I want to know which altitudes to choose when I decide on the 4 altitudes to plug into the DES FORECAST page.  From my understanding, inputting these winds into the FMC helps the system plan the best time to start the descent and the optimized angle of the descent to use, with the goal of minimizing use of engine thrust.  That being said, is the idea to use altitudes that I know ahead of time the engines will be on idle during the vertical flightpath?   I have learned from my experience in the NGX that I don't use the last cruise altitude or the altitude of the FAF of the ILS, I would pick the first altitude shown on the FMC legs page below cruise, an intermediate altitude (around 18,000ft) and a base altitude, perhaps 7000ft, where I most likely transition from the VNAV PTH descent to a level off or more shallow descent with engine thrust added.   

 

Another thing I learned, from watching Kyle Rodger's informative videos, is the 777 uses something called Geometric Descent logic during parts of the VNAV PTH descent.  This appears to refer to the part of the descent regime where the descent is shallower than the descent from cruise, and engine thrust and pitch changes could be used to maintain a set speed.  My interpretation of this descent logic and how it applies to the choice of altitudes used to populate the DES FORECAST leads to this question:  Do I look at the descent and only choose 4 altitudes crossed when i'm in IDLE descent?   See this screenshot below, taken from my in progress flight between KATL and OMDB using the DESDI6D STAR into OMDB ILS30R

 

 

 

As you can see, I am descending on idle power from FL370 to 14,638ft at 290 kts, but I then slow down to 230 kts shortly after passing the DESDI waypoint to meet the at/or below 13,000ft altitude restriction at the GITBO waypoint.  At this point, VNAV will command the nose to pitch up to arrest the rate of descent and the previously constant rate descent is interrupted.   Therefore, the idle segment of the VNAV descent is performed between FL370 and 13,000ft.   Does this mean I should only pick 4 altitudes between those two altitudes, FL370 and 13,000ft?

 

What is the commonly used procedure a pilot uses when making this determination?

 

 

I want to know which altitudes to choose when I decide on the 4 altitudes to plug into the DES FORECAST page.

 

Looks like I need to do a tutorial on this as this is the second question I've seen on it today.  The rule of thumb is that you want to choose the altitudes with the most amount of variation.  As an example (using wind right now over ROA):

 

FT  3000    6000    9000   12000   18000   24000  30000  34000  39000

 

ROA 2911 3214+16 3116+11 3112+05 2615-08 2917-18 340933 361644 350756

 

Say we're going to descend from FL350 over ROA.  Anything under FL350 is potentially useful for me, though 3000 and 6000 probably aren't, because most TRACONs pull you off of your VNAV path and assign you altitudes on their own instead.  So, we're left with this:

 

FT 9000   12000   18000   24000  30000  34000

 

ROA 3116+11 3112+05 2615-08 2917-18 340933 361644

 

As you can see, it's pretty linear, with not too much out of order.  There's a reasonably-sized jump between FL180 and 12000 in direction (about 50 degrees).  That's worth noting.  The FL340 speed is likely so close to our current altitude that it doesn't do us much good.  From there, I'm mainly concerned about the 50 degree jump (FL300-FL240), the 30 degree jump (FL240-FL180), and the next 50 degree jump in the wind (FL180-12000).  Since our current situation is probably Those altitudes are FL300, FL240, FL180 and 12000, so I'd use those.

 

Always look for jumps:

Big changes in direction

Big changes in velocity (in this case, there are some changes, but none too crazy)

 

As far as only entering them for the IDLE part of the descent is a good one.  It would certainly help to have more of the information in that part of the descent, but it isn't wholly worthless to add wind entries below that.

 

 

Another thing I learned, from watching Kyle Rodger's informative videos, is the 777 uses something called Geometric Descent logic during parts of the VNAV PTH descent.

 

...and thanks for watching my videos!  B)