Hi,A little help would be very welcome.I am using the PMDG load manager for configuring fuel and load requirements but I am getting some unusual results when approaching cruise altitude.Using FSbuild to define the flight plan with 747-400/RR engines specified, I estimate and uplift fuel for the trip, in this case EGLL - KJFK of around 154000 lbs.ZFW is around 500,000 lbs and actual takeoff weight around 652,000 lbs.I realise this is a little heavy at landing (I usually aim for around 8 tons) but I was anticipating a significant hold.Normal takeoff and departure, climb settings in FMC N1 thrust if I remember is around 1.56 during the climb but drops to 1.47 above FL240 using .77 mach to FL280 then 0.81 mach to cruise alt.Problem is with LNAV and VNAV set I get the "cannot cruise altitude message on the FMC.The aircraft eventually stabilises at around FL290 and mach 0.81 and no amount of coaxing will get it higher. Interesting TAT was showing between +41 and +50 at altitude which seems ridiculous to me and might account for the low ceiling, have I missed a setting somewhere?I do not think the FS-X real-time weather could come up with this.Thanks,

The high TAT is the problem. Higher temp means less dense air and the engines provide less thrust. My flight tests at 650,000 GTOW show a direct climb to FL390 in 146 nm.I like ActiveSky by HiFi Simulation. They have versions for both FS9 and FSX and provide their own weather server and depiction. I also recommend the latest version of FSUIPC with registration. This module does an excellent job of smoothing wind, pressure and temperature excursions that MSFS creates. It's too bad MSFS thinks all we are interested in is eyecandy.

Imagegear ( do you have a real name?)My guess is that this is online with FSX and FSInn? If so then turn the FSInn weather off for flights above about FL240, there is a known issue with this.

My apologies Gents have been out of the country for a time.Thanks, yes that all makes sense now. FSInn is grabbing hold of FS-X Weather settings and resets to the User Defined settings within a few seconds of trying to reselect anything else. I previously had quite good weather with an earlier version has FSInn recently gone pear shaped ?Ken DenmanBAW1232