Airbus laws: direct law
Join Date: Feb 2009
Location: Seattle
Posts: 379
Likes: 0
Received 0 Likes
on
0 Posts
Airbus Experts need to comment on Airbus details
AF330 - I am more familiar with the details for Boeing FBW so I will leave comments about Airbus system details to those experts.
AF330:
Yes, if I understand you correctly then you seem to be more or less correct, with the following comments:
Direct Law
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:
FCeng84:
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.
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...
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.
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.
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.
Last edited by BuzzBox; 6th Feb 2015 at 22:22.
Thread Starter
Join Date: Jul 2014
Location: Paris, France
Age: 23
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
:)
Good evening,
Interesting....
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!
Interesting....
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!
Join Date: Apr 2013
Location: everywhere
Posts: 128
Likes: 0
Received 0 Likes
on
0 Posts
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?
What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG?
AF330:
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.
I'm not familiar with the A320, so I'm not going there!
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!
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.
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?
1) So does this point keep changing during the flight? Where is that point generally located?
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?
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?
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?
The FWD CG is calculated by getting the CG calculated by FCMC?
CG calculated by FCMC - AFT CG calculated by FE = FWD CG?
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.
Last edited by BuzzBox; 8th Feb 2015 at 09:00.
Thread Starter
Join Date: Jul 2014
Location: Paris, France
Age: 23
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
Last edited by AF330; 8th Feb 2015 at 20:27.
Join Date: Jun 2009
Location: somewhere
Posts: 451
Likes: 0
Received 0 Likes
on
0 Posts
What happens if FCPC's/ELAC's stop receiving CG info? Do they revert to some default gains, or work on last received CG?
(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.
Last edited by A33Zab; 8th Feb 2015 at 14:25.
Thread Starter
Join Date: Jul 2014
Location: Paris, France
Age: 23
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
@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!
It will then give the fuel's weight which has been burned..
Confused!
AF330:
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/35785...uced-drag.html
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
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.
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.
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!).
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.
No. See above.
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!
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/35785...uced-drag.html
2) Sorry, what does MAC mean? Could you please explain?
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
Ok, the FCMC needs the fuel's volume and density to get GW.
But I didn't understand how is FOB calculated.
But I didn't understand how is FOB calculated.
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!
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?
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..
It will then give the fuel's weight which has been burned..
Join Date: Apr 2013
Location: everywhere
Posts: 128
Likes: 0
Received 0 Likes
on
0 Posts
@A33Zab
Hi A33Zab, thanks for reply - that sorts it out 
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?
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?
Thread Starter
Join Date: Jul 2014
Location: Paris, France
Age: 23
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Join Date: Jun 2014
Location: Village of Santo Poco
Posts: 869
Likes: 0
Received 0 Likes
on
0 Posts
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?
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?
Join Date: Feb 2009
Location: Seattle
Posts: 379
Likes: 0
Received 0 Likes
on
0 Posts
Basic Physics
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.
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.
Thread Starter
Join Date: Jul 2014
Location: Paris, France
Age: 23
Posts: 185
Likes: 0
Received 0 Likes
on
0 Posts
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
Got it now.
But does balance = Lift + Drag + Weight + thrust = 0?
So basically the plane flying at level and maintaining a speed??
Thanks