So i take the -800 on the KRNO - KLAS route a lot, simulating different conditions each time. As we are all aware, VNAV is giving a hard time on descent, and usually i find myself forced to use LVL CHG to achieve 16000 and 250KTS at FUZZY on the SUNST2 arrival.

 

The other day I was flying the exact same route as i always do with real world weather, but this time i checked the "download winds aloft data with real-world weather" box in the DISPLAY->WEATHER settings just to see what it does. I didn't input any winds aloft data to the FMC, and i was getting a steady 50 knot tailwind for the majority of the flight.

 

This time, on descent, VNAV worked perfectly ! descended smoothly out of cruise without too many changes in V/S, got me exactly at 16,000 and 250KTS at FUZZY, and I didn't need to switch to LVL CHG up until i was at CHIPZ.

 

So can anyone think why a 50 knot tailwind straightened up VNAV and made it fly right ? my instinct says it should only tamper with the descent, considering the fact that the FMC didn't account for the wind in calculating TOD since I didn't make any wind inputs..

I find that VNAV generally works well hitting speed and altitude restrictions on STAR's consistently no matter where I fly. VNAV needs a little baby sitting but I do not share your experience that VNAV is generally not flying the path and provides pilots with a "hard time". I have flown in VNAV many times all the way down to the final approach fix.

 

If your 50 knot tail wind was consistent, it seems calculating a vertical path with a constant ground speed somewhat easy for VNAV to accomplish.

 

Maybe other pilots that share your experience can help.

 

Don't expect any data source to automatically enter descent winds in the forecast page.

I don't have any problems with VNAV either. Ocassionally it will require drag to maintain VPATH and FMC Speed, but almost always that will be due to manual changes I've done to descent/STAR waypoints.

 

I use FS2Crew and that helps tremendously getting the jet configured well in advance of TOD, though I don't think there is anything in that particular process that would make an FS2Crew'd aircraft hold to the descent profile any better than a non FS2Crew'd one...

 

I'll experiment though and see if winds aloft data "Messes up" my VNAV, where it "helps" yours :) there are a million variables involved and maybe we're just in different areas of the tolerance range.

 

Additionally... now that you've used the phrase "Straighten up and fly right" in your post, I'll be hearing that song and "cool down poppa don't ya blow your top" in my head and all day today... :)