Hi guys I'm hoping this is a quick fix or something I'm doing wrong - I can't seem to get the a/c to reach cruise altitude. My past flights have been roughly 50% load with approx 50% fuel so we're way under MTOW. I set up everything correctly (as far as I know) and the a/c climbs on A/P nicely however when it gets to around FL210 or thereabouts, it slows to about 400 fpm and UNABLE CRZ ALT or something comes up. Any ideas?

If your using FSINN that is probably the reason. Sounds like you got the heat bug and its +40c outside and you can not climb in that kind of extreme. The solution is to disable FSINN weather and use some program like Activesky for Weather Generation.

is it only that? I use REX and thats where I get my weather from. surely it can't be that - I'm flying exclusively around Australia at the moment and we're in winter.

Yep, I bet it's weather. I use ASE and this happened for me when ASE quit in the background and I didn't realize it. My TAT was +55C at FL210 and that was as high as I could go. I'm not familiar with FSINN but if it has a "refresh" of the weather or something like that, try that.

Mr. Thomas, The first thing to check is your TAT. If it is abnormally high for your altitude I would venture (as previous posters already have) to say that is your problem. If not we need to dig further. Hope it helps,

Need to disable FSINN weather completely. All it does is cause problems. In the Control Panel of FSINN go to SET->Basic->Weather and check/tick the box that says "Disable all FSINN weather" or something to that effect.

awesome..thanks guys, I shall try that when I get home this evening.

Also, make sure you have indicated airspeed and not true airspeed in your FSX realism settings.