Wind velocity indication on ground??When?
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Wind velocity indication on ground??When?
Another little quirk bothering me please.Do u get wind velocity(speed and direction) on the PFD/ND display of a B-738 on ground.....say while taxying or say on take off and/or landing rolls?? Anw why not.Also WHEN would you get Wind Velocity on the PFD/ND??Say u are taxying at 30 kts into a 30 kt headwind.Or into a 30 kt tailwind.Would u get a Wind Velocity indication??All replies are MOST welcome pls.Thx.
Last edited by boeingdream787; 13th Feb 2007 at 20:49.
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The air data computer receives data about a number of things amongst which the aircraft heading and airspeed. It also receives information about the aircraft geaographical position and thus can calculate the ground speed and track. The difference between these two pieces of information can only give you the wind speed and direction.
As the track made good and gound speed when in contact with the ground the two will be the same, ie. the wind has no effect on the heading or ground speed whilst taxiing, the wind velocity wil no be shown or be spurious.
I have seen some strange displays of wind velocity whilst on the ground but this is only due to the computer's calculation and the occasional jumps in GPS position.
As the track made good and gound speed when in contact with the ground the two will be the same, ie. the wind has no effect on the heading or ground speed whilst taxiing, the wind velocity wil no be shown or be spurious.
I have seen some strange displays of wind velocity whilst on the ground but this is only due to the computer's calculation and the occasional jumps in GPS position.
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Hmmm....maybe not!
Miserlou,What u are essentially trying to say is that you would'nt EVER get ANY wind indications on ground.I beg to disagree to that.I have had wind velocities on t/o runs.Say u are taking off into a 100 kt headwind(hypothetically),would i get a wind speed or not.And at what IAS would that be??And say my VR is 150kts,i obviously would lift off at 50 kts G/S right?How do u explain that. And say i'm landing into the same headwind...?? However i fully agree with your explanation of the mechanics of the data input.ANY more input is welcome.
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If either of you are Professional Pilots.... get back in the books 
Whilst on the ground, you can get the HW/TW value, but no cross track element (since Trk = Hdg). It will require an ASI reading (v INS GS), so will only "click" in with some forward airspeed, hence why it appears on TO, but only as a 12/6 o'clock display...
As you get airborne, and drift comes in, then you get the full W/V display (INS GS/Trk vector v Hdg/TAS vector)...
HTH
NoD
Whilst on the ground, you can get the HW/TW value, but no cross track element (since Trk = Hdg). It will require an ASI reading (v INS GS), so will only "click" in with some forward airspeed, hence why it appears on TO, but only as a 12/6 o'clock display...
As you get airborne, and drift comes in, then you get the full W/V display (INS GS/Trk vector v Hdg/TAS vector)...
HTH
NoD
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Get off your high horse, Nigel.
How accurate is your ASI below 60kt? How often does your INS update? The computer is certainly not quick enough to keep up with turns or changes of airspeed in the air on a second to second basis and always needs a few seconds of constant state to come with the correct answer so the data you get on the ground will never be reliable.
Anyway, who's looking? What use is it whilst you're taxiing or bowling down the runway?
How accurate is your ASI below 60kt? How often does your INS update? The computer is certainly not quick enough to keep up with turns or changes of airspeed in the air on a second to second basis and always needs a few seconds of constant state to come with the correct answer so the data you get on the ground will never be reliable.
Anyway, who's looking? What use is it whilst you're taxiing or bowling down the runway?
Anyway, who's looking? What use is it whilst you're taxiing or bowling down the runway?
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Quote. "but it's really useful to know if the wind component on the takeoff roll is significantly different to that you've used for your performance calculation."
I've never seen any SOP where this is a stated procedure nor is it likely to be reliable because of the update rate and the changing variables.
I've seen wind speeds of 100 kt and varying wildly whilst taxiing and seen the ground speed calculated to be anything between 5 and 20 kt while we were parked at the gate!
With the conservative values used for performance calculations and safety margins, I think one would notice any relevant change in the wind conditions especially when ATC say "Wind is xxx, you are cleared for take-off."
I've never seen any SOP where this is a stated procedure nor is it likely to be reliable because of the update rate and the changing variables.
I've seen wind speeds of 100 kt and varying wildly whilst taxiing and seen the ground speed calculated to be anything between 5 and 20 kt while we were parked at the gate!
With the conservative values used for performance calculations and safety margins, I think one would notice any relevant change in the wind conditions especially when ATC say "Wind is xxx, you are cleared for take-off."
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Therefore if Nigel on a Horse were,oops Nigel on Draft were correct that would mean that IF say i were taking off into a 100kt HWC and were static on the runway,i should get the full wind velocity.Cause obviously,being factored ONLY against the IAS,which is reading 100kias,it would give me the correct component.Now say this wind were say coming from a "one thirty" position,ie 45 degrees right cross,would i still get a correct HW component....say cos 45 x 100 (whatever the HWC would be.....i forget the calculations),since i still have a reasonably legit IAS on the tape.Thx...
P.S.....Thx Miserlou for your input.
P.S.....Thx Miserlou for your input.
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One of the reasons that no wind is shown on the ground may be is that NAV is not engaged due wheels on the ground? No NAV = windspeed computations. The ADC is still doing it's sums but the nav computer won't be listening.
I've never seen any SOP where this is a stated procedure nor is it likely to be reliable because of the update rate and the changing variables.
I've seen wind speeds of 100 kt and varying wildly whilst taxiing and seen the ground speed calculated to be anything between 5 and 20 kt while we were parked at the gate!
With the conservative values used for performance calculations and safety margins, I think one would notice any relevant change in the wind conditions especially when ATC say "Wind is xxx, you are cleared for take-off."
On all the modern jets I've been on the wind readout seemed pretty accurate and instantaneous. If I was on the takeoff roll in some dark corner of the world and the displayed wind component was much more adverse than I thought it should be, I would check the IAS against the GS and consider a reject at low speed or 'firewalling' if at high speed. You're not going to get a windshear warning from a steady (or slowly increasing) tailwind, even though it might put you in the trees at the end of the runway...
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Seems to be a tendency for people nowadays to rely on the aircraft for information which is freely available elsewhere.
Having done your take-off calculations you know roughly how fast you will be going at various points down the runway and you know how your own mount usually accelerates. If it's not accelerating as quickly as it should be you'll notice and check the ENGINE parameters surely.
And you know the distance you need so if you're not close to V1 at a certain point on the runway then you know there's something wrong.
I don't deny that the wind velocity may be a useful piece of information but how many pieces of useful information do you need or have time to process.
Having done your take-off calculations you know roughly how fast you will be going at various points down the runway and you know how your own mount usually accelerates. If it's not accelerating as quickly as it should be you'll notice and check the ENGINE parameters surely.
And you know the distance you need so if you're not close to V1 at a certain point on the runway then you know there's something wrong.
I don't deny that the wind velocity may be a useful piece of information but how many pieces of useful information do you need or have time to process.
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Miserlou....
Not very... 
Hence why I said
I don't know your company SOPs, but mine tend to suggest not turning during TO 
Disagree... certainly on the Airbus and 757/767...
I've never mentioned taxiing 
I have looked at it in the latter stages of Takeoff for (potentially excessive) TW... Not sure how I would "action it", but it sure is useful to report back to the tower the actual tailwind on takeoff when they are giving incorrect figures (deliberately or otherwise ).
I reiterate my point about getting into the books My ATPL notes, from 1994, clearly state that to get a valid W/V solution, all you need is an INS and a TAS input. The latter, as I had already suggested, and you confirmed, is unlikely to be valid below ~@50K.
The topic did start off with a request
and
. I suggest the questioner wanted some technical answers rather than you just slagging everybody off who tries to help 
How accurate is your ASI below 60kt?
Hence why I said
It will require an ASI reading (v INS GS), so will only "click" in with some forward airspeed, hence why it appears on TO,
The computer is certainly not quick enough to keep up with turns
or changes of airspeed in the air on a second to second basis and always needs a few seconds of constant state to come with the correct answer so the data you get on the ground will never be reliable
What use is it whilst you're taxiing or bowling down the runway?
I have looked at it in the latter stages of Takeoff for (potentially excessive) TW... Not sure how I would "action it", but it sure is useful to report back to the tower the actual tailwind on takeoff when they are giving incorrect figures (deliberately or otherwise ).I reiterate my point about getting into the books
My ATPL notes, from 1994, clearly state that to get a valid W/V solution, all you need is an INS and a TAS input. The latter, as I had already suggested, and you confirmed, is unlikely to be valid below ~@50K.The topic did start off with a request
All replies are MOST welcome pls
ANY more input is welcome
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"If either of you are Professional Pilots.... get back in the books."
Nigel,
I don't think I was slagging any-one off and suggesting you get off your high horse is entirely reasonable when you wade into a technical request with the above statement.
Perhaps, if you stop to think at all, you may realize that the (not very) technical explanations I have given, together with examples of other instances of inaccuracy ('not quick enough to keep up with turns') serve to help solve the 'quirk' the poster queried.
And my reference to asking of the relevance of the WV vector during TO is supported by mcdhu's note on the A320 ('W/v comes onto the ND at 100KIAS')
Having appointed yourself the cleverest amongst us, please give the poster a full explanation of the wind velocity indication on the B-738. Otherwize, your opinion is only as relevant as everyone else's.
Nigel,
I don't think I was slagging any-one off and suggesting you get off your high horse is entirely reasonable when you wade into a technical request with the above statement.
Perhaps, if you stop to think at all, you may realize that the (not very) technical explanations I have given, together with examples of other instances of inaccuracy ('not quick enough to keep up with turns') serve to help solve the 'quirk' the poster queried.
And my reference to asking of the relevance of the WV vector during TO is supported by mcdhu's note on the A320 ('W/v comes onto the ND at 100KIAS')
Having appointed yourself the cleverest amongst us, please give the poster a full explanation of the wind velocity indication on the B-738. Otherwize, your opinion is only as relevant as everyone else's.
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boeingdream787, quick search through my ops manual says the wind direction speed arrow displayed if wind magnitude is greater than 6 knots and blanked if wind becomes less than 4 knots.Blank until TAS is greater than 101 knots.
So you wont get any indication of wind other than a visual look at a windsock, smoke, trees blowing over unless it is reported by the tower. As for conservatism well that is all about airmanship and how long is a piece of string? If you have any doubt about the accuracy of wind reports then make your mind up on during a walkaround and use nil wind or a tail wind factor.
Our company charts are already factored for 50% of any headwind and and 150% of any tailwind.
Knowing your aircraft is important.I have been off for 3 months but I think that on classic and 800 737's you should achieve full thrust and 80 knots just after the 1500' markers have been passed. I will have to time it myself but V1 is normally reached at 35 secs I think so don't quote me on that one.
We are in the habit of starting a clock for the takeoff to know how long the engine has been at t/o thrust anyway.
So you wont get any indication of wind other than a visual look at a windsock, smoke, trees blowing over unless it is reported by the tower. As for conservatism well that is all about airmanship and how long is a piece of string? If you have any doubt about the accuracy of wind reports then make your mind up on during a walkaround and use nil wind or a tail wind factor.
Our company charts are already factored for 50% of any headwind and and 150% of any tailwind.
Knowing your aircraft is important.I have been off for 3 months but I think that on classic and 800 737's you should achieve full thrust and 80 knots just after the 1500' markers have been passed. I will have to time it myself but V1 is normally reached at 35 secs I think so don't quote me on that one.
We are in the habit of starting a clock for the takeoff to know how long the engine has been at t/o thrust anyway.
I've seen wind speeds of 100 kt and varying wildly whilst taxiing and seen the ground speed calculated to be anything between 5 and 20 kt while we were parked at the gate!
As TAS is needed to account for windspeed, and a few knots of wind (especially if not straight down the pitot tube) won't give an indication or meaningful data to account for TAS, and as the aircraft can't actually sense the direction or value of the wind when taxiing...how do you suppose the aircraft data systems can come up with a realistic wind value when on the ground? Short answer...they can't.
The values you're seeing are in error.
But if you really believe you've been taxiiing in 100 knot winds, time to step inside and wait it out. Seriously.
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It was a KT1000 or some such.
As I thought I'd made clear, this was a spurious reading due the update rate of the GPS position as this is the only information being received.
As I thought I'd made clear, this was a spurious reading due the update rate of the GPS position as this is the only information being received.