take-off CG A320

[avionneta]

Hello guys,
Anybody knows which method is used to calculate the take-off CG. Is it an extrapolation from the ZFW CG entered in the MCDU or a result from measurements via sensors, or something else ?

[FlightDetent]

FCOM 2.01.40 p3 has a sample manual LDS/W&B diagram that illustrates the process nicely.

1 get ZF index
2 get index correction for tkof fuel
3 combine 1 and 2 to obtain index at takeoff
4 get TOW (GW + TO Fuel Weight)
5 enter chart with TOW and TO index to obtain TO CG.

I suspect though, that you are asking about how FMGS calculates the GW CG at FUEL PRED pg MCDU? The description is at FCOM 4.03.25 p 16. But the princple must be the same.

1) you enter ZFW and ZFCG
2) machine reverse calculates ZF index (not displayed)
3) machnie reads the amount of fuel from sensors
4) machnie enters a stored fuel index table to work out actual index (not displayed)
5) machine figures out GW (displayed) and from a stored formula calculates CG position (displayed) that applies for this weight and index.
6) if you happen to have the exact amout of fuel in tanks as you will have for takeoff then GW CG = TOCG.
Or something like that. :D

BTW, what use is FMGS calculated TOCG? That's too late to set the trim and besides you need to figure it out before you start loading pax / cargo, to make sure you do not end up with TO CG that is beyond the limits, right?

FD (the un-real)
PS: Of course, "weight" is incorrect term to describe what in fact is "mass" as long as the units are kg/lbs.

[Honkozzie]

Your load controller, or equivalent should have given you a ZFW and ZF Cof G, (expressed as a MAC) on the loadsheet. Once inserted, the FM merely adds the fuel index and weight(it knows where the fuel is from the sensors,) and produces a TOW and MAC for TO.

Merely crosscheck this against the Takeoff Weight and MAC loadsheet figures, and provided they are within a nominal limit (usually 2% for the bigger airbuses) this will give you a stab trim for takeoff.
This can all be done within a few minutes of startup. ( unless you have to do it yourself, but even then a laden index ZFW will be easy enough to calculate, using the FCOM tables):ok:

The big catch is.... if the entered ZFW data is in error... the aircraft, and your good self... will be unaware until the aircraft tries to fly. Nasty if it's way out!

Once airborne, it WILL attempt to cross check itself by using a backup CofG computation. It applies a little reverse logic by checking STAB angle, thrust setting, IAS, Angle of Attack etc, to work out if the whole picture is valid. And if it isn't, the bells and whistles start.

Garbage in... garbage out.

Hope this clarifies things a bit.

[FlightDetent]

Your load controller, or equivalent should have given you a ZFW and ZF Cof G, (expressed as a MAC) on the loadsheet. Once inserted, the FM merely adds the fuel index and weight (it knows where the fuel is from the sensors,) and produces a TOW and MAC for TO. Err.. that's not exactly what our box does... (320 CFM Honeywell Pegasus 2+). What is your hardware configuration?

[Iceman49]

Make sure its in the green!

[capt.anjum.chhabra]

You see ZFW value is needed for the following things and these values will be incorrect if in wrong ZFW entry is entered.

GW displayed on ECAM and Fuel Prediction page
CG displayed on Fuel prediction page
FM predictions
Speeds displayed on MCDU (F, S, O, Vls, Vapp) as GW and C of G is required for computation of these speeds. Note if values are taken from FMGC, these values if wrong may lead to wrong V1, V2, and Vr computation.
So is it right to say if no ZFW entry all the above 4 entries wont be displayed?

But in air except for Vapp all the values are calculated by FAC and displayed on PFD will be correct and will not match with FM but on ground these values on PFD are displayed by FM as FAC does not work on ground.

Rather notice apart from ZFW we also need Fuel qty form FQI and Fuel Flow from FADEC so if any of these are not working fine then also these values will be wrong. But I think not too much impact as these are generally correct but it is the ZFW which is entered by pilot and an error is likely.
Also note Vsw, V alpha prot and V alpha max are not affected as these are based on aerodynamic data.
So we can say protection speeds are not impacted by ZFW/ZFWCG as they are on PFD.


Now where do we actually need CG value inflight..i know it is needed for 2 kts decrease in Vapp speed if forward C.G may be in some failures we need this CG value i am not sure of..