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To calculate the V approach the FMS is using a wind either taken by the ADIRS or inserted on APPR PERF page. At first I will say that it takes the wind inserted by the crew, but I am not completed sure about this assumption since the MINI FUNCTION on A320 will need as well wind info taken by ADIRS to be able to cope with sudden loss of headwind / increase tailwind.

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It uses the wind entered in the PERF APPR page to correct the computed VAPP:

Takeoff, performance and go around speeds

[...]

The FMGC uses the performance model and either the predicted landing weight or the current gross weight at transition to the approach phase to compute approach speeds (VLS, VAPP, F, S, Green Dot). On the PERF APPR page, the selected LDG CONF determines the applicable VLS and VAPP, the latter being updated by the WIND correction that the pilot enters on the same page.

(A320 FCOM - Autoflight - Flight Management - Performance, emphasis mine)

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  • $\begingroup$ I'm a bit disappointed. I imagined the aircraft was able to evaluate wind component based on difference between trajectory as defined by inertial sensors and trajectory as expected by air mass sensors (static and dynamic pressure, AoA, side slip sensor,...) $\endgroup$ – Manu H Apr 10 at 7:49
  • $\begingroup$ @ManuH The aircraft is capable of doing exactly that and it will display this wind component on the ND (Navigation Display). You don't want to use this current and local wind for VAPP though, because this should be based on the wind on the ground at the airport, which you can only know in advance from the ATIS. If you slow down to VAPP at 1000ft you still have very different wind as measured by the aircraft compared to ground level. $\endgroup$ – Bianfable Apr 10 at 8:45

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