This is the first time this has happened to me, and I've flown the 747-400 A LOT.Scenario: In KDEN, on VATSIM using SB3, FSUIPC 3.71, 747-400F P&W about 2/3 loaded, 65,000 lbs of fuel, for flight to KLAS. Total weight was calculated at about 660,000 lbs. Set cruise to FL380 (max was calculated to be FL383).Weather at KDEN was poor. About -6 degrees C, heavy fog, and gusty wind. Taxi out, and takeoff seemed relatively normal. Get up to about 15,000 feet and things start to go wonky! The plane starts have one #### of a time climbing. CLB trust is indicated on EICAS, and verified by the EPR setting. As I slowly, slowly climb, its clear that I will NEVER make it up to FL380.I remember reading about similar performance issues people have had with ActiveSky. Although I'm not using AS, that got me thinking. So, I manually forced SB3 to update the weather with conditions at KLAS. And... Lo and behold! The aircraft performance improved somewhat - I wam then able to get up to FL380.However... the entire time during cruise, the aircraft is struggling to keep the speed up - whereas I would normally expect an N1 of 90-something, it's pegged at the limit almost the entire time.Clearly, something is out of whack here, and I'm suspecting it has to do with weather, or how FS (or FSUIPC??) (or SB3??) deals with weather.Anyone have any ideas?- Bill

First thing that came to my mind regarding wx was temperature, if the simulation was trying to run with unrealistic TAT then performace would be unrealistic. Wind variation also comes into play when changes in speed or direction or unrealistically rapid but in your case the even seemed to last a long time so I'd take a look at the temps.By the way, KDEN was closed last night and remains closed due to the blizzard... but I guess you knew that.

Couldn't resist doing a short KSFO-->KDEN PMDG 747 flight yesterday with the blizzard going on. 1/8 visibility and snow, you pretty much don't see the runway until touchdown. However, a flawless autoland with the QOTS. Much fun indeed.As for the performance issue, I'm using Active Sky and didn't see the issues that you described. However, I did get bounced around pretty good during climbout of KDEN on the next leg to KORD. But above FL150 things were OK.The flights in and out of KDEN were about 4PM to 5PM local time.

I imagin there was a bounce. However, even with ActiveSky I have found I need to use FSUIPC to control wind variation. There was a deep anticyclone in the area creating the blizzard. Perhaps this was causing an almost constant change in wind direction for you flight and hence the performance hit.

Was watching the wind pretty close, and it remained at a relatively constant 55-60 kt headwind - very little variation.At this point, seems like a very real possibility the preformance hit was temperature-related. When I have some spare time this weekend, I'll try to reproduce by playing with temperatures manually.Thanks for the feedback, guys!- Bill