Hey guys,I´ve just made my flight back from Denver to Portland on a 737-900WL. It was pretty the same routing as the one before from Portland to Denver. I configured the plane correctly but it gave me an optimum flight level of only FL375 whereas I flew the other one at FL380. When I attempted to put in FL380 as cruise flight level, it refused and said that it´s unarchievable. But we had an cruise flight of arround 650 MN, so why isn´t it archievale?Would these additional 1000ft make a that big difference in economy?Can someone clear that up?

Upwind vs downwind perhaps?

Upwind vs downwind perhaps?

Hmm, yeah, if I recall correctly, I had a pretty stiff headwind on the flight back to Portland.

Nomally this shouldn't be a problem. Something must have been different than on the flight befor. Have you checked you load? If you have a 1000kg more fuel (or passengers or cargo) onboard it is likely that you max altitude will be reduced.

Well, the fuel was the same (23.400lb) but I changed the payload. I didn´t know that this would have a that big influence.

It would, also, the length of the sector would not. OPT and MAX levels are a function of weight. TRIP level IIRC is what FMC would suggest to be the most optimal throughout the flight (and will be higher than initial OPT often).In your case I guess you were just simply too heavy for 380, although that seems a bit strange if OPT was 375. Are you sure it was not MAX that was 375?

The weight of the plane is mainly what determines the maximum cruise altitude

For the 737 even 2,000-3,000 kg (5,000-6,000 lbs) make difference. Put 5 tons plus, that's almost 10% of the MTO weight...Maybe you can look into some of the FSX flight planner tools (TOPCAT etc.), they will give you a very good idea about the matter.

Keep in mind that with a strong headwind, a higher cruise speed will provide more fuel at landing. (Higher cost index)