FMC and ND Winds Differ

[bradrcfii 3]

I noticed the reported wind direct differs between the FMC and ND. Spped is always the same. Is one magnetic and the other true?

[Flexman 30]

I've tried to find information for you but I couldn't find any.One thought though:Maybe the wind shown on the ND is calculated by the whole FMS, taking into account the information coming from INS. And maybe the wind shown on the CDU is only considering GPS position as you can see in PROG 2/4.

[victorlima01 2]

FMC wind on the prog page is true. The ND one is magnetic. At least I remember reading this somewhere. I could be wrong though. But if Omar (the real deal) couldn´t find it I´m betting I won´t be able to confirm that either. By the way what does frozen atpl mean Omar?

[Flexman 30]

You have a frozen ATPL when you have your licenses expiring in the fridge cause you've had enough of paying for flying instead of flying for getting paid :) Also, it's when you have a CPL-IR but have also completed the theoretical module for the next license (ATPL). That theoretical part, which are mainly passed exames, get frozen until you meet the flight time requirements for the ATPL flight check (usually done in airline). The ATPL is the license required under JAR for operating as captain, plainly speaking. I think in South America it's similar to what some countries call "first class commercial pilot license".

[oneill875 1]

Victorlima01 is correct, FCOM Vol2 says that the FMC wind direction is True, but the ND wind display is :Indicates wind speed and direction, with respect to display orientation and heading/track reference. Displayed if wind magnitude is greater than 6 knots and blanked if wind magnitude becomes less than 4 knots. Blank until TAS is greater than 101 knots. PLN mode displays speed/direction only.So I guess that means Magnetic.Dan.

[Flexman 30]

Victorlima01 is correct, FCOM Vol2 says that the FMC wind direction is True, but the ND wind display is : Indicates wind speed and direction, with respect to display orientation and heading/track reference. Displayed if wind magnitude is greater than 6 knots and blanked if wind magnitude becomes less than 4 knots. Blank until TAS is greater than 101 knots. PLN mode displays speed/direction only. So I guess that means Magnetic. Dan.

Makes sense. I was thinking that the one on the CDU was, as for the leg time prediction, a mix between forecast wind downlinked or manally put in, and actual INS wind. The farther from the airplane, the more forcast winds were taken into account. And when closer to the airplane, then INS wind was the reference. But the mag and true explanations make far more sense. Now all we have to do is convert to magnetic, and see if the numbers add up.

[oneill875 1]

Yeah, the IRS uses true heading and true wind direction so should just be a case of comparing the FMC or IRS to the ND, they should not be the same if the MD is magnetic.I don't think that the FSX Mag Variation is up to date, Im sure there is a sticky about this?Maybe just flick the ND to True and see if the wind direction changes at all?Dan.