How does the PFD show (calculate) wind velocity during the takeoff roll ?
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Gosh, if it gives a_pilot some sort of warms and fuzzies to check the ND on T/O, who are we to tell him not to? We all have our idiosyncrasies, chief among them being that we became pilots in the first place.
re Uplinker's post 50
Why would the aircraft be more difficult to rotate with a tailwind, assuming rotation was commanded at the "bugged" airspeed?
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An aircraft can never measure vertical windspeed. The systems rely on the component of the wind in the direction of flight. Winds are not a static flow, but turbulent and gusting, especially though the column near the ground.
Component is the operative word, for both horizontal and vertical winds.
I would have to disagree here, just as the ac cannot provide reliable wind information while crabbing, or in a turn while descending. There is not enough data from the flap settings, latency, and associated lift components to provide vertical winds. (especially when the sensors can be shielded by the manoeuvre such as rotation.)
I believe you will find that AB has some issues with this system, especially on final in winds while it is a start, I would call it far from accurate or reliable.
However the displayed nose / tail wind components will be accurate and could be used to confirm suspected wind shear during take off.
it is also possible to calculate the vertical wind from TAS, flight path angle (Pitch angle - average(AOA)), Vertical speed. But i'm pretty sure this is not displayed.
Which is unfortunate in my opinion.
Which is unfortunate in my opinion.
I believe you will find that AB has some issues with this system, especially on final in winds while it is a start, I would call it far from accurate or reliable.
Last edited by underfire; 13th Sep 2016 at 02:24.
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Bonjour KayPam.
On A320/A321/A330, the wind strength and direction is displayed on the ND from 100 kts on the take-off roll - I think that is IAS/TAS
Like the OP, I would be very interested to know how these older aircraft calculate the wind direction information while still on the runway, and why the wind vector information is displayed from 100 kts and not later.
Would it be possible to ask your Airbus colleagues?
On A320/A321/A330, the wind strength and direction is displayed on the ND from 100 kts on the take-off roll - I think that is IAS/TAS
Like the OP, I would be very interested to know how these older aircraft calculate the wind direction information while still on the runway, and why the wind vector information is displayed from 100 kts and not later.
Would it be possible to ask your Airbus colleagues?
In the absence of a sideslip probe, sideslip can be estimated from (left AOA - right AOA).
There have been tests performed and a model or a table has been derived from these tests, the aircraft can then refer to this model or table to compute sideslip, and then wind direction.
underfire : yes, there are many known/admitted limitations to wind calculations.
However, you can derive yourself the formulas and check that what I said is correct if you assume that all measurements are done correctly.
So i wouldn't know any more details about the models/tables used to compute the sideslip angle from the AOA difference, but i'm pretty sure it would lead to a relatively high inaccurracy.
(However, the effect in itself is well documented and should even be included in the flight mechanics part of the ATPL theory : part of the dihedral effect is due to an AOA difference between the two wings in sideslip)
As mentioned earlier, if you have time in the sim ( and yes, certainly on the Triple it does replicate what happens on the aircraft ).
Fly an approach using crab technique..take a look at the ND wind..then transition to the low wing/sideslip technique (FWIW the Triple will do this on autopilot anyway) and watch what happens to the ND wind as you align the heading with the track..
Then dependent on results consider whether you can really rely on the ND wind display to tell the full story all of the time.
Fly an approach using crab technique..take a look at the ND wind..then transition to the low wing/sideslip technique (FWIW the Triple will do this on autopilot anyway) and watch what happens to the ND wind as you align the heading with the track..
Then dependent on results consider whether you can really rely on the ND wind display to tell the full story all of the time.
Why would the aircraft be more difficult to rotate with a tailwind, assuming rotation was commanded at the "bugged" airspeed?
Had we been given the correct wind, we would have used the opposite runway, or a lower flex or TOGA (more engine power) and different Vspeeds. I would also have been very much more 'go' minded had I been aware of the unexpected tailwind.
Like I say, Airbus blank the wind vector below 100kts, so if it was unreliable or if it was a problem for pilots to see it, surely they would blank it until airborne? Easy enough to do.
In the absence of a sideslip probe, sideslip can be estimated from (left AOA - right AOA).
There have been tests performed and a model or a table has been derived from these tests, the aircraft can then refer to this model or table to compute sideslip, and then wind direction.
There have been tests performed and a model or a table has been derived from these tests, the aircraft can then refer to this model or table to compute sideslip, and then wind direction.
Last edited by Uplinker; 13th Sep 2016 at 17:05.
I don't doubt something happened to you that day but I question the relevance to the wind on ND discussion.
If the tower wind was wrong for your calcs, then you would have 'just' rotated normally further down the runway and had slower acceleration.
If you had a shift mid runway, you'd have picked the trend arrow dropping and reacted then.
If you got windshear (or a localised thermal or dust devil sans dust) at Vr, then I'm not sure what checking anything inside is achieving. There is no time to analyse the cause- it might be wind shift, wrong FMS data etc. It's TOGA and out of there.
If the tower wind was wrong for your calcs, then you would have 'just' rotated normally further down the runway and had slower acceleration.
If you had a shift mid runway, you'd have picked the trend arrow dropping and reacted then.
If you got windshear (or a localised thermal or dust devil sans dust) at Vr, then I'm not sure what checking anything inside is achieving. There is no time to analyse the cause- it might be wind shift, wrong FMS data etc. It's TOGA and out of there.
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kaypam, yes, what we look at is the myriad of input variables and combinations that keep the ac straight, horizontally and vertically. The effect of shielding and directional flow on the sensors, and latency in the system.
The sensors that combine the pitot and AOA are particularly, well, an issue. The potential for the pitot and static combination for shielding, especially in crosswinds at loer speed such as on final.
will pm you.
The sensors that combine the pitot and AOA are particularly, well, an issue. The potential for the pitot and static combination for shielding, especially in crosswinds at loer speed such as on final.
will pm you.
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Anyone flying with honeywell smart probes will be aware of sideslip compensation.
This could certainly be used for a crude wind calculation while on the ground. I wouldn't be looking at it though.
This could certainly be used for a crude wind calculation while on the ground. I wouldn't be looking at it though.
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The existence of an aerodynamic effect that could theoretically allow to measure things, does not necessarily mean that in practice the measurement can be done with reasonable accuracy.
However, Airbus will sometimes compute inaccurate things and use them to know "tendencies" i.e. get some insight into how a parameter is changing.
For instance, an increase of 10kts in the measurement of something can be three times higher than the real value, in magnitude, but will most likely represent a real increase of the real value.
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For those who wanted an airbus document that suggest the wind direction and velocity can be use in windshear detection; can take a look at the Procedure Data Package, Supplementary Information, Windshear, Detection Clues.
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Togue
The issue here is on take off run. All airbus documents including the latest PDP gives detection clues in the air obviously rapidly changing wind speed/direction is definitely a clue in the air. All that the latest PDP says on take off wind shear is:
At take off before V1, only reject take off, if significant speed variations occur below indicated V1, and if the flight crew determines that the runway is sufficiently long to stop the aircraft". Point to note is it only mentions about speed variations and does not ask you to confirm it from ND or any other source. If you want to know about FCOM then below: PRECAUTIONS FOR SUSPECTED WIND SHEAR
FCOM A to B → 18 SEP 12 PRO-SUP-91-20 P 1/2
BEFORE TAKEOFF
Delay takeoff until conditions improve.
Evaluate takeoff conditions :
• Using observations and experience.
• Checking weather conditions.
Select the most favorable runway (considering location of the likely windshear).
Use the weather radar or the predictive windshear system before commencing takeoff to
ensure that the flight path clears any potential problem areas.
Select TOGA thrust.
Monitor closely airspeed and airspeed trend during the takeoff run for early signs of windshear.
Again no mention of wind speed/ground speed. So those who do are doing on their own and say so. So be it. You won't find anything in airbus documents.
The issue here is on take off run. All airbus documents including the latest PDP gives detection clues in the air obviously rapidly changing wind speed/direction is definitely a clue in the air. All that the latest PDP says on take off wind shear is:
At take off before V1, only reject take off, if significant speed variations occur below indicated V1, and if the flight crew determines that the runway is sufficiently long to stop the aircraft". Point to note is it only mentions about speed variations and does not ask you to confirm it from ND or any other source. If you want to know about FCOM then below: PRECAUTIONS FOR SUSPECTED WIND SHEAR
FCOM A to B → 18 SEP 12 PRO-SUP-91-20 P 1/2
BEFORE TAKEOFF
Delay takeoff until conditions improve.
Evaluate takeoff conditions :
• Using observations and experience.
• Checking weather conditions.
Select the most favorable runway (considering location of the likely windshear).
Use the weather radar or the predictive windshear system before commencing takeoff to
ensure that the flight path clears any potential problem areas.
Select TOGA thrust.
Monitor closely airspeed and airspeed trend during the takeoff run for early signs of windshear.
Again no mention of wind speed/ground speed. So those who do are doing on their own and say so. So be it. You won't find anything in airbus documents.
Last edited by vilas; 15th Sep 2016 at 08:03.
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Reproduced Below is what airbus answered in a symposium about accuracy of displayed ND wind
Q. You gave a possible error for the ND wind. Doyou have Data for a possible error in winddirection indicated on the ND? Thanks for theanswer
Session:Runway Lateral Excursions:The Crosswind Landing Technique
A. wind value on ground or during the flow displayed on the ND may have an error of 5 to 8 kts in speed and up to 10 degrees. The reason of this could be IRS drift and/or the aircraft entering in ground effect.
Q. You gave a possible error for the ND wind. Doyou have Data for a possible error in winddirection indicated on the ND? Thanks for theanswer
Session:Runway Lateral Excursions:The Crosswind Landing Technique
A. wind value on ground or during the flow displayed on the ND may have an error of 5 to 8 kts in speed and up to 10 degrees. The reason of this could be IRS drift and/or the aircraft entering in ground effect.
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Speaking as an engineer, you could possibly design a system to estimate sideslip on ground to estimate wind vector. But you would have to ask yourself why anyone would do such a thing. It's of little practical use to the pilot and you are only adding more complication to the system for the relatively brief period between 100 knots and Vr. Also, how would you transition from this estimated on ground reading using sideslip to the rather more accurate ADIRS calculations in flight. It would be just one more thing to test and certify with almost no possible use.
The answer to the question, as has been pointed out several times before, is that it doesn't calculate wind vector on the ground. All it is doing is making the best of the data it is receiving and solving the velocity triangle as best it can. It may look like a sensible wind direction reading, but appearances can be deceptive.
The answer to the question, as has been pointed out several times before, is that it doesn't calculate wind vector on the ground. All it is doing is making the best of the data it is receiving and solving the velocity triangle as best it can. It may look like a sensible wind direction reading, but appearances can be deceptive.
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Back to topic:
Hasn't anyone of you thought about the idea that maybe the computers do just the same as they do in-flight?
Just look up any crosswind takeoff on youtube. You will notice that maintaing centerline will require a considerable amount of crab, hence sideslip. My bet is, that it just simply doenst care about if its still on ground or airborne already. It just does the usual dumb computation.
Hasn't anyone of you thought about the idea that maybe the computers do just the same as they do in-flight?
Just look up any crosswind takeoff on youtube. You will notice that maintaing centerline will require a considerable amount of crab, hence sideslip. My bet is, that it just simply doenst care about if its still on ground or airborne already. It just does the usual dumb computation.
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zahnpastaesser
You maintain crab on take off run? You aircraft wheels swivel side ways?
You will notice that maintaing centerline will require a considerable amount of crab
You maintain crab on take off run? You aircraft wheels swivel side ways?
But you do need a slip angle to generate enough reaction force from the tyres otherwise you’ll drift off the runway. So yes, there’s a bit of crabbing on the ground...
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One would certainly not notice while doing the takeoff-run. At least I haven't so far. But its true. As I mentioned in my post: look it up, you will be impressed about how much angle any a/c will habe to maintain to remain on centerline. I might link a picture later when I have WIFI.
