A319 - FAC & FMGS calculated green dot and holding speed

[DesiPilot]

Hi Folks,

Today arriving at DEL at FL150 we were asked to hold, the green dot speed calculate by FAC was around 209 kts and calculated on the FMGS (Approach phase) was 205 kts. We were already flying selected speed of 220 kts, as we entered the hold, managed the speed, the speed target became 205 (margenta), which was of course below green dot speed but above VLS speed. Now the questions:

1) The holding speed is calculated by FMGS which should be equal to calculated green dot speed in clean config. Now my questions is, the FMGS calculated speed that is displayed in approch phase is that the green dot speed for present condition or does it take in account the distance and time (hence the fuel burn) and displays the speed that will be good for APPR phase?

2) Although it was only 4 knots difference would you fly selected speed of 209 knots rather than managed speed of 205 knots? I know its only 4 knots but I wanted to know other peoples view- our company policy is not to fly below green dot in clean config so we ended up selecting the green dot speed.

Thanks in advance.

[tom775257]

Answer after much beers but:

FAC calculates the current green dot.

FMGC calculates green dot at expected approach including flight fuel burn and thus approach weight.

[OPEN DES]

Are you using authorised weights for baggage as opposed to actual weights?

The discrepancy was probably due to the aircraft GW being higher than as per the loadsheet and hence the FMGS through INIT B.

The speeds on the PFD are calculated by the FAC, published accuracy I believe is +-3kts due to AOA-sensor error. However in reallife probably more accurate.

You can easily 'manage' the speed when you have a discrepancy as the A/THR will fly the PFD(FAC) or FMGS(loadsheet) green dot/S/F whichever is higher. Only when in landing config the aircraft will fly the FMGS Vapp or VLS(FAC, speed-tape) whichever is higher, hence you will not always have a 5kt split. Some people do bump the Vapp up by a few knots.

[calypso]

I thought, but may be corrected, that while clean and on managed speed the A/C would not fly below green dot. This means that the magenta bug may be a few knots below green dot but the aircraft would still fly green dot hence there is no need to fly selected speed.

In fact you may as well activate the approach fase and still fly managed speed at gren dot until ready to start the approach.

[FlightDetent]

FAC calculates the current green dot.
From AoA sensors and displays is on PFD. Error as much as 3%. (not 3 kts!).

FMGC calculates green dot at expected approach including flight fuel burn and thus approach weight.

MCDU displays the predicted Vls and Vapp only until you activate the approach phase. Once activated, the Vls is frozen to the present weight.

[OPEN DES]

I thought, but may be corrected, that while clean and on managed speed the A/C would not fly below green dot. This means that the magenta bug may be a few knots below green dot but the aircraft would still fly green dot hence there is no need to fly selected speed.

That is correct. The a/c will fly the PFD speeds rather than the FMGS speeds (whichever higher)

[OPEN DES]

From AoA sensors and displays is on PFD. Error as much as 3%. (not 3 kts!).

I stand corrected. Haven't read the FCOM bulletin for a while. :)

However I do think that in daily life the accuracy is much higher than that. The error should work in both ways, and I have very rarely seen a discrepancy which would lead to believe that we are lighter than we actually are. But I guess it's not down to me to comment on published accuracies.

[DesiPilot]

Thanks everyone.

Yes, the plane indeed flew the green dot speed and not the margenta speed as expected.

I just wasnt too sure about the VLS and VAPP speed calculated, if it was calculated considering fuel burn till approach phase is activated (perhaps the margenta decelarate point).

Once again thanks a lot.

PS: We do use actual weights for our Init B page.