Calculating GW inflight A320

[Flyman35]

during a flight one of my friend showed me a method to calculate the GW from the Green Dot speed by dividing on 85 (constant) or multiplying by 85 I don't actually remember,then they can figure out the Gross Weight, does any one know any reference for that method.

[Iamneon]

Isn't it twice your weight plus 85.
so 60 t plane would have a green dot of (2*60)+85=205.

but I always mix up the 85 or 80.

best climb rate is green dot plus 80 (or 85 if the previous was 80).
You can check this by selecting CI 0 during climb.

ciao

[OPEN DES]

A319:

(2xGW)+85 = green dot

Or to be more precise:

(2xGW)+85+ correction (1kt per 1000ft above FL200) = Green dot

So when in cruise you can deduce your GW (well at least your FAC calculated GW) by taking Green dot from the PFD. Substracting a correction of 1kt per 1000 ft above FL200. Substracting 85. And then divide by 2.

(Green dot (PFD) - correction (1kt per 1000ft above FL200) - 85) / 2 = GW (FAC)

You can compare this GW vs. GW on the status display or FUEL PRED page to assess FAC accuracy and/or loadsheet accuracy.


CI=0 will give you best rate of climb speed indeed. (ref: Getting to grips with a/c perfomance (airbus)) There is no rule of thumb but the SEVERE TURBULENCE speeds are a good approximation. (ref. FCOM)

[Flyman35]

thanks OPEN DES for your help.It seems like you are experienced in this type.

[Caudillo]

You can compare this GW vs. GW on the status display or FUEL PRED page to assess FAC accuracy and/or loadsheet accuracy.

Then do what next?

[OPEN DES]

Hi there!

Nothing at all. Just nice to know..... FOR INFO
A large enough discrepancy should attract attention anyway and would probably become evident quite soon.

CHECK GW message should come up on MCDU if difference is more than 7t.
FCOM 4.6.20 p7 tells you what to do in this case:

"CHECK GW MESSAGE" appears on the MCDU when the gross weight computed by the FM and that computed by the FAC disagree by more than seven tons (16055 LB). When this message appears :


CALL UP the MCDU FUEL PRED page and CHECK the current GW (= main engine start weight from load sheet - fuel used) and correct it if necessary.

If the current GW on the MCDU is correct. (FM characteristics speed displayed on the MCDU are correct).

DISREGARD the computed FAC characteristics speed (VIs, F, S, Green Dot) and refer to speeds given in the QRH.
(FM characteristics speed displayed on the MCDU are correct).

If the load sheet seems to be the cause of the discrepancy

COMPARE FAC and QRH speeds, and select the most appropriate.

[Dan Winterland]

Or try this.

MCDU Menu > AIDS

AIDS > ALPHA PARAMETERS

Type in GWFK for Metric or GWFP for imperial into an available line.

Read out the GW as computed by the FACs. As the FACs compute GD from the angle of attack once airborne, this is essentially the same as what has been described earlier, but more accurate. But I'm not sure all customer options allow access to the AIDS in flight, so It may not work on your aircraft.

[396.8T]

Dan.Just to add the FMGS weights are used for speeds up till 14800 feet approx above this the FAC computed weights are used

[OPEN DES]

Dan.Just to add the FMGS weights are used for speeds up till 14800 feet approx above this the FAC computed weights are used

I suppose you're talking about the A330/A340.....
The A320 will always have the FAC computed speeds on the PFD.

[toby320]

Or try this.

MCDU Menu > AIDS

AIDS > ALPHA PARAMETERS

Type in GWFK for Metric or GWFP for imperial into an available line.

that's correct in old A320's but in new A320's you type GW and FWC compute your actual weight

[OPEN DES]

I think you are confusing different things toby. GW can always be called up, that's the same one as on the FUEL PRED page and the SD; in other words: the loadsheet. Dan was referring to a call-up of the FAC computed GW, which is basically what the aircraft thinks it weighs, as computed by the FAC using INIT B CG, alpha (AoA), IAS.

So to wrap it up:

FM computed speeds are called by GW
FAC computed speeds are called by GWFK