A320 wind computation

[CVividasku]

Hello,

I noticed that the 320 almost always displays tailwind, just after takeoff, when wind on ground is supposed to be 0.
I am starting to have an idea about why that is.

I took a picture of colleagues during a flight, here are the values :
GS 178
TAS 170
This should give 8kt tailwind
Wind : from the back, 20° cross component, 13kt. This gives 12kt tailwind.

Why is that ?
We also have at this time : 2000fpm climb. 12.5° pitch angle.
This gives a FPA of 7° and AOA of 5.5°.

If we assume the TAS to be aligned with pitch angle, we get horizontal TAS = 166 and tailwind component = 12kt. As computed here.
If we assume the TAS to be aligned with the flight path angle, we get horizontal TAS = 169 and tailwind component = 9kt. Much closer to the difference between TAS and GS.

Also, if we assume 15° pitch up, 150kt, no wind in reality, and AOA of 6°, with 9° FPA we get :
TAS 150kt
GS 148kt
Tailwind : 0kt
Horizontal speed if we incorrectly assume the speed is along the pitch angle : 145kt
Imagined tailwind : 3kt.

Can anyone concur ? It would be interesting to have a picture of a PFD+ND during a takeoff without wind.

I know for sure that the crosswind component is wrong whenever there is a significat sideslip angle, that the plane doesn't know. So I wouldn't be too surprised if there was a mistake here ? Or am I the one in the wrong ?
It seems like the airplane doesn't know AOA, as it doesn't know sideslip, when computing wind.

[Fursty Ferret]

Wind : from the back, 20° cross component, 13kt. This gives 12kt tailwind.



i mean, it does at the point where the wind is measured. The runway is often different.

[CHAPARRAL]

Hi CV.
As all the data (TAS, GS, pitch, FPA, etc.) is coming from the IRS’s it is really puzzling, and I always assumed that the values are linear.
Are you sure about your observations?