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Brent

Different 'wind aloft' direction on 747 instruments

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FolksI've often wondered why I consistently see different wind direction in the inboard Capt CRT versus what is shown on the CDU progress page 2. Just curious why this would be the case! Example below shows 287/81kts in the CRT and 302/81kts in the CDU....more or less the same but why the difference?llbew.jpgCheersBrent

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Is it always like that? The FMC acts like a real FMC, and so it only updates the received winds every few seconds or so, meaning it might be a few degrees off if your wind is shifting rapidly. If after it updates again it matches the current reading of the ND then you are just the victim of unlucky timing.If it doesn't you have a problem I don't know how to solve so hopefully someone else can jump on this.

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EricI tried paying more attention to the observation I made and found that on a 1700 mile trip today at FL370 I consistently saw a 18 deg difference (exactly the same speed) between the two displays. As I travelled west-east the difference reduced to the point where there was no difference (this was very gradual and took the 3 hours of travel time). Odd, but I wondered if there was an explanation. The manual indicates that the FMC calculates the wind and direction and wondered if there was a reason that I might be seeing this and if anyone else has noticed any differences between the two. It doesn't minimize the enjoyment I get flying the Queen but I was curious if there was an explanation.Thanks for the interest!

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The answer is not clearly explained in the manual that came with the product but from referencing a real B744 FCOM....its pretty straightforward....The wind vector info on the ND indicates wind bearing, speed, and direction with respect to display orientation and heading/track reference - so I suspect these are referenced to MAGNETIC North.It would appear the wind info displayed on PROG P2 displays current wind direction and speed referenced to TRUE North.

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The answer is not clearly explained in the manual that came with the product but from referencing a real B744 FCOM....its pretty straightforward....The wind vector info on the ND indicates wind bearing, speed, and direction with respect to display orientation and heading/track reference - so I suspect these are referenced to MAGNETIC North.It would appear the wind info displayed on PROG P2 displays current wind direction and speed referenced to TRUE North.
Thanks Steve! That makes sense....now I can stop chewing my lip thinking about it.Whew.gif

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