FAC weights and Vls

[Altcrznav]

Any ideas as to why Airbus would use FAC weights (which can be off by as much as 5-10k pounds) to determine Vls?

Our procedure has always been to bump Vapp by at least +5 above Vls.

What is the FAC weight even used for?

[Goldenrivett]

What is the FAC weight even used for?

​​​​​​​As a cross check against rubbish speeds generated by inserting a rubbish ZFW during INIT.
See this OEB from 2004.
http://www.blackholes.org.uk/PP/oeb-819-1.pdf

[Altcrznav]

Yes, I have that. But we’ve been told by Airbus that FAC weights can be off by as much as 10000 lb.

Not that great of a cross check.

[halfofrho]

Have to regularly adjust Vapp on the 319's but for whatever reason the 320's are always fine. No idea why there is a difference between the two bar different software.

[Check Airman]

I have more confidence in the FAC weight than the weight from our load closeout, which uses ludicrously low average weights.

[EGPFlyer]

Originally Posted by halfofrho

Have to regularly adjust Vapp on the 319's but for whatever reason the 320's are always fine. No idea why there is a difference between the two bar different software.

The A320 display the speeds derived from the FMGC weights (as opposed to aerodynamic data) so you’ll always get a 5kt split but it doesn’t necessarily mean it’s more accurate.

[Check Airman]

Originally Posted by EGPFlyer

The A320 display the speeds derived from the FMGC weights (as opposed to aerodynamic data) so you’ll always get a 5kt split but it doesn’t necessarily mean it’s more accurate.

Not all airplanes do that. It's been a relatively recent modification to the planes in my fleet.

[pineteam]

The old birds use the FAC to determine the characteristics speeds and the MCDU to compute the VAPP. On the newer models, all the speed displayed are from the data inserted in the MCDU. And the FAC are only there as a back up. In case of major discrepancy, a « CHECK GW » message will be triggered after airbone.

[Altcrznav]

Originally Posted by Check Airman

I have more confidence in the FAC weight than the weight from our load closeout, which uses ludicrously low average weights.

i wouldn’t and Airbus says you shouldn’t.

[seven3]

Originally Posted by Altcrznav

i wouldn’t and Airbus says you shouldn’t.

I step in here and am interested.

do you have a reference for that ?

[jaja]

@ Altcrznav
Have a read in this excellent document from Airbus : ISI 22.00.00061 "FMGC & FAC - Gross weights and associated characteristic speed computations on Single Aisle"

@ Goldenrivett
Your attachment is not an OEB, it is an outdated FCOM BULLETIN

[Lantirn]

Originally Posted by pineteam

The old birds use the FAC to determine the characteristics speeds and the MCDU to compute the VAPP. On the newer models, all the speed displayed are from the data inserted in the MCDU. And the FAC are only there as a back up. In case of major discrepancy, a « CHECK GW » message will be triggered after airbone.

I was reading this on FCOM recently and there is no explanation why airbus did that.
Effectively what we see in PFD below 14600 and 240 is still dummy and what we entered in MCDU? Unless there is a 7 ton error? Where in this case it will show what in PFD...

[pineteam]

Hi Lantirn,

Try to have a look in the FCTM-Aircraft Systems-Flight Management- ZFW-ZFWCG entry errors.
You will see that for the newer models the speed displayed on the PFD are always from the data inserted from the MCDU even with the « Check GW »message
But like in the older models, V&Prot, V&Max and Vsw are still based on aerodynamic data.

[halfofrho]

The A320 display the speeds derived from the FMGC weights (as opposed to aerodynamic data) so you’ll always get a 5kt split but it doesn’t necessarily mean it’s more accurate.

Good to know, thanks!