A20N LDG CG trim.
Thread Starter
Joined: Jun 2018
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From: Vladivostok
A20N LDG CG trim.
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.
Joined: Apr 2010
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From: IRS NAV ONLY
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.
Joined: Jun 2007
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From: Wanderlust
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.
Joined: Jul 2006
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From: Somewhere over the rainbow
Thanks
Joined: Jun 2007
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From: Wanderlust
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
Joined: Jul 2006
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From: Somewhere over the rainbow
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.
Thread Starter
Joined: Jun 2018
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From: Vladivostok
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.
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.
Only half a speed-brake
Joined: Apr 2003
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From: Commuting not home
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).
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).
Joined: Jul 2006
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From: Somewhere over the rainbow
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?
Joined: Mar 2006
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From: USA
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.
Joined: Jul 2006
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From: Somewhere over the rainbow
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?
