A320 Wind Indication on PFD during Takeoff-Roll
 

Probabably discussed already...But can anyone supply information and source to the question how wind speed and direction is calculated without drift on takeoff-roll ( A320 ) ?

Thanks RvB

Mmmmm........what a good question. I can't come up with the answer. Surely someone can help?

Cheers,
mcdhu

I haven't actually looked to see what the indication is on take-off.

But I suppose it doesn't calculate it properly and constantly uses the relationship between the aircraft heading/IRS ground track and the TAS/IRS groundspeed to calculate the wind.

It will then sort itself out once airbourne when the aircraft crabs into wind.

FIS.

I don't see why it couldn't generate a wind vector - the system I use does generate a wind vector. As Field In Sight stated, all the inputs that the aircraft normally uses to generate the vector are available. The only problem is artificially imposed constraints and therefore the vector is probably not going to be correct.

The aircraft should be using track and heading to calculate the wind direction. No difference between them during the takeoff roll so (if I recall correctly) you'll more than likely get a direct headwind or tailwind on the vector until you crab after lifting off.

No, you do get a pretty accurate wind arrow and readout which comes in at 100kts on the top left of the ND. I have no idea how it does it - but it does. It's Honeywell stuff in the Airbus if that helps.

Cheers,
mcdhu

Rainboe
 

I think you are pretty much correct, Though it's certainly not suppressed on the 744 'cos I've watched this on the N/D (as heavy :) ) of the 744 a few times.
...what seems to happen is that whilst the aircraft is firmly on the runway the wind speed is vaguely credible ( I guess it's just using IAS minus Inertial G/S to derive windspeed), however wind direction is pants, with the wind arrow showing direction from straight ahead, since aircraft heading equals track. However once the aircraft unsticks and weather cocks into wind then often as not Track no longer equals heading and the wind "arrow" slews quite rapidly around to the correct bearing.

Most interesting information during T/O for me is windspeed, especially in an ATC-Environment where you ask: 'What's the wind?' and they reply 'What do you need?' And the Head/Tail-Wind-Component is correct, whereas the direction becomes only when approaching liftoff.

If you're using reduced Thrust an calculated with L/V-Wind and then you see 10 knots or more from behind, maybe it's a good idea to go up to TOGA Thrust.

Regards, MAX

the wind vector is true wind direction and speed provided by the ADIRS. when departing on a runway equipped with an ILS this freq is auto-tuned. when take-off power is set and no trk/hdg preselected, the aircraft track can be seen on the ND at 30kts as a yaw bar ( the LOC)and 'RWY' on the FMA, so I guess the a/c is tracking magnetically along the runway with a true wind indication courtesy of ADIRS. :ugh:
FLYJETS, not sure what you mean-hope this helps: avionics cooling;
Once takeoff power is selected, the inlet and extract valves close (closed configuration)with avionics cooling coming from the a/c airconditioning.

have you got the answer?
 

A320 can know the wind from head or tail use the diffrence between IAS and GS .
but how to know the drift ?

Unless Airbus has retro-fitted a wind-direction vane on top of their fuselages or tail-fins, there is no way the ADIRS can possibly calculate the wind direction (as opposed to headwind component) while the aircraft track is being dictated completely or even partially by its main wheels on terra-firma.

I'm shocked that any professional pilot posting on this thread could think otherwise. Do you not study the basics of air navigation - including the vectors of heading, track and wind - for your licences nowadays? :ugh:

Well Chris Scott....if you understood the question...

Airbus aircraft (A318, 319, 320, 321, 330, 340) display a wind vector on the ND when groundspeed > 100 Kts. And it doesn't just display it as a head or tailwind, it will show a direction as well as a speed.

The original poster was asking how it does this.

It's a great question.

Sounds like your professional piloting is quite limited.:ugh:

Perhaps it uses rudder or rudder pedal displacement? I do look at it after the 100 kts call to check on the hwc/twc.

I'm guessing it's mainly just sensing the headwind/tailwind component, not actual winds. The exact direction is likely unreliable and fluctuates with even tiny heading changes during the takeoff roll.

Quotes from 767-300ER:
(1) "Airbus aircraft (A318, 319, 320, 321, 330, 340) display a wind vector on the ND when groundspeed > 100 Kts. And it doesn't just display it as a head or tailwind, it will show a direction as well as a speed."
Yes, and - as wiggy has explained - once you get airborne it'll give you an accurate wind direction and speed. Prior to that, forget it!

(2) "The original poster was asking how it does this.
It's a great question."
I'd say it was a very fair question, perhaps with a hint of sceptical surprise?

(3) "Sounds like your professional piloting is quite limited. :ugh: "
Love it! :rolleyes:

Now, if you are convinced the displayed wind direction is accurate before the a/c becomes airborne, can you answer the OP's question?

I don't know anything Airbus specific but generally speaking groundspeed displayed on MFDs (and some PFDs such as 777) is calculated by the navigation system utilizing both heading information as well as TAS information generated by the ADC (or whatever your flavor name for ADC is, be it ADIRU, ADIRS etc).

To solve for TAS, you need a good measurement of either TAT or OAT. In transport category jets these days TAT is what's used by means of the TAT probe in concert with the data generated by the static and pitot ports.

Usually TAT isn't very reliable on the ground and def not until it's heated up by some real friction and not by the metal just sitting there being either cold by the outside temperature or being heated by the sun in warm places because of its metallic color.

I believe and I could be entirely off base here, that because the TAT isn't reliable on the ground and at very low airspeeds that some of the data generated by its mean is suppressed purposefully because of that. Wind drift/velocity would be a good example of that.

Now if you ask why do they still display TAS next to GS showing them equal... I have no idea. One would think they'd suppress that too but I know some airplanes display that along with SAT which is obviously wrong too since it's a derived calculation based on TAT.

In particular as an answer to your drift question. It's based on heading course and TAS. Usually provided to you seamlessly by the FMC or whatever is doing the navigation portion of your overall system/fabric that is in the end displayed to you as one picture on both MFD+PFD.

Chris: your critical poster should be aware that ANY heading fluctuations such as are typical for a cross wind takeoff will allow the FMS to try to compute a wind direction - however it will be in error!
This feature was often seen during base training when the trainee had insufficient/too much rudder in for asymetric flying. You could watch the W/V readout change continually with the rudder changes! This often led the trainee to suspect, when in the sim. that "sir" had tweeked up the wind without telling. Not so, just poor rudder use. I occasionally used to demo this on the downwind leg at low speeds.

It's a great question and I've asked training captains many times how it does it but nobody seems sure. One suggested some info was derived from rudder inputs during take off roll but I'm not convinced. The arrow at 100knots is pretty accurate though........

As far as pilot FCOMs are concerned there are only a couple of F.Ctl inputs to the ADIRS system. ELACs and the BSCU. Given that it should not (it is Airbus after all) be ELAC it could well be the BSCU. Agreed it would be a very coarse way of measuring the input required to maintain track but it would appear to be all we have. Logically by the time we reach 100 knots we should have the thing vaguely under control so a measurement about then would be roughly correct.

Logically that is and as I said it is Airbus.

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.