A320 Wind Indication on PFD during Takeoff-Roll

[Chris Scott]

Thanks Meikleour,
I think rudder movements on the T/O run may well have something to do with the displayed wind-direction fluctuations you and others describe. **
(BTW, I'm not sure the sim is always representative. I seem to remember a F/O once telling me that in the sim the secret of tracking the ILS localiser accurately on the approach was to use the nosewheel steering...)

Quote from the OP, RvB (my emphasis):
"...can anyone supply information and source to the question how wind speed and direction is calculated without drift on takeoff-roll ( A320 ) ? "

Was hoping not to resort to didactics, but... back to basics. The way an ADIRU calculates W/V is the same as a navigator would, isn't it? Except it does it instantaneously. The relevant triangle of vectors includes HDG(T)/TAS and TRK(T)/GS, with W/V (wind bearing and true speed) completing the triangle.

The IRU measures HDG(T), TRK(T) and GS. The AD unit calculates the TAS from IAS, TAT and pressure altitude.

So the difference between HDG and TRK (i.e., drift), is an essential datum to calculate W/V. On T/O, If the aircraft was mounted on rails like a V1 (doodlebug) from WW2, there would be zero drift, even in a strong crosswind. In that case the ADIRU would show the wind direction as being either the same as the a/c HDG or the reciprocal, depending on the comparison between TAS and GS.

In practice, the tread grip and profile of the (radial) tyres can only do so much in a crosswind to try and achieve that. A small discrepancy between HDG and TRK is going to develop, particularly on a wet runway. This will constitute drift. Also, the PF will be making frequent changes of rudder angle, and tiny changes of HDG, to stay near the runway centreline. The resulting lateral accelerations will lead to momentary fluctuations of the drift, which probably explains the rapid changes in the wind direction displayed on the ND.

BTW, the title of this thread refers to "wind indication on PFD." Am I correct in assuming that the OP meant the indications on the ND? If not, things have changed since my day.

** [By edit]
As you suggest, Meikleour, in flight any asymmetry-induced sideslip (including bad flying) will presumably be interpreted by the ADIRU as drift (difference between HDG and TRK). downwindabeam also raises an interesting point about the possible difficulty in calculating TAS using the TAT from a heated probe (fully heated for T/O?) at low speeds.