737 descend forecast

[Olendirk]

Guys,

having heavy tailwinds during descend, i enter them and how does the path apply for those winds?

And how do you compensate the deceleration in the V/S the fmc is offering for the descends?

Many comments appriciated


Talking about a classic 737 by the way!

OD

[Norwegian]

The best way to think about this is what You have to do if you have Strong tailwind aloft before Your descend. You have to start descend earlier.
And thus will the fmc tell you. If You put in strong tailwinds during the descend the TOD will move furter away from the 0-wind TOD meaning closer to your A/C.
It will calculate a new "Path" with deceleration segments according to the restrictions in your flight plan route/etc. If Your path is to steep it will tell you to increase drag or that your next altitude restriction is to steep ie "unable next altitude".

Either delete next alt restriction or deploy your speedbrakes or even increase your speed: Levelchange.

The Fmc is doing this even if it is a Classic or a NG

[RYR-738-JOCKEY]

If you put your active way point on top again and execute, the box will recalculate your descent path interpolating between actual wind at your current level and a) zero wind at E/D, or b) any winds entered in descent forecast page. So what I normally do is to have a look at the wind charts, and take note of any large changes in W/V and more importantly establish early if you can expect tailwind during initial approach. Put this into forecast page.
Then, the V/S information is only useful for a geometrical path without any decelleration. For an approach it works perfect. It basically converts your vertical bearing to the next descent "target" waypoint. You will often find that using this V/S will gradually bring you higher and higher, but you will end up at the waypoint at the correct altitude/FL but with excess speed.

[Denti]

Descent winds should be entered latest enroute some time before you start your actual descent. On our OFPs we have descend wind forecasts for FL310, FL200 and FL100, just right to fill the three forecast lines in the Descent Forecast Page.

On route sectors with less than 100NM remaining to the next waypoint putting him on top and execute again will indeed recalculate your VNAV profile using spot wind and interpolations between either zero zero wind at EoD or your entered descent winds.

Anyway, it is much easier to correct for a wrong descent wind on the classic than it is on the NG, the difference is quite big, even more so between a small classic (500) and a big NG (800/900).