small sidestick input commands a small rate of roll. Zero sidestick commands zero rate of roll (ie a constant bank angle).

Also, in ground mode the aileron/spoiler movement for a given sidestick input reduces with speed, however, the elevator movement is not modified for speed - there is a direct relationship between the sidestick position and the elevator position.

Thanks to everyone!
 

Ok, so to complete this:

1) How do you express "roll of rate"? Like speed can be km/h, roll of rate????

@ Buzzbox: In ground mode, each SS deflexion has a aileron/spoilers movement. But are you sure that each SS deflexion has an aileron/spoiler movement at each speed? I had thought that speed was not counted.... So it gives:
2) In ground mode, when you take speed, the aileron/spoiler movement reduces with stick deflexion. True? So each side-stick deflexion at a given speed has a programmed aileron/spoilers movement. Right? So if you turn at 15kts or 25kts with same Side-stick deflexion, you will make a same turn because aileron/spoilers movement will reduce at 25kts. Am I right?

3) Direct law is same as Normal law's ground mode. Each SS deflexion has a programmed aileron/spoilers movement at each speed. But in direct law, this is true on ground AND flight. Right?

Thanks

Rate of roll would normally be expressed in degrees per second - if you are rolling at 15 degrees per second, it will take you 3 seconds to change the angle of bank by 45 degrees, for example.

Think of the Airbus acting like a non FBW aircraft on the ground, where the movement of the side-stick is directly reflected by movement of the associated control surfaces (this is not entirely true, as you've stated above it's mediated by airspeed, but close enough). Once the transition is made into flight mode, the movement in control surfaces is whatever is required to achieve the commanded maneuver - e.g. referring to roll, if a 5 degree per second rate of roll to the left is requested by the pilot (by displacing the SS slightly to the left), the related control surfaces (ailerons and flight spoilers if applicable) will move to the extent that a 5 degree per second rate of roll will result.

Roll rate is the rate of change of the angle of bank, ie it's a measure of how fast the angle of bank is changing. It is usually expressed in terms of 'degrees per second'.

I can't speak for all Airbus types, but from the A330 FCOM:
The first part is right, but I'm not sure what you're asking in the second part. The ailerons/spoilers are not used to 'turn' the aircraft on the ground. Apart from the flight control check, the ailerons/spoilers are only used to keep the wings level during take-off with a strong cross-wind.

Direct Law is similar to Normal Law's Ground Mode, but it is not exactly the same. In Direct Law, the aileron/spoiler deflection is proportional to the sidestick deflection, regardless of speed and regardless of whether the aircraft is on the ground or in flight.

To complicate things, in Direct Law the maximum elevator deflection varies as a function of CG, to ensure adequate controllability. With an aft CG, the maximum elevator deflection with full sidestick will be less than that with a forward CG. Similarly, the maximum aileron/spoiler deflection varies according to the slat/flap configuration. When the aircraft is clean, the maximum aileron/spoiler deflection with full sidestick will be less than that when the slats/flaps are extended.

Function of CG in Direct Mode?
 

I'm curious as to what signal is used in Direct Law to schedule the gain between stick and elevator. What is the source of CG data to effect this scheduling?

Thanks to everyone!
Have just read all your great replies!

Have to go to school now so will reply in the evening!

Let's try to relate to conventional aircraft and talk about in air characteristics as it is more relevant to flying. In an oversimplifying attempt of explanation.

In air for conventional aircraft the amount of pitch in cockpit controls determine the amount of response. The more you pitch or roll, the more the aircraft pitch or roll. The faster you do it, the faster the aircraft does it.

On the Airbus in the air, the more you move the cockpit control, the FASTER the aircraft response. It does not matter whether you do it fast or slow, it is the amount you move your sidestick that matters.

Fundamentally this FBW algorithm will work in tandem with autotrim. Picture a turn, conventionally you turn an amount to get your bank angle and hold it there until you want to level off then you level off your control.

In Airbus you will neutralise your control when the bank angle is achieved. You DO NOT hold the displacement. Aircraft is trimmed in roll. When you want to level off you have to ask the aircraft to turn the opposite side, you need to input a roll in the opposite side to neutralise. However when the aircraft reverts back to Direct Law, the normal conventional behaviour of your ab-initio aircraft (as discussed earlier) prevails.

But it may not be conducive to think or analyse too much in flying. You wanna go left you turn left, wanna go right you turn right. Just remember there is autotrim and when you get what you desire, neutralise.

Having understand this fundamental philosophy then you can get into the protection (bank beyond 33deg, full deflection limitation of direct law during high speed take off roll regime...) part. Step by step.

As one very smart instructor once told me, "this ain't rocket surgery, son".

Not rocket surgery?
 

Maybe not flying it, but understanding how it all works? I'd say that's pretty close to rocket surgery!

What is the source of CG data to effect this scheduling?
 

The FMGS calculates this based on the ZFWCG entered on the INIT B page and the remaining fuel on board.

Expanding on m39462's comment, on the A330 there are two Fuel Control and Monitoring Computers (FCMC) that control the fuel system. One of the functions of the FCMC is to calculate the aircraft's Gross Weight and Centre of Gravity. That calculation is based on the Zero Fuel Weight and the ZFWCG entered by the crew during pre-flight initialisation of the Flight Management Guidance and Envelope Computers (FMGEC [i.e. the FMC for you Boeing types!]), and the sensed fuel quantity and distribution.

The FMGEC normally uses the GW and CG information calculated by the FCMC. That information is passed on to the Flight Control Primary Computers (FCPC, or 'PRIM'). If both FCMCs fail, the Flight Envelope (FE) part of the FMGEC is also able to compute the GW and CG as a backup.

Simple really (not!!).

CG estimate dependent on pilot data input?
 

Thanks for the explanation of CG estimate calculation. It sounds like this is dependent on the crew entering the initial weight and CG and then the system monitoring fuel management during the flight. Is there anything in this process to guard against a data entry error? If the CG is actually 30% but the crew enters 10% does the system spend the whole flight thinking CG is near the forward limit when in fact it is much closer to the aft limit? How about if the initial ZFW entry is off by 30%?

I would imagine there are checks or limits to guard against an entry that is way out of bounds. I'm interested in whether or not the system is able to determine that the data entry, while within operational limits, is in error and if it can make such a determination what corrective action does it take?

Hi

Sorry, was very busy yesterday.... (and tired!)
Will try to reply today...

The short answer to your question is NO.

The following Airbus document explains how the various Airbus types calculate GW and CG, and the effect of incorrect ZFW/CG entries:

Effect of ZFW/ZFWCG on Aircraft Operations

Both pilots should independently check all the data input to the aircraft.

This includes ZFM, ZFM CG, Trim setting etc. etc.

CG estimate for control laws independent of pilot input
 

BuzzBox - thanks for the reference document.

I see that there is an independent computation of CG and GW based on aerodynamics that is used for the control laws. My interest is mostly in that functionality as control law gains defining closed loop stability and handling qualities are impacted.

From the reference it appears that for the single aisle models the CG and GW estimates used by the control laws are completely independent of pilot input of ZFW and ZFCG. For the long range models the picture does not look quite as clear with the stated impact on control laws characterized as "slightly affected".

I realize that the aerodynamics based estimates will only be available in air and then only after a certain period of time to allow the associated estimation algorithms to converge. If the control laws are dependent on CG and GW during takeoff roll and/or initial climbout I suspect that either default values coded with the control laws or the data input by the pilots will have to be used.

Is that a threat? Should we be worried?

Good evening,

No Amadis...I had said that I will answer on the 5th of feb, but I couldn't.
So I just wanted to excuse myself. I have started this thread so If I leave it...it doesn't look good (at least for me!). Hope you understand.

I first would like to thank everyone to have helped me to understand how the plane reacts. I also looked to F-COM, some questions were answered, some came!

Normal law

Ground mode

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, for example, we know that:

At 0 kts - full side-stick deflexion = Aileron + spoiler
You can check it during check-list.

As you take speed, aileron/spoiler movement will reduce for each side-stick deflexion - for example:
If at 12kts, at full side-stick deflexion, you have a certain amount of aileron and, let's say spoilers X, Y and A (I am sorry, I don't know their real number - have to learn them!), at 18kts, you will get less amount of aileron and only X and A.

At take-off (after you have selected TO CONF?), you don't have lot of aileron/spoiler because as you have speed, even small amount of aileron can keep the wing to level.

Flight 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 roll rate (roll demand) by using aileron, spoilers and rudder at any speed.

Direct law

Direct Law is similar to Normal Law's Ground Mode, but it is not exactly the same. In Direct Law, the aileron/spoiler deflection is proportional to the sidestick deflection.

So basically it doesn't take the speed.

So if you turn the SS at full deflexion, on ground, in flight, you will get the same programmed aileron/spoiler movement.
So if you turn the SS to 3°/s, the computer has a programmed value (doesn't care of speed) of aileron/spoiler movement.
So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY...

Before going any further, do you all agree? :)

Lateral / Directional Control Inputs on Ground
 

For all airplanes lateral input (stick or wheel) is used on the ground only to keep the wings level when in cross winds. Without cross wind, no lateral input required or desired.

There are four ways to turn an airplane on ground:
1. Use the tiller to steer the nose gear
2. Use the rudder pedals to move the rudder thus generating yawing moment if speed is sufficient.
3. Use differential braking to generate yawing moment.
4. Use differential thrust to generate yawing moment. (This assumes you have wing mounted engines that can generate significant yawing moment.)

Great! Is everything else right sir?