Real world ops question...

[kevinh 409]

With the exception of the 747, which can weigh itself if the optional Weight and Balance System is installed. The system uses strain gauges in the landing gear struts to calculate weight and CG position.

[willywonka 36]

Neat! But then, the sensors can only tell you the total aircraft weight, right? It doesn't know what of the total makes up the ZFW and cargo. Even then, why do we enter an independently calculated ZFW? I can imagine having that info to verify the plane's sensors.

[scandinavian13 5,192]

 

 

It doesn't know what of the total makes up the ZFW and cargo.

 

I can somewhat see your confusion here, but it's stated a bit awkwardly. Cargo is part of your ZFW. Fuel isn't. The sensors calculate the total weight and CG. The fuel tank sensors can get a decent reading of what amount of fuel is in there. GW - Fuel = ZFW. That's all you really need.

 

 

 

Even then, why do we enter an independently calculated ZFW? I can imagine having that info to verify the plane's sensors.

 

Trust but verify. Always.

 

Plus, sensors aren't error free. Remember that we have tons of that to deal with in aviation - IAS and CAS is only one of those issues. CAS exists because we realize that there are errors in collecting the data to determine our airspeed.

[willywonka 36]

 

 

I can somewhat see your confusion here, but it's stated a bit awkwardly.

Sorry, English isn't my language but you got my meaning. But yes, it almost seems like the real FMC could have a "ZFW autofill shortcut" like PMDG  B), then let the pilot verify against the loadsheet, of course.

 

Still though, while the cargo+baggage are weighted the pax and their carry-ons are only averaged or estimated. At 300 pax, assuming average weigh of 180lb*300 = 54000lb, and if the pax are closer to 170lb then it's 51000lb. Operationally it makes little difference of course (3000lb out of, say, 555000lb is less than 1%!), my point is only that the loadsheet's figure can be different than actual weight.

[scandinavian13 5,192]

 

 

Still though, while the cargo+baggage are weighted the pax and their carry-ons are only averaged or estimated. At 300 pax, assuming average weigh of 180lb*300 = 54000lb, and if the pax are closer to 170lb then it's 51000lb. Operationally it makes little difference of course (3000lb out of, say, 555000lb is less than 1%!), my point is only that the loadsheet's figure can be different than actual weight.

 

Yep, which is the entire reason I have a video about Weight and Balance. Too many simmers try and get into the gritty details of flight numbers because "the dispatch says X" when in reality, that dispatch is using planned numbers and not actual.

 

Still, the ZFW shortcut is already provided because the pilot simply confirms the GW, keeping in mind that he or she should also be verifying the other numbers.

 

It's like this:

X + Y = Z

 

If you verify X, you must also verify Y, but you should also verify that X + Y = Z. Either way, confirming Y (the ZFW, as an example) is only confirming one aspect of the equation that brings you to Z.

 

If you verify Z, you must also verify that Y and X bring you to Z (primarily because X - the fuel - changes over time). Confirming Z is only confirming one aspect of the equation that is made up of other variables.

 

However...since all of the variables are related (ZFW + Fuel = GW), confirming one value is essentially confirming the others (though you should confirm each individually). Moreover, the stress sensors are calculating the plane as it sits at that moment, which is the GW. In my mind, since the sensors are calculating the GW, then you should be confirming that value on the LSKs of the CDU, if that makes any sense.

[willywonka 36]

Yep, make sense to me. Pretty done as a check and balance.