I learned that as part of the descent planning you should recalculate your gross weight before you start your descent by comparing your fuel quantity and the calculated amount of fuel left at your destination seen on the PROG page and then substract this from your gross weight found on the INIT REF page.

 

This to get a more accurate descent path and landing data figures but is there a rule exactly at what point you would normally do this recalculation in a B738?

If you play with it you'll find the extra effort is not worth the time and distraction.  Anytime less than 100 nm before TOD I will set up the FMS landing V-speed page using the GW that it provides. If you compare those V-speeds with what you get after going through the adjust for fuel used in descent and landing you will probably not see a 1% change. Try it.

Fact is I already did play with it and found exactly what you say, that the difference looking at the V-speeds is minimal.

 

What I did notice though is that the descent profile seems to be more effective/correct when I do this recalculation about 150 nm from my calculated ToD because the a/c feels much less slippery during the descent and I don't need to use the speed brakes all the time.

That could be a placebo effect LOL. I'm not sure how landing Vref selection affects descent profile, the 738 is always slippery.

The weight value on the INIT/REF page has no effect on the descent path, only the approach vref speeds.  I'll usually wait until I'm within 200 NM of the destination to select the approach speeds.  On the way down through 10,000 I'll look at it again and if the speeds have changed by a couple knots just reselect it.

I dont think it does Dan as the FMC keeps track of the weight based on fuel burn ? And adjusts performance figures accordingly.

Ok, thanks for the input guys. Guess it must have been a placebo effect when I felt the a/c being less slippery when I recalculated the GW.

 

Feels good not having to worry about this any more and will be interesting to see if I see a change in the descent profile.

In real life I don't really worry about it that much. I do it by time. At about 30 minutes prior to arrival I do all my calculations and ask the lead FA if they need any pax assists, lav service, etc. Then I send the in-range report and at the same time send out for my ref speeds and all that good stuff. Once I get it, we both verify it, and input it. 

 

In the sim I try to do the same but my dog Fuego verifies it

At 60-40nm to TOD, I do the Descent Checklist which includes setting landing data I.e Vref.

 

To set landing gross weight I simply add the FMC predicted arrival fuel to my ZFW (Arrival fuel + ZFW) and input that weight to generate Vref speeds. Re-check anytime before 3000ft above destination.