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mike wess
1st Oct 2007, 18:16
On A-320 by 100 kts on the takeoff roll the wind data will start showing on the ND and its very close to tower wind. How is this wind calculated? Any help would be appreciated.

fantom
1st Oct 2007, 19:24
PFM, courtesy the French.

The Flying Cokeman
1st Oct 2007, 20:05
Hi there,
The wind displayed on your ND corresponds to the instant wind. It is computed by the IRS after 100 kts.
The computation of the wind direction and force uses the real time difference between the heading and track of the aircraft, as seen by the IRS. Therefore as long as your aircraft is on the ground, the display corresponds to longitudinal component of the wind. Moreover during the take off roll anemometric disturbance may cause an erroneous wind value to be displayed on the ND.
Once airborne, since lateral drift can be measured, the IRS are able to compute and display the wind direction and force.
I hope this helps anwering your question :)
TFCM

mike wess
1st Oct 2007, 20:45
Thanks for your responce, I have no problem with the winds info during flight however on ground all is equal GS/IAS and Track/HDG therefore as you stated for the IRS to measure wind direction there must be an offset between track and HDG and for wind velocity obviously there must be a difference between GS and TAS and of course this does not happen on ground. Can you please elaborate more on the longitudinal component of the wind. Many thanks
Mike

TyroPicard
1st Oct 2007, 21:01
Can you please elaborate more on the longitudinal component of the wind

however on ground all is equal GS/IAS

Are you sure about GS and IAS being equal on the ground? Think about what you wrote and we will not need to elaborate.
yro

mike wess
1st Oct 2007, 21:12
You are right about the GS my mistake I meant TAS, but still where is the wind direction coming from?

TyroPicard
1st Oct 2007, 21:19
The wind velocity indicated is head or tail component until after lift-off, because there is no drift for practical purposes. The calculation performed by the FMGS is no different - the value of drift is zero.
Once airborne, the wind velocity becomes valid in both speed and direction.
TP

mike wess
1st Oct 2007, 21:21
so you're saying that the wind direction on the ND with aircraft on ground is not accurate?

TyroPicard
1st Oct 2007, 21:43
Correct, and nor is the speed. Speed is head or tail component only (IAS minus GS). The direction arrow varies either side of (in a headwind) straight ahead as you steer the a/c on the ground.
Only tells the truth when airborne.
Don't get fixated on it - look out of the window instead - far more important things to think about on every take-off!:ok:
TP

Don Coyote
1st Oct 2007, 22:54
Wind speed and direction displayed on the ND are not accurate on the ground or the air for the following reason.

When the IRS is set up you tell it where it is, the IRS knows the TAS and heading and time flown and therefore knows where it should be in relation to where it started. The difference between where it thinks it should be and where it calculates it actually is it assumes is due to the wind. This vector is displayed as the wind on the ND.

This would be true if the IRS did not have any error of any kind; however the IRS does have a drift error. The drift error is added to the position error and is displayed as a wind on the ND. Normally this is very minor but it also explains why you may see 10 kts groundspeed on the ND when sat on the stand with the parking brake set.

Apparently new aircraft will have a wind on the ND derived from GPS data so the error will be negligible when compared to that derived from IRS data.

TyroPicard
1st Oct 2007, 23:12
When the IRS is set up you tell it where it is, the IRS knows the TAS and heading and time flown and therefore knows where it should be in relation to where it started. The difference between where it thinks it should be and where it calculates it actually is it assumes is due to the wind. This vector is displayed as the wind on the ND.

Can this be true? Surely this vector would be the average wind affecting the a/c from ramp to PPos... ~Bolleaux~ is what I think.

ND displays an instantaneous wind - the accuracy of the calculation will vary with the accuracy of the inputs.
Tyro

mcdhu
2nd Oct 2007, 14:57
Answer from Airbus:
9. WIND DISPLAYED AT 100 KNOTS DURING TAKEOFF ON THE ND
During take-off roll, where is the wind derived from at 100kts on the ND? As the
aeroplane is still on the ground, how does it work?
ANSWER:
The wind displayed on the ND corresponds to instant wind. It is computed by the
IRS, only from 100kts.
The computation of the wind direction and force uses the real time difference
between the heading and the track of the aircraft, as seen by the IRS. Therefore,
as long as the aircraft is on ground, the display corresponds to the longitudinal
component of the wind. Moreover during the take off roll, anemometric disturbance
may cause an erroneous wind value to be displayed on the ND.
Once airborne, since lateral drift can be measured, the IRS are able to compute
and display the wind direction and force.
Cheers
mcdhu

WaterMeths
2nd Oct 2007, 15:25
blimey - we've gone full circle !! :confused: