View Full Version : A320. FMGC (MCDU) VLS and FAC (PFD) VLS
7th Jun 2012, 17:47
Sometimes I see that VLS in the PFDs is significantly higher than that of the PERF page. It is easy to notice when there is little wind because there is less than 5 knots between VAPP and VLS in the PFD. When there is wind or MCDU VLS is higher than PFD's VLS, GS Mini masks the difference.
I have got used to routinely check the difference during every approach. Sometimes the effect in airplane's handling is very noticeably. Airplane will fly more during flare or it will sink, depending on which VLS is higher.
Most of the times, PFD VLS is a few knots lower than MDCU VLS, or they are the same, and only a few times PFD VLS is higher. I try to adapt the flare to that piece of information. If PFD VLS is higher, I retard somewhat later. If it is lower, I retard earlier.
However sometimes things don't happen as I expect.
And some particular airplanes in the fleet seem to always have a difference in the same sense, which makes me think there is some problem with the probes or something.
Some pilots add a few knots to the VAPP when MCDU VLS happens to be too slow compared to PFD's. But now we are not allowed to do that and I wonder why.
7th Jun 2012, 17:59
FCOM DSC 22-40-40
Above 14500ft VLS is computed out of the gross weight, so if you've not been as accurate as you could have been when it comes to updating the gross weight or FOB, you'll get a slightly erroneous calculation of VLS and hence VAPP will appear closer than 5kts.
Below 14500ft VLS is computed from current angle of attack, speed/Mach, altitude, thrust, and CG. There's by definition an inaccuracy in this method, which Airbus quote as +/- 3kts in config full.
Policy at my previous airline was to add a couple of knots to VAPP to ensure a 5kt split in this situation.
7th Jun 2012, 18:01
It is now SOP at our company to make sure that Vapp inserted into the MCDU is minimum 5kts above the PFD fac calculated VLS. Apparently since this change we are seeing far fewer hard landings, this is especially evident on the A321s where our sensed weight is almost always higher than loadsheet.
We were having landings in the A321 close to VLS where the flare didn't seem to do much to arrest ROD.
7th Jun 2012, 18:04
As per FF the company I work for have recently introduced this into our procedures to ensure that once configured Vapp is Vls + 5. If not then we input an appropriate speed into the perf page.
Strange how different companies have different views on this.
Edited to say my company is not the same outfit as tom775257 (no A321's)
On the 31st May our A321 aircraft just had a FAC software update which, from reading the info fixes this issue.
8th Jun 2012, 05:52
In my airline that is now forbidden. Go figure...
In the case of a particular airplane being always "FAC heavier" it is obvious that the problem is in the FAC or the AOA probes. These will lead to longer float landings.
But in other cases where there is a significant ocassional difference between both PFDs and FMGC VLS, it is a good thing to add a few knots.
The reason in my airline for banning that practice is increased landing distances. We never land in any runway shorter than 3,500m. :ugh:
Now I just adapt my flare to the circumstances: sharper with early lever retarding or more progressive with late lever retarding, according to circumstances.
8th Jun 2012, 12:34
If you go into the Alpha Codes in the AIDS and insert GWFK (or GWFP if you work in lbs), the FAC computed gross weight will be displayed. This is what your GD, VLS, Alpha Prot and Alpha Max is computed from when airborne. If your FMGC weight is different, just add or subtract one knot per tonne difference for the FMGC compted VAPP and you will have the 5 knots above VLS.
Vary rarely are loadsheets accurate when using notional weights. People are getting heavier and their handbagge often weighs above the allowed maximum- the aircaft is nearly always heavier than the loadsheet says.
8th Jun 2012, 16:51
airbus says FMGC is more accurate than FAC... If load sheet is accurate.
Green dot will change by 2kt per ton. VLS by 1kt per ton
Sometimes FAC is accurate bcause I can feel how the airplanes sinks in the flare. But sometimes, however, FMGC is good. There is no way to be sure...
8th Jun 2012, 18:27
Airbus published a Flight Crew Bulletin where you can find most answers to your questions. It includes a good explanation and some recommendations how to handle this issue.
Please check FCB-FCB17: CHARACTERISTIC AND PROTECTION SPEEDS for more information.
8th Jun 2012, 19:51
Not sure, and inviting response ... but I don't think the "reduced split between Vls and Vapp" is down to differences in weight. I think it's down to differences in C of G. Quite often, it's most accurate when you fill the aeroplane up - but when you don't fill it up, that's when there are most likely to be discrepancies in C of G position compared to what's programmed. If it were down to discrepancies in weight, the error would be biggest when the aeroplane was fullest.