B737NG FMS - Cruise Wind entry?

[ATRflyer]

Hi all,

Just have a few questions about the cruise wind entry in the Performance Initialization page of the 737NG FMS.

If you put the forecast wind for your cruising level in the CRZ WIND line, would I be right in saying that the FMS will use the actual wind sensed by the ADIRS for calculating the ETA for the active waypoint; and all ETAs for waypoints along the other legs of the route will be calculated using this entered wind data?

The boeing manual says that if no entry is made, the FMC assumes zero wind for preflight predictions. This sounds very straight forward but what happens once your in flight? Would it use current wind for the active waypoint and zero wind for all other waypoints along the route?

Thanks in advance,

Regards,
Atrflyer

[jonny dangerous]

ATRflyer: The NG FMS will use the entered route winds on the RTE DATA page, or it will default to the cruise wind entry on the PERF INIT page. With neither of those supplied it will use zero winds for the predictions.

Once in flight, the the FMS will periodically update ETA's using the current groundspeed data. I'm not sure what the tolerance is, but it will update the ETA on its own, or whenever a selection is made that requires an EXECUTE, such as a "direct to". It will also update the predictions as it passes a waypoint.

If you play around with the RTA (Req'd Time of Arrival) page, without executing, you'll be able to see the ETA evolve with the winds (3rd or 4th line on the right side). To do this, enter the active waypoint in the RTA field, and it will bring up the current ETAs. With winds that are changing rapidly, and/or varying from the predicted winds, you can watch the ETA in line 3 or 4 change fairly quickly. If you're bored...

Hope that helps. But it sounds like you've got a pretty good handle on it.


Jonny

[Old Smokey]

ATRflyer,
would I be right in saying that the FMS will use the actual wind sensed by the ADIRS for calculating the ETA for the active waypoint; and all ETAs for waypoints along the other legs of the route will be calculated using this entered wind data?
The first part is half correct, the latter half pertaining to future legs is correct.

For the CURRENT leg, the FMC/FMS uses the mean of the present wind and the entered wind for the next waypoint. If the next waypoint is more than 30 minutes away, this averaging applies only for the next 30 minutes, whereafter the entered forecast winds prevail.
what happens once your in flight? Would it use current wind for the active waypoint and zero wind for all other waypoints along the route?
Once any wind is entered, all subsequent legs assume the last entered wind. (Don't you just love that "INSUFFICIENT FUEL" message when youve put in the 100 knot headwind for the first part of the flight, but haven't yet entered the lesser winds for the later part of the flight out of the jetstream?)

It's a bit more complex than all that, it's not linear averaging for one thing, but all of the above is a good practical approach.

Generally, this explanation is not found in the Boeing handbooks, you'll have to go to the FMC/FMS manufacturer's handbook for a complete explanation of this.

Regards,

Old Smokey

[ifleeplanes]

The main purpose of the CRUISE WIND entry on the PERF INIT page is to optimise the ECON climb calculations and adjust the climb speeds. If you havn't put an entry in on the ground the FMC assumes climb winds to be zero on the ground and actual wind when airborne. If it hasnt been entered on the ground the ECON climb speed will not consider the wind component.

The cruise wind will also propogate to the cruise waypoints if entered after the route entry, it does not displace the forcast winds. It will blank when you reach the cruise.

[ATRflyer]

Thanks a million to all of you for your help.
Much appreciated.

Happy New Year to you all by the way, not long to go now!!!:D

Regards,
Atrflyer