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A320 Seat Assignment on A321 Causes Late Rotation

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  • Gabriel
    replied
    Yes to all, except this: "It would have to be very accurate."

    No it wouldn't.
    Remember that the manual entry used today is done by estimating pax weight and considering that all pax or cargo that is put in a section of the airplane is assigned the same location (in the center of that section). An average-accuracy weight measuring system associated with the landing gear legs will be quite more accurate that that, both in the weight and especially in the location of the force.

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  • Evan
    replied
    Originally posted by Gabriel View Post
    Exactly, that's why I was (and still am) confused when you said "Then it just has to know the weight-on wheels (mains)... "
    We had been speculating on a system that might measure main gear strut compression or strain to determine GW. The 747 actually had an optional system like this and it did reportedly work accurately, so it doesn't seem implausible to add that feature to modern airliners. However, AFAIK, none of the Airbus models are so equipped.

    Generally, the zero-fuel weight and CG must be provided to the FMC by pilots on the INIT page. That leaves it entirely prone to human error.

    ATL is suggesting that there is an optional or updated system function whereby the aircraft can determine CG on its own. There is an updated system to catch trim errors but I still believe that, on the ground, it must calculate everything from those human entries on the INIT page. I can't fathom how it could otherwise determine either weight or CG.

    The Airbus lit I posted above states that the safeguard system gets CG from the fuel management system. That system uses an array of fuel tank sensors to measure the fuel levels and convert them to weight (FQIS) and features a separate system with its own dedicated sensors to alert the crew when fuel levels drop below a dangerous threshold. The only wildly far-fetched possibility I can come up with is that perhaps there is a way it can use these sensor readings to calculate the angle the plane is resting at and convert that to a CG value. But that could only work it the tarmac is known to be perfectly level to begin with. So, kind of useless...

    But I think a system with compression sensors on both MLG and NLG could conceivably work. It would have to be very accurate and very reliable though. Personally, I think the redundancy is better created through added human procedural checks (i.e. ensuring that an A320 load sheet never gets carried over to an A321, and that the preflight entry procedure requires the type to be cross-checked).

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  • Gabriel
    replied
    Originally posted by Evan View Post

    I'm not aware that the A320 has weight on wheels sensors on the NLG. If that were the case, we wouldn't be scratching our heads over this.
    Exactly, that's why I was (and still am) confused when you said "Then it just has to know the weight-on wheels (mains)... "

    The Airbus doc tells us that the TOS1 function is using CG data provided by the fuel management system. That's a headscratcher.
    My guess is that the fuel management system is in charge of calculating the CG throughout the flight but that this calculation is based on a fixed empty operative weight and CG, fuel quantity en each tank (with known positions), and payload weight and position data entered for each flight*. I just don't see any other possible way around, at least with the plane on the ground.

    * (or the entry may be weight and CG at take-off, or weight and CG at zero-fuel, in any case, when you have one you can compute the others)

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  • Evan
    replied
    Originally posted by Gabriel View Post

    I don't get you. If you know the weight-on-wheels (independently for each leg) on a static plane (and the location of each leg, of course, which is fixed and known even before the airplane is built so it can be easily pre-programmed in a computer) you don't need any additional information to determine the total weight and the location CG. You don't need to know the distribution of the load. You don't need to know the fuel quantity and location. That is for the initial CG. Afterwards (like throughout the flight), you would need to asume that the payload doesn't move and adjust by fuel's consumption and change of location.
    I'm not aware that the A320 has weight on wheels sensors on the NLG. If that were the case, we wouldn't be scratching our heads over this. The Airbus doc tells us that the TOS1 function is using CG data provided by the fuel management system. That's a headscratcher.

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  • Gabriel
    replied
    Originally posted by Evan View Post

    I'm wondering if the TOS1 function ASSUMES that the load has been correctly assigned and more or less balanced (that people have done that job correctly). Then it just has to know the weight-on wheels (mains) and the fuel quantity and location, right?

    I mean, that's all I've got...
    I don't get you. If you know the weight-on-wheels (independently for each leg) on a static plane (and the location of each leg, of course, which is fixed and known even before the airplane is built so it can be easily pre-programmed in a computer) you don't need any additional information to determine the total weight and the location CG. You don't need to know the distribution of the load. You don't need to know the fuel quantity and location. That is for the initial CG. Afterwards (like throughout the flight), you would need to asume that the payload doesn't move and adjust by fuel's consumption and change of location.

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  • Evan
    replied
    Originally posted by Gabriel View Post
    So, with the airplane on the ground, it is impossible in principle (in the strong sense) for a computer to determine the CG unless either a) there is an external input of the weight and location of the payload or b) there is information of the weight on the wheels.
    I'm wondering if the TOS1 function ASSUMES that the load has been correctly assigned and more or less balanced (that people have done that job correctly). Then it just has to know the weight-on wheels (mains) and the fuel quantity and location, right?

    I mean, that's all I've got...

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  • Gabriel
    replied
    Originally posted by ATLcrew View Post

    Great question for an engineer. The document I have has formulas I'll never decipher.
    Any chance that you can share this document?

    I am clearly not an expert on this kind of system but, conceptually, the CG is the location around which the sum of moments (torques) due to weight add up to zero. There are only 2 ways to calculate it:

    One is to know the weight and location of each thing ("thing" can take a bunch of known things together if you know how much they weight collectively and their aggregated center of gravity, for example the "empty airplane" can be one "thing".

    The other one would be to measure the necessary moment (torque) done by forces other than the weight to add up to a TOTAL moment of zero (total meaning these other forces + the weights), that is, to achieve angular equilibrium.

    For the first method, a computer can know the weight and CG of the empty plane (as recorded by the manufacturer or maintenance during an empty weight-and-balance check) and the fuel (nd other fluids) by sensors measuring the quantity of each fluid (for which the density is known) in each container or tank (for which it's location in known). But additional human input would be required in each flight to enter the weight and location of people, cargo, catering items, etc. It makes sense to have the fuel management system manage this since fuel is the only thing whos quantity and location will change significantly throughout the flight. And I know that in some planes that management goes beyond simply measurement and determination, to actively moving fuel from one place to another to put the and CG at a specific location.

    For the second method, when the airplane is in flight "all the forces other than the weight" is the other 3 forces of flights: lift, thrust and drag (the 4'th force being the weight). These 3 forces (and their center of force, i.e. the point about which each of them make zero moment) can be calculated (if you have a good enough model and good enough sensors) from parameters like speed, air density, angle of attack, position of each control surface (including the stabilizer) and engine parameters like fuel flow and EPR.
    When the airplane is on the ground the main non-weight forces would be (ironically) the "weight on wheel" forces (which are not a real weight, but rather the "normal" force). That is, for example, how the CG of an empty plane is determined during maintenance checks, by placing the plane on scales under each gear leg. (There may be other forces like lift and drag if there is wind, thrust if the engines are running, and friction if the plane is sitting on a surface that is not fully level, but let's say that we can ignore all these ones.)

    So, with the airplane on the ground, it is impossible in principle (in the strong sense) for a computer to determine the CG unless either a) there is an external input of the weight and location of the payload or b) there is information of the weight on the wheels.

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  • ATLcrew
    replied
    Originally posted by Evan View Post

    So the question is, how does the fuel management system determine the CG?
    Great question for an engineer. The document I have has formulas I'll never decipher.

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  • Evan
    replied
    Originally posted by ATLcrew View Post

    Apparently yes, they have that capability on the ground, at least on some serial #s and Mod #s. The ECAM would be what we discussed above and, again, it will only pop up when the T/O config button is pushed. I don't think it would pop up "when the thing won't rotate" since T/O INHIBIT mode will be active.
    Here's the full link if you're interested. It also specifies serial numbers by configuration beginning around 2009. This is a supplemental update to what was published in Safety First Issue #8:

    https://safetyfirst.airbus.com/takeo...ing-functions/

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  • Evan
    replied
    Originally posted by ATLcrew View Post

    Apparently yes, they have that capability on the ground, at least on some serial #s and Mod #s. The ECAM would be what we discussed above and, again, it will only pop up when the T/O config button is pushed. I don't think it would pop up "when the thing won't rotate" since T/O INHIBIT mode will be active.

    Trim Check (TOS1)


    Originally posted by safetyfirst.airbus.com
    TOS1 also compares the trim setting entered by the crew into the Takeoff PERF page with the actual Trimmable Horizontal Stabilizer (THS) position and with the trim computed by the FAC/FMGEC/FE based on the CG value provided by the fuel management system. If an inconsistency is detected, this will trigger the F/CTL PITCH TRIM/MCDU/CG DISAGREE (A320/A330/A340) or F/CTL PITCH TRIM/FMS/CG DISAGREE (A350/A380) ECAM message.
    So the question is, how does the fuel management system determine the CG?

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  • ATLcrew
    replied
    Originally posted by Evan View Post

    They can do this on the ground, as in, before the take-off roll?



    What is the ECAM message?



    If it could alert them of a CG issue before rolling, then of course, but if it throws up an alert about the same time they are realizing that the thing won't rotate, then thanks for nothing ECAM.
    Apparently yes, they have that capability on the ground, at least on some serial #s and Mod #s. The ECAM would be what we discussed above and, again, it will only pop up when the T/O config button is pushed. I don't think it would pop up "when the thing won't rotate" since T/O INHIBIT mode will be active.

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  • Evan
    replied
    Originally posted by ATLcrew View Post
    It appears that while FACs do not sense CG directly when on the ground, they can derive it from other data, such as pitch angle, weight, and even the amount of thrust that it takes to move the airplane (so FADEC is involved also).
    They can do this on the ground, as in, before the take-off roll?

    Since the FACs know what the CG range should be for a given weight, if they get information from all those sources that the CG may be out of that range, the above ECAM will be triggered.
    What is the ECAM message?

    Whether or not that would have helped this particular crew, I know not.
    If it could alert them of a CG issue before rolling, then of course, but if it throws up an alert about the same time they are realizing that the thing won't rotate, then thanks for nothing ECAM.

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  • ATLcrew
    replied
    So, I got an Airbus document from my Fleet Chief. It's a good 30 pages, most of it heavy engineering stuff that's so far beyond my feeble mind, it gave me cerebroalgia It appears that while FACs do not sense CG directly when on the ground, they can derive it from other data, such as pitch angle, weight, and even the amount of thrust that it takes to move the airplane (so FADEC is involved also). Since the FACs know what the CG range should be for a given weight, if they get information from all those sources that the CG may be out of that range, the above ECAM will be triggered.

    Whether or not that would have helped this particular crew, I know not.

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  • ATLcrew
    replied
    Originally posted by Gabriel View Post
    Could it be that it is the "pitch trim value calculated by the FAC based on the CG entered by the crew"?

    (ok, I'll wait)
    That's just it, there is a check for THAT discrepancy AS WELL, and on some serial numbers that value and the actual trim value are all that are cross-checked. It appears, however, that there are software (and possibly hardware) versions that check for those two PLUS that "FAC-calculated value".

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  • ATLcrew
    replied
    Originally posted by Evan View Post

    Standing by... but the above says the trim value is calculated by the FAC based on the CG (which isn't calculated but is a known value). I suspect that refers to the CG entered on the INIT page. I'd love to be wrong on this...
    That's what I thought, too, and that appears to be the case on certain serial numbers. But on others, however, it appears the FACs are able to somehow calculate the CG themselves with the FQIC being some sort of an accessory to that process.

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