Question about fuel consumption
 

Why does a jet engine have a lower fuel burn rate at higher altitudes?

Same reason a piston engine has a lower fuel consumption at altitude: there is less oxygen available for combustion, thus less fuel required to achieve proper stoichiemetric combustion. The turbine engine behaves rather like a normally aspirated, fixed-pitch piston engine. As such it is producing much less power at altitude. This is compensated for by the low atmospheric density, the airplane "indicates" 300, but it is actually travelling at 500 kts "true" airspeed. This what you were looking for?

I answered this question rather comprehensively on this thread, now contained in the Tech Log Archive:

Why are jet engines more fuel efficient at high altitude?

Checkerboard and Cardinal,

Thanks for the excellent responses. I have people at work who are arguing, betting money, and fighting over this. I hope I have your permission to cut, print and post your responses on the board at work. Talk you you again sometime.

Roger (Rojop)