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OPEN DES
2nd Jul 2007, 11:22
A320 tech q.

Hi everyone,

Was thinking about this the other day. The PFD as you all know displays the characteristic speeds as calculated by the FACs (aerodynamic data). While the FMGS has got the speeds based on estimated weight (ZFW input+FOB).

What is the actual commanded target speed for the A/THR when the approach phase is active in managed speed? Will it aim for the FMGS speed? Allowing the aircraft to fly below S and F as displayed on the PFD? (when the FAC calc. speeds are higher than the FMGS speeds). Or will it hold the speed at the PFD displayed S, F and GD.
What about vice-versa. If the FAC calculated speeds on the PFD are in fact lower than the ones in the FMGS. Will the A/THR hold the FMGS speed or will it go down to the PFD (FAC) speeds?

My guess is that it will fly the one speed whichever is higher. But couldn't find any reference.

I have read the FCOM bulletin regarding the calculation of characteristic speeds but it doesn't talk about A/THR speed targets in case of discrepancies between FAC and FMGS calculated speeds.


Thanks

S.

hetfield
2nd Jul 2007, 12:16
It will definetly flythe "calcualted" approach speed. I've obseved many times that margin between Vapp and the VLS-band was almost nothing. Means we were actually heavier. So I assume that concerning F,S and greendot speed it's the same, just FMGS-calculated speeds and that's what A/THR will command.

OPEN DES
2nd Jul 2007, 12:31
Hi Hetfield,

Thanks for the reply. I observe Vapp being close to VLS (pfd) very often as well, but I am sure that the A/THR will never be allowed to command below VLS(pfd), regardless of FMGS calculations. (unlikely because requires a 5kt discrepancy anyway). I was wondering if the same is true for F, S and GD. It will be very subtile anyway. Just give or take a few knots. With the A/THR performance you wouldn't even notice lacking the magenta target bug in managed speed.
Thx again

TyroPicard
2nd Jul 2007, 21:31
This may be of interest, even if irrelevant.....
The FAC accuracy for VLS in CONF FULL is about +/- 3 kt.(FCOM 1.22.40) This corresponds to about +/- 3 tonnes on Gross Weight for the A320 at MLW. I reckon most loadsheets (and thus FMGC speeds) are more accurate than that.
The only FCOM reference which is vaguely relevant to your question says the FAC computed speeds "also go to the FMGC to be used as limits for various guidance modes". I think the A/THR uses FMGC speeds but respects VLS in CONF FULL .. but I could be wrong!
Cheers, TP

Carnage Matey!
3rd Jul 2007, 01:01
The aircraft will not fly below Vls regardless of what the FMGC calculates as the approach speed. This was demonstrated by a colleague who flew an aircraft packed with rather sizeable passengers and a loadsheet based on standard weights. When they went to managed speed the FMGC calculated Vapp was actually several knots below Vls!!!!!

OPEN DES
3rd Jul 2007, 01:11
Thanks guys,



@carnage

That's my understanding as well. I was just wondering if the same would apply for F, S and green dot?


Cheers

Dan Winterland
3rd Jul 2007, 03:57
I for one don't have much faith in notional weights. For example, the last time I was at the notional weight for an adult male, I was 14! And how often these days is cabin baggage less than 3kg per person! One aircraft I used to fly had Weight and Balance Computers which calculated the aircraft's mass and C of G from the pressure in the oleos. WABC mass was always greater than loadsheet mass. If you saw a 747 full of average size Americans who had to strain to get their '3kg baggage' in the overhead locker, you could understand why the WABC mass was once 8% greater than loadsheet!

The A320 series allows you to access the FAC computed mass. Go to MCDU menu > AIDS > Parameters > ALPHA Call Up and the FAC computed masses will be displayed. There are restrictions and I think .1 degree of pitch equates to 3 tonnes, but in my experience they are more accurate that the loadsheet figures. I change the Vapp to allow for this allowing 1 knt per tonne. This isn't necessary on the 320s I fly, but the 321s are always heavier that loadsheet by about 3 tonnes. On the approach, I check the 'gap' between VLS and Vapp and make it a minimum of 5 knts.

Also, if the aircraft is struggling to get to height, sometimes putting the FAC computed Mass into the MCDU on the Fuel Prediction page will give you more accurate Optimum and Rec Max Alts. On more than one occaision, the new weight has shown we couldn't get to our planned cruising Alt.

mcdhu
3rd Jul 2007, 09:00
This was further reinforced for me when the recent security procedures were inflicted on our long suffering customers and much more (almost all) baggage therefore went into the holds. The computed (mcdu) Vls much more closely matched the PFD (FAC) figures because the load form/sheet was suddenly more accurate.

Because our guests are now permitted to bring their huge trolley bags on board again, the figures tend to disagree.

As an exercise if you have a minute in the cruise, work back from the PFD Green Dot figure, via the QRH 4.01 to see what mass the ac thinks it is at and compare with the ECAM figure.

Cheers all,
mcdhu

OPEN DES
3rd Jul 2007, 19:26
Thanks guys. :ok:

An Paddy Eile
3rd Jul 2007, 21:32
OPEN DES,

Yes. It will be the same for S, F and GD. That is usually the way that I will discover the error between FAC and FMGC calculated targets and add the appropriate amount to Vapp.

With the Perf APP page displayed and any characteristic speed in view, you can see any differences easily. e.g. S speed 190 in the MCDU, but S speed is at 193 on the PFD. So add 3 knots to Vapp and you'll be covered!!!