A20N LDG CG trim.

[Sergei.a320]

Hello pilots! Recently I've found smth interesting. Maybe I'm missing something obvious, so help me find the answer. During des prep I checked that LDG CG was around 31%, according loadsheet it was the same. But during final and until landing actual pos of pitch trim was around 20-25%. The same situation occured during next flights. Can someone explain this?(with reference please)As I know FMGS always use 25%CG to calculate your Vapp, and thats why we add our actual LDG CG in EFB ldg performance app to correct our Vapp.(+-1/2kt difference). Why do we do that if actual ldg cg differs from calculated))thanx.

[FlyingStone]

It's been a while since I flew the Airbus, but I believe the CG indications on the pitch trim wheel will indicate setting used for takeoff, where aircraft is trimmed for V2. During landing, the aircraft is in a different configuration than takeoff (well, at least in CONF FULL) as well as at different speed, so trim setting will be different for the same CG.

[vilas]

After takeoff pitch trim doesn't represent the CG. The CG is given on the FUEL PRED page. In the air Trim wheel keeps moving to keep the flight path. Can CG be changing so rapidly? Not possible. Next time check it on FUEL PRED page.

[pineteam]

As I know FMGS always use 25%CG to calculate your Vapp, and thats why we add our actual LDG CG in EFB ldg performance app to correct our Vapp.(+-1/2kt difference). Why do we do that if actual ldg cg differs from calculated))thanx.
Any reference for this? I never heard we need to correct the VAPP. I always thought the FMGS will used its computed CG.
Thanks

[vilas]

Although that's true he is talking about this.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1069x1563/psx_20210430_104915_2_21cdd6fd0361f6409e9b9582f7d417aba9a755 74.jpg

[vilas]

As I know FMGS always use 25%CG to calculate your Vapp, and thats why we add our actual LDG CG in EFB ldg performance app to correct our Vapp.(+-1/2kt difference). Why do we do that if actual ldg cg differs from calculated))thanx No! FMGS doesn't always use 25% to calculate Vapp. The GW andCG has been entered in FMGS and it just does the subtraction of fuel used to calculate new GW and CG. After approach is activated it uses actual weight. It's all automatic. After takeoff trim wheel is just that its no longer representative of the CG. If you are correcting Vapp for trim wheel as CG then it's incorrect.

[pineteam]

Thank you Vilas. I just played with the numbers on the ground in the MCDU and the VAPP will indeed vary depending of the ZFWCG inserted. I went from 25% to 10% and the VAPP increased by 3kt. I think if you leave it blank it might assumes 25%. I remember on the older Honeywell FMGC it was by default 25% the ZFWCG value.

[Sergei.a320]

OK thanx. Thats what I wanted to hear. First of all After takeoff CG vs trim are not in the same relation as before takeoff (takeoff trim setting). Than I think you dint understand me. As described in ISI 00.00.00391 Vapp is calculated by FMS in a way of: 1. A fixed CG of 25% is used before approach and,
2.Current acft CG during approach
Thats why the values of Vapp before/after initiating the approach might be different. And Fly Smart v5.0 gives us opportunity to add the expected CG to performance app during des. preparation and get real Vapp.

[FlightDetent]

Hello. I have a hunch the before / during approach Sergei is talking could be in fact "activating the approach phase".

The FMS changing from predicted to real values at that point is easily witnessed for immediate return after take-off. The approach speed changes once you press the approach phase (assuming the original destination and its fuel-to-burn is still in the F-PLN A = for demonstration of the feature).

[pineteam]

I’m a bit confused about what you guys are talking about. Are you saying after airbone if you activate VAPP it will assume a VAPP for an immediate landing even so the flight plan is still sequence for your destination? I thought VAPP will vary according to the predicted landing weight and CG. If you activate approach phase and hold for 2h surely VAPP will decrease no?

[Check Airman]

The PERF APPR page displays predicted values based on your landing weight until 14,500 in the descent or approach phase is activated. At that time, it'll switch to actual values. I'll have to look up the reference for that, but I'm 99% sure that's right.

[Sergei.a320]

yes!you're right!;)

[pineteam]

Ok thank you guys! Interesting topic. I get it now but Sergei I don’t undestand about the assumption of 25%CG you mentioned earlier. From what I saw during flight preparation if you change the ZFWCG value it will affect the VAPP on the Perf page based on the estimated landing CG. Then from what Check Airman said, when you pass 14500 feet or when you activate approach phase the actual CG. Isn’t it?