A320-characteristic speeds discrepancies

[OPEN DES]

Characteristic speeds of the A320: VLS, F, S, GD are calculated by the FAC. The FAC estimates the gross wt using inputs like alpha, aircraft attitude, CG and IAS. From there on it takes the GW and displays the corresponding 'characteristic speeds' on the PFD. Airbus advertises that the FAC will estimate the GW such that characteristic speeds are accurate within +/- 3 kts.
However the speeds in the MCDU PERF pages are based on the crew entered GW, Vapp is calculated using the VLS we have here.
Max. expected discrepancy between PFD and FMGS speeds should be +/- 3 kts.

In our operation when we have such a discrepancy it's always the FAC speeds which are higher than the FMGS speeds and never vice-versa. While you'd expect an equal distribution of error. This has led us to believe that in many cases we are heavier than we actually 'think'. This assumption is supported by pitch-attitudes observed when we have a situation like this, which make me believe that the FAC is far more accurate than advertised. For many it is now common practice to 'top-up' Vapp to make up for the discrepancy (2-3 kts max). Which is common sense when the A/THR is getting nervous approaching VLS.


I wonder if anybody here has any similar experiences or can offer any thoughts on this. Comment very appreciated.

[Zeke]

Originally Posted by OPEN DES

I wonder if anybody here has any similar experiences or can offer any thoughts on this. Comment very appreciated.

FMGC/FMGEC predictions will rely on the ZFW & ZFW CG that the crew put in. It suggests that your load sheet is not accurate.

The actual CG position can be monitored in flight with the MCDU MENU with a ALFA callup. If you were to compare that after takeoff it will give you an idea of the error in your load sheet.

Load sheets are normally done with zone loading, with assumed passengers weights, assumed baggage weight distribution, and assumed hand carriage distribution, and an assumed galley/catering weight, giving the pilots a ZFW and ZFW CG with a lot of assumptions.

It is only due to the FACs (or PRIMs on the bigger ones) that you can make this comparison. This sort of error is accepted across all manufacturers.

[Dixons Cider]

The actual CG position can be monitored in flight with the MCDU MENU with a ALFA callup

On that note....and sorry to hijack the thread.... but any idea where I can find a list of the ALPHA PARAM codes?? I know a few of the basic ones but would be handy to know more, the above example of CG included.

Now back to the thread, hope I havent killed it!!!

[Zeke]

Originally Posted by Dixons Cider

any idea where I can find a list of the ALPHA PARAM codes??

Send me an email or PM with a valid email address.

[FlapsOne]

Perhaps average weight values (including the ever increasing amount of cabin baggage!) are in need of an update!!

On another point, if A/THR gets "very nervous" (although VLS is no big deal really) then just disconnect it and park the thrust in a sensible place and stop the nervousness!

[captainpaddy]

If your company uses standard bag weights then I would expect the discrepancy to be very large for a full 320 or 321. I doubt even one of the bags loaded are 13kgs. The interesting thing is that when someone shows up at check-in with a heavy bag, they are charged for excess baggage and their bag gets sent to the aircraft regardless. The loadsheet will still say 13kgs. Take 200 bags each weighing the 20 kgs generally allowed and you've already got 1,400kgs of weight not showing on your loadsheet. Not to mention the errors in standard weights for passengers........

[Dixons Cider]

Cheers Zeke, PM enroute.....

[Dan Winterland]

VLS is computed by the FAC. GD, s anf F come from the FMGS for the data entered into the MCDU on initialisation. This is wht GD doesn't change (computed as 2 x GW + 80 + 1 Knt/1000' above 20,000') but VLS will.

To fing the FAC computed GW, go to MCDU menu, then AIDS, the Aplha Param Call Up. The GW computed by the 2 FACS should be on page 1, although this may vary with your company's configuration.

[captainpaddy]

Originally Posted by Dan Winterland

VLS is computed by the FAC. GD, s anf F come from the FMGS for the data entered into the MCDU on initialisation. This is wht GD doesn't change (computed as 2 x GW + 80 + 1 Knt/1000' above 20,000') but VLS will.

I understand that new version of the 'BUS have a different logic to older versions, but certainly for the aircraft I am flying this is not the case.
The speeds displayed on the PFD (GD and F and S and VLS) are all from the FAC. The speeds targeted by the autothrust come from the FMGC (i.e. the speed targeted when clean, flap 1, flap 2, etc. and the VAPP when using managed speed in the landing configuration). These FMGC speeds are calculated from the entered GW and fuel load.

Often the first indication I have that I may need to overwrite VAPP is when I notice that GD, F or S speed as displayed on the PFD is higher than that shown on the PERF APP page of the MCDU. In these cases, and given very smooth conditions (because the A/THR hasn't a hope of accurately keeping a constant speed otherwise!!!) you will notice that the A/THR actually maintains the speed a few knots below the PFD characteristic speed for the flap setting. For example, the FMGC A/THR will target 187 knots in Flap 1 cos that's what it thinks S speed is, but the PFD shows a more accurate S speed of 190 knots.