On takeoff while accelerating through approx 100 kias the nd displays wind speed and direction. I know the manual says it will display when true airspeed is greater than 100 kts. But how is wind speed and direction determined while still on the ground?

On takeoff while accelerating through approx 100 kias the nd displays wind speed and direction. I know the manual says it will display when true airspeed is greater than 100 kts. But how is wind speed and direction determined while still on the ground?

I think it's by the tower control.

On takeoff while accelerating through approx 100 kias the nd displays wind speed and direction. I know the manual says it will display when true airspeed is greater than 100 kts. But how is wind speed and direction determined while still on the ground?

Same way it's determined while in the air, by comparing the air data and the inertial reference data. The difference between your TAS and your GS gives you the headwind component. The crosswind component can be worked out by your crab-angle. Those two components together can be combined (with some trigonometry) to give you the wind direction and speed. The crosswind component might be a bit difficult to determine while still on the ground though, as the friction of the tyres will also play a part.If you mean, how do you know the wind speed and direction before you reach 100 TAS, then indeed you ask the tower or listen to the ATIS.

Thanks. On the ngx, I can set up a takeoff with a direct crosswind and the nd shows the correct wind speed and direction. Just wondering how it does that because while still on the ground the track and heading are still the same. Would seem like the aircraft would need to be in the air to determine wind based on groundspeed, tas, heading and track.