wind calc during T/O roll- how?

[Mats_J 2]

I agree John, but we were discussing a highly theoretical scenario.Cheers,

[Guest Stefan Bergert]

Hi all, thanks for the replies.So as conclusion I would say that it is NOT possible to have the wind info (deg., spd, arrow) on ground (takeof roll, ldg-roll). This would also mean there is a small bug in the 747 (don't know about the NG) - but it doesn't really matter that much. "True windspeed and direction readouts are displayed on the ND, but the arrow is magnetically oriented."Thanks for that, Q. What about the FMC Prog2 readout? And where did you find that?THanks to everybody,Stefan

[Check_Airman 3]

>Paul:"I believe the way the system works is that until>liftoff, the arrow only shows headwind/tailwind component, as>there is no lateral drift">>Hmm, what's the sense in that? This would be a really bad>indication for the pilot making him think the wind blows from>somewhere not true - I guess it's better then to leave it>disabled until after lift-off.The information is very useful, particularly at limiting runway lengths. I was talking to an A343 pilot the other day, and he HATES when ATC tells him winds are CALM. He says even if it's 3kts tailwind, he needs to know, especially when operating out of his (usual) 9000ft RW at ISA+15. It's not a bad indication at all. If the pilot knows how to read his instruments properly, he would never assume that there is a direct head or tailwind. It's better than having no information at all.

[michal 44]

>>Paul:"I believe the way the system works is that until>and he HATES when ATC tells him winds are CALM. Because in fact in aviation 3 kts or less qualifies as CALM. I would say that if he makes such close-call takeoffs that 3 kts wind figures in his technique or decision making - then there is something wrong with his airline that sets his/her procedures. Michael J.

[Check_Airman 3]

I don't think there's anything wrong, but if I flew an A343 (or any plane for that matter, but especially the A343) at a runway limited weight, I'd want to know the exact wind too.

[Qavion 19]

"What about the FMC Prog2 readout? And where did you find that?"It's mentioned in the Big Boeing User's Guide..."PROGRESS...PIP/747/777/PegasusWIND bearing is degrees true, and relative to aeroplane's true heading.FMC winds are resolved along aircraft's true heading as opposed to track. Caution is advised when referenced during approach if in a slip.The error increases with increasing drift angle."Cheers.Q>

[michal 44]

>but the arrow is magnetically oriented.The arrow is "oriented" it makes no sense to say it is true or magnetically oriented. It point to the direction wind is blowing.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest Stefan Bergert]

Hi everybody,thanks for replying, especially thx to Q. ...forgot that I have the 2 FMC guides as well in my bookshelf.BTW: Just returned from 4 jumpseat-flights in an A319: at 100kts (so still on GND) it gave a wind direction and speed-readout which was quite accurate.Stefan

[CF104 0]

>I don't think there's anything wrong, but if I flew an A343>(or any plane for that matter, but especially the A343) at a>runway limited weight, I'd want to know the exact wind too.Usually winds below 5 knots are light and variable. A 270/5 wind at one end of a 12,000' runway can have several direction shifts over its entire length. It's best to look at the windsock and have your calculations based on the ATIS or tower info. A 3 knot tailwind has more effect on landing than it does on takeoff. Count on at least an extra 100' stopping distance for every 1 knot of tailwind. I don't know how you can determine a small tailwind component at low airspeeds while on the ground. Especially stopped. Even a small headwind of say 3 knots won't show up via the air data computers as their accuracy is not good below 50 knots.As for an A343. At runway limited weight it still gives you a single engine out V1 cut performance minimum altitude of 35' at the departure end of the runway. If 3 knots tailwind affected the A343 that much, it wouldn't be certified.I personally feel the manufacturers and regulating authorities know more about this than we could surmise here so I don't understand the big question regarding this. All airlines calculate their takeoff performance based on the most recent ATIS or tower info so I don't know why some here feel that the aircraft should know better.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

[Check_Airman 3]

I wasn't suggesting that pilots knew better than the manufacturers at all (those guys who do that scare me). The question came up of why the pilot would only want the "partial" wind info that is available before rotation, so I was just explaining that half a loaf is better than no bread.

[michal 44]

>Usually winds below 5 knots are light and variable.This is the key. This "nut" pilot shoud have walked onto the runway, pull out his handkerchief and start 'measuring' his 3 kts wind. And since such wind is usually very much time-dependent he should actually have someone stand at the end of the runway giving him hand signals when he is about to take-off. If such a person is actually flying for living - I am afraid I would prefer to not be his passenger.Michael J.

[Qavion 19]

"It points to the direction wind is blowing."But it has to be referenced to something... I guess it depends on your ND's heading reference...Cheers.Q>

[michal 44]

>"It points to the direction wind is blowing.">>But it has to be referenced to something... >>I guess it depends on your ND's heading reference...Nope. Only when you care about actual numbers (for example is it 217 or 200 deg) when "magnetic" or "true" plays the role. I can stand in the middle of a desert and point in the direction of a palm tree and not worry about magnetic or true ;) When simply drawing a vector you don't have to deal with magnetic/true stuff.Michael J.

[Qavion 19]

"When simply drawing a vector you don't have to deal with magnetic/true stuff."I see your point, Michael... but I believe you can use the arrow to determine the wind's magnetic direction (if your display is currently showing magnetic), by comparing it with the compass rose.Having said that, I'm just wondering if the arrow would be painted on the screen differently if could change your ND from a PMDG style "TRK UP" display to a (say, British Airways) "HDG UP" display.e.g. TCAS is relative bearing (to the aircraft's heading)... This can cause confusion on a TRK UP display because the TCAS targets are not where you think they should be (when you look out the window) during crosswinds.Cheers.Q>

[michal 44]

>Having said that, I'm just wondering if the arrow would be>painted on the screen differently if could change your ND from>a PMDG style "TRK UP" display to a (say, British Airways) "HDG>UP" display.>Ian, Yes, in this particular case the arrow will change position because you are changing your "UP" reference, but it won't change position if you switch between true/magnetic because this only changes the numbers on the compass rose scale.Michael J.