A320- S spd difference between MCDU and PFD???

[FlywithPark]

Hello.
today i experenced that there was a difference between indicated S spd on PFD and S spd on PERF-APPR page(about 5-10kts) while F and O speed were matched.
I've thought they have to be exactly same but captain told me that was allowable situation.(for S spd)
is there anybody who can explain why and where i can find it in FCOM?
Thanks!

[Server too busy]

Try FCOM 3.04.10, Operating speed definitions in Supplementary Techniques. There is also a good read in the blue bulletins at the back of FCOM 3.

I have seen it once where there was a big split between FMGC and PFD display that couldn't be accounted for and the engineers eventually found there was a problem with one of the SFCCs.

[longobard]

which blue bulletin?
Do you mean that S speed on PFD is limited to VFE F1 - 2kts?

[Jimmy Do Little]

It may have something to do with the way that speed ("S") is derived. FCOM "... Equal to about 1.22 tp 1.25 Vs of the clean configuration..."

Assume that you have 70 Tonnes in your MCDU at take-off, but your actual FAC computed weight is several tonnes more, BUT not enough more to cause a "Check Gross Weight" message.

MCDU is computing speed from the wieght you entered...
PFD is reading FAC computed speed...

This may explain what you saw.

[FlywithPark]

Thanks for your kind replies.
BTW, can you let me know the reason why only "s" Spd is different while "F" "O" spds are same with MCDU?

[Jimmy Do Little]

FCOM 1.22.20 It computes using actual weight (FAC) for S, F and Green Dot.

[aulglarse]

To add to Jimmy's first post, the weight diff can be up to 7 tonnes before a 'check gross weight' is generated. FCOM 4.03.35.

[toby320]

FAC compute the weight and in base of that the speed are display on PFD but could be differnet on mcdu the way to check the weight wich FADEC do the calculation are via AIDS page on FMGC, and you can up date the weight via FUEL page on FMGC, and no more dfferences.

[FlightDetent]

I would be interested if anybody had anwswer to FlywithPark's question. ?

Also please note that the FAC speed calculation is referenced in 1.22.40 p4 and it concludes that accuracy is +/- 3 kt for Vref, based on errors in calculated GW, which according to QRH speed table translates as +/- 3000 kg. Why would anybody try to substitute certified loadsheet data with such coarsely calculated value I do not understand.

Yours,
FD (the un-real)

[Server too busy]

which blue bulletin?


The bulletin I was referring to was the one about the differences in speeds coming from FMGC and FAC and their respective inaccuracies.

[FlightDetent]

Ok, fine piece of condensed knowledge; definitely worth reading.

FlywithPark asks: FMGS' F and O speeds indetical to those on speedtape. S speed different by about 7 knots. Any ideas?