A320 wind computation
 

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.

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

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?