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Entering Wind Direction/Speed In FMS Question

Sun May 21, 2006 8:21 pm

I am slightly confused, I didn't believe you had manually to enter wind speed and direction in the FMS system while cruising - I thought that the FMS would automatically detect the wind speed and direction as you fly and deal with it, but according to something I have read a pilot is required to enter the wind speed and direction.

What actually happens?

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RE: Entering Wind Direction/Speed In FMS Question

Sun May 21, 2006 9:47 pm

The FMC uses the sensed winds for some computations. However, the reason the pilot enters the winds for the route is that without a predicted wind entry the FMC assumes forecast wind as ZERO for it's time and fuel calculations. Often we see this when flying a long international segment. We most likely will download our winds from our company computer via the acars directly to the FMC once we are airborne. On a flight from Europe to the states with strong winds the initial FMC ETA can be significantly different than the flight plan due to no winds being loaded. Once the winds are in the computer and it can then use them in determining the predicted winds the FMC ETA will be more in line with the Flight Plan.
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RE: Entering Wind Direction/Speed In FMS Question

Sun May 21, 2006 9:50 pm

Hmmm, must type more quickly  Yeah sure

[Edited 2006-05-21 14:57:21]
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RE: Entering Wind Direction/Speed In FMS Question

Sun May 21, 2006 10:42 pm

Here is some information I was able to get from my Boeing FMC guide. This is applicable to the FMC's on the 747-400,777 and 767 at my company.

" The FMC uses a mixing algorithim to determine the predicted wind at points infront of the aircraft. The predicted wind is a mix of the forecast wind and the measured wind.

At the aircraft's actual position, the computer uses 100% of the actual winds.

At 200nm in front of the aircraft, the FMC is using 50% of the actual wind and 50% of the forecast wind.

At distances over 200nm, the FMC will use successively less measured wind until the winds used is near 100% forecast.

In the climb and descent phases, the wind is mixed the same way except that the equal weight distance is 5,000' rather than 200nm and the distance from the aircraft axis is in feet rather than nautical miles."
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RE: Entering Wind Direction/Speed In FMS Question

Mon May 22, 2006 1:40 am

If you look at the FMS of the Airbus aircraft you will see this anomaly.

On a long leg, look at time of your next waypoint on the FMS, also, look at the next waypoint time on your ND. They may not be the same. The difference?

The FMS uses the winds you have manually input and is using them to estimate your next waypoint time. The ND next waypoint time is using actual winds, and is assuming (perhaps incorrectly) that winds will not change to the next waypoint. Therefore the difference.

It is something one has to consider when sending position reports .... which of the two is the more accurate.
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