757/767 fuel/eta predictions
I have seen this a few times on the 767 going across the pond and flying way above optimum due to ATC etc..the fuel and eta's disappear.
This is due as several peple have suggested already to it beng outside of its performance parameters and usually only occurs with a variable cruise speed set in the fmc ie ECON or LRC. if you intervene at your desired econ/lrc speed as commanded and enter that speed in the fmc it will recalculate everything as it now has a constant speed to work from.
It does not ean however you are going to run out of fuel.
As for the descent profile situation. I hven't come across that one yet, come t thik of it i cant think when i have ever wanted to be 10000' above a star hard height either..