Why do I get 'MAX ALT FLXXX' in CDU?

[martinlest2 58]

Hi. Anyone able to tell me why, when I select a cruise altitude in the initial setup of the PMDG (here 737NG) CDU do I sometimes get a message there telling me that the Max Alt allowed is lower than the one I want to set? I expect that on short flights, but on a long haul flight (over 1500 kms) I should be able to select, say, FL350. I can certainly fly up to FL350 and stay there for 90% of the flight duration before I need to start my descent.So why does the CDU object and tell me I can't programme in over FL315? What are the parameters that decide what MAX ALT is for the CDU? I don't get the message on every flight.Thanks.Martin

[downscc 1,327]

Max cruise depends on the weight, not directly on distance; however, on longer flights you lighten as fuel is burned but the key is weight.The 737-700 can be flown up to FL410 in most configurations... how about a little more info such as GTOW and variant? Are you checking the cruise performance screen where optimum and maximum altitudes are provided? Is this message during preflight or does it appear during the flight?

[Jack C 8]

FL315? Thats odd. I have never seen an altitude that low in the real machine. I have flown the -800 from CYVR to PHNL (approx 2300 miles) at near gross weight and we were still able to initially climb to 360 and thats and a cooler than ISA day.Jack

[martinlest2 58]

I'll have to post back with specific numbers next time it happens. I get this message during preflight programming. If I ignore the message and fly up to, say, FL360 anyway, the message still keeps appearing. My TO weight is well below maximum. Could just be a glitch on some flights, but as I say, I'll post back with some specific information when I next get the problem. It's odd to me too ..M.

[Jack C 8]

What temp at cruise are you inputting?Jack