wind calc during T/O roll- how?

[Guest Stefan Bergert]

Hi everybody! One small question: How can I get a wind-direction and -speed readout during the takeoff-roll? AS far as I know the FMC calculates the direction and speed from the Groundspeed(vector) (by IRS) and the TAS from the Air Data Computer (like the wind-triangle one can do with a mechanic Flight Computer). But during the takeoff-roll you don't have a GS-vector different to the TAS, you are not drifted off by the wind. Wind-data and windarrow are displayed on ND at 96kts in PMDG.BTW: Is the NE wind-direction true or mag? Is the wind in the FMC (POS pge2) true or mag?Thanks for any ideas, Stefan

[Mats_J 2]

Stefan,That is a good question. I've asked it myself since the NG was in flight testing. Would love to hear if it is possible to get wind read outs while still on the ground.Cheers,

[CF104 0]

On the real aircraft you won't get wind speed/direction on the ND until 101 knots air speed. Also the wind is displayed if the magnitude is greater than 6 knots and blanked if the wind drops below 4 knots.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com

[Mats_J 2]

John,>On the real aircraft you won't get wind speed/direction on>the ND until 101 knots air speed. Also the wind is displayed>if the magnitude is greater than 6 knots and blanked if the>wind drops below 4 knots.OK. That's all fine and dandy. But do we get wind indications while still on the ground and if so how does the a/c compute it?Cheers,

[Check_Airman 3]

The plane has no way of knowing wind direction until it is moving. From the 100kt limitation, it has to be moving at a considerable speed before this is possible. Until then, rely on ATIS.FYI. I believe the way the system works is that until liftoff, the arrow only shows headwind/tailwind component, as there is no lateral drift. Once airborne, the arrow smoothly swings around to show the actual wind direction as the IRS can now calculate drift angle.I welcome corrections from those in the know if I'm incorrectPaul

[Mats_J 2]

Paul,Makes sense. Lets see if we can get someone to confirm this as well? ;-)Cheers,

[Guest tmetzinger]

you don't. You get that from the ATIS or looking at the windsock.

[Mats_J 2]

Thanks Tim. That's what I've been suspecting all along.Cheers,

[michal 44]

>But during the takeoff-roll you don't have a GS-vector>different to the TAS, you are not drifted off by the wind. You answered it yourself - it is theoretically impossible to compute wind before wheels lift off the ground.Michael J.

[Guest Stefan Bergert]

still we have it *grin* ...just tried it with the 744 (PAX)...don*t know about the NG, think it's the same. Same with other add-ons (like PSS Airbus'BTW: Is the NE wind-direction true or mag? Is the wind in the FMC (POS pge2) true or mag?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.Michael:"it is theoretically impossible to compute wind before wheels lift off the ground"At least you can't get a wind direction / vector. I guess one could take the total press coming into the pitot-tube compared with the sensed acceleration data from the IRS to get a windcomponent?The question actually came up when my avionics-teacher told about a training-FO (Airbus) who had this question. Anybody with access to a full-motion sim? ;-)Really thanks a lot so far for the replies,Stefan

[michal 44]

>I guess one>could take the total press coming into the pitot-tube compared>with the sensed acceleration data from the IRS to get a>windcomponent?No. Just think about it - the error would be so huge that would make data useless. Acceleration data would have to be compared against the identical situation (rotational friction, power setting, runway condition, weight, etc.) without any wind. What you are asking is simply impossible.Michael J.

[Mats_J 2]

>>I guess one>>could take the total press coming into the pitot-tube>compared>>with the sensed acceleration data from the IRS to get a>>windcomponent?>>No. Just think about it - the error would be so huge that>would make data useless. Acceleration data would have to be>compared against the identical situation (rotational>friction, power setting, runway condition, weight, etc.)>without any wind. What you are asking is simply impossible.Michael, I don't get it. To me the above scenario is very much theoretically possible. After all the pitot sensors should be quite accurate at a 100 knots. And the accelerometers are very sensitive. Never mind it is not how it works in real life.The pitot systems has the ground speed plus any wind component covered and the accelerometers measures the ground speed only. It's just a subtraction of one from the other and you'll have your wind influence.Or am I totally out on the wrong limb here?Cheers,

[michal 44]

Mats,In fact you are absolutely correct. Pardon me, I failed to understand the original point, my mistake. Yes, you could at least get an idea what is the component of the wind that is aligned with the runway. I apologize for misunderstanding.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Qavion 19]

"I guess one could take the total press coming into the pitot-tube compared with the sensed acceleration data from the IRS to get a windcomponent?"Why do you need the IRS's? The pitot-static system gives you your forward wind component (airspeed display), which is what is needed for assessing whether the wind under your wings is fast enough for flight, not TAS. The pitot tubes are narrow tubes rigidly affixed, parallel to the fuselage, not "windsocked".As previously stated, with the tyres on the ground, the IRS's are not going to sense any lateral drift, so there will be no wind vector. Heading = track.True windspeed and direction readouts are displayed on the ND, but the arrow is magnetically oriented.Cheers.Q>

[CF104 0]

It's theoretically possible but in the takeoff phase the info is rather useless while on the ground. The wind arrow desn't even show up until the TAS is higher than 101 knots.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpgwww.SSTSIM.com