Airbus laws: direct law [Archive]

AF330

3rd Feb 2015, 22:49

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.

FCeng84

4th Feb 2015, 01:11

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.

BuzzBox

4th Feb 2015, 02:12

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).

Tatarin

4th Feb 2015, 06:32

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

AF330

4th Feb 2015, 12:04

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!

Uplinker

4th Feb 2015, 12:23

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.

AF330

4th Feb 2015, 12:40

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)

vilas

4th Feb 2015, 12:53

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.

AF330

4th Feb 2015, 13:48

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?

vilas

4th Feb 2015, 14:05

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.

FCeng84

4th Feb 2015, 17:40

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.

AF330

4th Feb 2015, 19:36

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?

FCeng84

4th Feb 2015, 20:48

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!

BuzzBox

4th Feb 2015, 21:23

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.

AF330

5th Feb 2015, 00:13

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

Cloud Cutter

5th Feb 2015, 00:56

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.

BuzzBox

5th Feb 2015, 01:03

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

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'.

But are you sure that each SS deflexion has an aileron/spoiler movement at each speed?

I can't speak for all Airbus types, but from the A330 FCOM:

'When the aircraft is on the ground (in “on ground” mode), the sidestick commands the aileron and roll spoiler surface deflection. The amount of control surface deflection that results from a given amount of sidestick deflection depends upon aircraft speed.'

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.

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.

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?

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.

FCeng84

5th Feb 2015, 05:29

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?

AF330

5th Feb 2015, 05:41

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

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

titaniumwings

5th Feb 2015, 06:48

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.

Amadis of Gaul

5th Feb 2015, 09:49

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

Cloud Cutter

5th Feb 2015, 19:02

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

m39462

5th Feb 2015, 22:01

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

BuzzBox

6th Feb 2015, 03:24

FCeng84:
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?

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!!).

FCeng84

6th Feb 2015, 04:21

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?

AF330

6th Feb 2015, 05:37

Hi

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

BuzzBox

6th Feb 2015, 08:31

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?

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 (http://www.smartcockpit.com/docs/Effect_Of_Zero_Fuel_Weight_On_Aircraft_Operations.pdf)

Uplinker

6th Feb 2015, 08:47

I would imagine there are checks or limits to guard against an entry that is way out of bounds.

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

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

FCeng84

6th Feb 2015, 17:15

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.

Amadis of Gaul

6th Feb 2015, 17:24

Hi

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

Is that a threat? Should we be worried?

AF330

6th Feb 2015, 18:59

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? :)

FCeng84

6th Feb 2015, 19:16

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.)

AF330

6th Feb 2015, 19:27

Great! Is everything else right sir?

FCeng84

6th Feb 2015, 19:43

AF330 - I am more familiar with the details for Boeing FBW so I will leave comments about Airbus system details to those experts.

BuzzBox

6th Feb 2015, 22:11

AF330:

Yes, if I understand you correctly then you seem to be more or less correct, with the following comments:

Direct Law

So it can be dangerous during take-off to move the SS, one of your wings can touch the RWY...

Yes, theoretically, if you used a large aileron input with the sidestick. However, remember that Direct Law is a 'reconfiguration' flight law - you would not take-off in Direct Law intentionally.

You would only encounter Direct Law after multiple in-flight systems failures. On the A330, the system reconfigures from Normal Law to Alternate 1 Law to Alternate 2 Law and finally to Direct Law, depending on the level of failure. The failures that would lead to Direct Law include:

Triple IR failure,
Triple PRIM (Flight Control Primary Computer) failure,
Dual elevator fault, or
All engines out with PRIM 1 failed

FCeng84:

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.

WRT the long range Airbii, my understanding is that the control law gains are normally modified using the GW/CG values calculated by the FCMC from the ZFW/ZFWCG data input by the pilots. The GW/CG values calculated independently by the FE are used as a backup in the event of a dual FCMC failure, and are also used to trigger the 'Excess Aft CG' warning. I don't know if there is some kind of comparison going on behind the scenes in-flight - there is nothing mentioned in the books.

During the take-off roll, the flight control system is in Normal Law Ground Mode. There is a direct relationship between the sidestick deflection and elevator deflection, with full authority. As far as I'm aware the control law is not modified for GW/CG during take-off. The stab position is set manually (automatically on later aircraft) after engine start, according to the GWCG value (or loadsheet GWCG) and there is no auto-trim. Consequently, during take-off the pitch control operates exactly the same as it would in a conventional aircraft. It transitions to Flight Mode shortly after lift-off and it's only then that the FCMC derived CG value starts to affect the control laws.

C_Star

7th Feb 2015, 07:49

Where is A33Zab when we need him... :)

A33Zab

7th Feb 2015, 09:46

In - read only - Law.:(

C_Star

7th Feb 2015, 10:50

Hi A33Zab!

Good to have you here! Can you post some definitive info on how/where from Flight Control Computers obtain C.G. data for Direct Law gains?

A33Zab

7th Feb 2015, 13:37

See Buzzbox's reply, all I can add are some images.

http://i474.photobucket.com/albums/rr101/Zab999/CW_CG_zps6a522af9.jpg

http://i474.photobucket.com/albums/rr101/Zab999/Elevatordefl.jpg

AF330

7th Feb 2015, 17:11

Good evening,

Interesting....

http://i474.photobucket.com/albums/rr101/Zab999/CW_CG_zps6a522af9.jpg

Ok so the center of gravity is the point at which the entire weight of the plane may be considered as concentrated so that if supported at this point the plane would remain in equilibrium in any position.

1) So does this point keep changing during the flight? Where is that point generally located?

Ok, so let me take all the computers on your picture:

FMGEC = Flight management, guidance and envelope computer
FCMC = Fuel Control and Monitoring Computer
FWC = Flight warning computer
FCPC = Flight Control Primary computer

2) Ok, as I don't know all of these computers, I just would like to compare with the A320:
a) I have heard about the FMGS/FMGC about the A320, but never about the FMGEC. Has the A320 a FE?? If not, who controls GW and C.G?
b) Can we say that FCMC = FQI + FLSCU?
c) Does an A320 have a FWC? So each time a computer detects a problem, it sends it to the FWC (and the FMGEC maybe, no?)
d) FCPC...hmm... can we say that FCPC is kind of ELAC on Airbus? ELAC + SEC? ELAC + FAC? FAC + SEC?

3) Ok, so the FCMC is controlling the gross weight and the center of gravity. But is it calculating the fuel's GW? Is it calculating the plane's C.G?

4) So you have a AFT CG Monitor inside the FE.
The FWD CG is calculated by getting the CG calculated by FCMC?
CG calculated by FCMC - AFT CG calculated by FE = FWD CG?

It's all for now! Thanks a lot A33Zab, BuzzBox, FCeng84, titaniumwings, Uplinker, vilas and everyone else! :)

Amadis of Gaul

7th Feb 2015, 17:48

Are you asking if the -320 has a Flight Engineer?

AF330

7th Feb 2015, 18:10

Ha ha Amadis! If required, yes! :) No just joking, of course.

But I think that A33Zab and others know everything about all this! :)

C_Star

7th Feb 2015, 19:05

Thanks A33Zab and Buzzbox. I am quite surprised that Direct Law, which is the most basic form of reversion (save the Mech Backup) still needs data from additional "boxes" like FCMC/FMGEC, or FAC in case of A320.

What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG?

BuzzBox

8th Feb 2015, 07:22

AF330:

Ok so the center of gravity is the point at which the entire weight of the plane may be considered as concentrated so that if supported at this point the plane would remain in equilibrium in any position.

1) So does this point keep changing during the flight? Where is that point generally located?

Yes. Fuel is kept in the outer tanks as long as possible during the flight to provide wing bending relief. That means fuel is normally burnt from the centre tank first (if the aircraft has one), then from the wing inner tanks and finally from the wing outer tanks. On a swept wing aircraft the centre tank is obviously forward of the wing inner & outer tanks. Consequently, the CG moves aft during the flight as fuel is burnt by the engines.

To complicate matters, the long range Airbii have a trim tank in the tail. Fuel is transferred from the centre/inner tanks to the trim tank after take-off to move the CG aft. The fuel system aims to keep the CG close to an aft CG target, approximately 2% forward of the the aft CG limit, to reduce drag.

Where is the CG located? It depends how the aircraft is loaded and what stage of flight you're talking about, so it could be anywhere between the forward limit (typically about 15% MAC) and the aft limit (typically about 40% MAC). The fuel system of the long range Airbii aims to keep the CG at about 38% MAC (roughly) during the cruise.

2) Ok, as I don't know all of these computers, I just would like to compare with the A320:
a) I have heard about the FMGS/FMGC about the A320, but never about the FMGEC. Has the A320 a FE?? If not, who controls GW and C.G?
b) Can we say that FCMC = FQI + FLSCU?
c) Does an A320 have a FWC? So each time a computer detects a problem, it sends it to the FWC (and the FMGEC maybe, no?)
d) FCPC...hmm... can we say that FCPC is kind of ELAC on Airbus? ELAC + SEC? ELAC + FAC? FAC + SEC?

I'm not familiar with the A320, so I'm not going there!

3) Ok, so the FCMC is controlling the gross weight and the center of gravity. But is it calculating the fuel's GW? Is it calculating the plane's C.G?

The FCMC isn't 'controlling' the aircraft's gross weight. The aircraft is loaded to a certain GW while it is on the ground. The GW then reduces during flight as fuel is used.

The FCMC calculates the weight of fuel on board by measuring the fuel volume and density, and displays that calculated figure on the ECAM. It also calculates the fuel on board by subtracting the fuel used from the fuel on board at engine start and generates a caution if there is a discrepancy between the two calculated figures.

The FCMC also calculates the aircraft's CG using the ZFW and ZFWCG input by the pilots during the preflight, and the sensed fuel quantity & distribution - as discussed previously!

4) So you have a AFT CG Monitor inside the FE.
The FWD CG is calculated by getting the CG calculated by FCMC?
CG calculated by FCMC - AFT CG calculated by FE = FWD CG?

I'm not entirely sure what you're asking, but the aircraft only has one CG at any one time. That CG moves during flight (as discussed above) and can be anywhere between the forward CG limit and the aft CG limit. Those limits are defined when the aircraft is certified.

The FE does a GW/CG computation independently of the FCMC, using various aerodynamic inputs (speed, thrust, altitude, THS position, elevator position). The GW/CG values computed by the FE are used as a backup in case of dual FCMC failure, and are also used to generate an 'Excess Aft CG' warning if the CG is found to be too far aft.

AF330

8th Feb 2015, 10:33

Thanks a lot,

So I understand that during the flight, C.G moves AFT as fuel get's burned.

1) So does the plane tend to pitch-up as you burn fuel? But how do you reduce drag? If you pitch up, you reduce drag!

2) Sorry, what does MAC mean? Could you please explain?

Ok, the FCMC needs the fuel's volume and density to get GW.
But I didn't understand how is FOB calculated.

3) You know the weight of fuel. Why does it take fuel used during engine start??? What are the 2 calculated figures? I had XXX lbs at departure. Now, I know my fuel quantity by knowing the fuel density, no? Confused! ;)

4) The ZFWCG is given by the pilot. But does the plane calculate it again (and changes the value)? Like for fuel: when you enter "block", the FMGS can automatically change it If it wrong.

5) The FWD CG is also controlled by FE?

A33Zab

8th Feb 2015, 11:28

What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG?

That would mean dual FCMC, dual FMGEC and degraded (DIRECT) control law as FCSC in MASTER.
(as you can see in the diagram there is NO GW/CG connection to FCSCs).

Gains are defaulted at a safe value and depends on S/F position.

The FM section memorizes the last GW/CG value for internal purposes in case of power shutdown of FMGEC but AFAIK this is not available to other equipment.

AF330

8th Feb 2015, 20:04

@BuzzBox: Do you mran that it calcualtes FOB by subtracting weight of fuel before engine start - weight of fuel now?
It will then give the fuel's weight which has been burned..

Confused!

BuzzBox

9th Feb 2015, 00:10

AF330:

1) So does the plane tend to pitch-up as you burn fuel? But how do you reduce drag? If you pitch up, you reduce drag!

If nothing else changes, then yes, the aircraft would tend to pitch up as fuel is used and the CG moves aft. In a conventional aircraft the CG is forward of the centre of lift, creating a pitch moment that tends to pitch the aircraft nose down. To counteract that moment, the horizontal stabiliser must produce a down force to keep the aircraft balanced. That downforce adds to the aircraft's weight, which also acts downwards, and the wings must therefore produce more lift (i.e. upwards force) to keep the forces in balance so that the aircraft can maintain level flight. If the wings produce more lift they also produce more induced drag.

As the CG moves aft, the amount of downforce the stab must produce reduces and so does the drag. The Airbus fuel system tries to keep the aircraft's CG as far aft as possible during the cruise to reduce drag and improve performance.

The effect of CG on induced drag is discussed in the following link:
http://www.pprune.org/tech-log/357859-cofg-induced-drag.html

2) Sorry, what does MAC mean? Could you please explain?

MAC = Mean Aerodynamic Chord

The location of an aircraft's CG is usually expressed as a certain percentage of the MAC, e.g. 35% MAC means the CG is located 35% of the distance along the MAC.

The following link has a simple explanation:
Mean Aerodynamic Chord (MAC) - SKYbrary Aviation Safety (http://www.skybrary.aero/index.php/Mean_Aerodynamic_Chord_(MAC))

Ok, the FCMC needs the fuel's volume and density to get GW.
But I didn't understand how is FOB calculated.

Fuel weight (i.e. FOB) = Fuel volume x Fuel density

The FCMC measures the volume of fuel in the tanks and its density and calculates the FOB. The result is displayed on the ECAM and fuel system page.

3) You know the weight of fuel. Why does it take fuel used during engine start??? What are the 2 calculated figures? I had XXX lbs at departure. Now, I know my fuel quantity by knowing the fuel density, no? Confused!

The FCMC performs two calculations of the fuel weight. The first is described above. The second calculation is performed using a different method - it subtracts the fuel used (by measuring fuel flow) from the FOB at engine start. The two figures are then compared and if there is a discrepancy the FCMC generates an ECAM caution. A discrepancy could be an indication of a fuel leak.

4) The ZFWCG is given by the pilot. But does the plane calculate it again (and changes the value)? Like for fuel: when you enter "block", the FMGS can automatically change it If it wrong.

No. The ZFWCG is determined by the way the aircraft is loaded with passengers, baggage & cargo, etc. The ZFWCG doesn't change after the aircraft is loaded on the ground (if it does, you're in trouble!).

5) The FWD CG is also controlled by FE?

No, the FE provides an 'EXCESS AFT CG' warning if the CG is too far aft, but there is no such warning if the CG is too far forward, simply because it's not necessary. If the aircraft is loaded correctly on the ground, then the CG can't move too far forward - it can only move backward as fuel is used or transferred to the trim tank in the tail. The reason an 'EXCESS AFT CG' warning is provided is because the fuel system tries to keep the CG just forward of the rear limit during the cruise. If there is a problem with the system (too much fuel pumped aft, or not subsequently pumped forward as the CG moves aft) then the CG could move too far aft.

Do you mran that it calcualtes FOB by subtracting weight of fuel before engine start - weight of fuel now?
It will then give the fuel's weight which has been burned..

No. See above.

C_Star

9th Feb 2015, 11:43

Hi A33Zab, thanks for reply - that sorts it out :ok:

I undestand that the kind of failure required to lose the CG info on the A330 is fairly remote. OTOH, on the 320, FAC1+2 Fault and Gear Down should be enough if I understand the system correctly...

What about FCPC gains variability with respect to S/F position? Why is that implemented? Is it due to additional pitch authority required co counter the pitching moments after S/F extension? (though this should be catered for by the THS if available) Or are there some other considerations?

AF330

11th Feb 2015, 17:31

Thanks a lot BuzzBox! ;)

1) What do you mean by "balance"? If the plane is already pitching down, why do you want to create a down force with the horizontal stabilizer????
You want the plane to pitch even more down????!!!!

2) Yes, but is the ZFWCG RE-calculated by FCMC? FE? ...No? Just entered by pilots?

3) So FCMC calculates FOB and tries to figure out EFOB at a given WPT that it then sends to FMGEC?

Thanks a lot! ;)

PS: I am not able to get "balance"!!! COG is the point where the aircraft is balanced.
So what is balance? Aircraft at level? Aircraft in which position?

Thanks again!

Amadis of Gaul

11th Feb 2015, 17:46

AF330,

You're not paying attention. If the H-Stab moves down, which way does the nose go?

The elevators deflect upward, which creates downforce on the H-Stab, which then moves down, and the nose does what?

FCeng84

11th Feb 2015, 17:49

AF330 - If you want to better understand how airplanes fly and maneuver I suggest that you find a very basic book on the subject. There are many available on line.

For the very basics of trim and balance here are a few thoughts:

In order for any object to be in steady state equalibrium the forces and rotational moments applied to that object must balance. The sum of the forces must be zero as must the sum of the rotation moments be zero.

As for up/down forces on the tail, these generate rotational moments. A tail down force causes the airplane to rotate in the tail-down / nose-up direction. This is what BuzzBox is referring to. If the airplane has a tendency (due to other rotational moments) to rotate in the nose down direction, down load on the tail is needed to bring it into rotational equilibrium.

AF330

11th Feb 2015, 18:13

Oh yes....forgive-me....
Got it now.

But does balance = Lift + Drag + Weight + thrust = 0?
So basically the plane flying at level and maintaining a speed??

Thanks

BuzzBox

12th Feb 2015, 00:58

AF330:

No, for balanced or steady state flight the sum of the vertical forces must be zero and the sum of the horizontal forces must be zero. In other words, the forces must be equal and opposite to cancel each other out:

Lift = Weight, and

Thrust = Drag

But as FCeng84 said, that's only one part of the problem, because the Lift/Weight and Thrust/Drag forces normally don't line up with each other. That difference causes rotational moments that tend to pitch the aircraft either nose up or nose down. The horizontal stabiliser's job is to balance out those rotational moments to keep the aircraft in a steady state.

The following link has a basic explanation of the forces and moments in straight and level flight:

03 - Straight And Level - X-PPL - Learn to Fly - X-Plane.Org Forum (http://forums.x-plane.org/?showtopic=41729)

As FCeng84 suggested, if you want to learn more about aerodynamics there are plenty of excellent text books available. One that's been around for a very long time is called 'Mechanics of Flight' by AC Kermode.