Fuel Flow in cruise versus at idle on the ground
 

From an old story in a magazine about the Lockheed U-2.

"Fuel flow at 72,000' was 910 lbs/hr, less than it was at idle power on the ground."

http://aviationweek.com/site-files/a...20Reportp1.pdf

Older technology engines, sounds about reasonable.

Long time since I was in the seat but my recollection on the 727 was that F/F at the holding point wasn't all that much different to that in the cruise ..

Did some research on fuel flows for the missing 777 last year. Basically there is no difference between fuel flows at LRC and holding speed at cruise altitudes. The endurance will be the same but the distance flown will vary. That is basically true if the altitude changes. The difference between LRC at FL350 (OPT ALT) and the lowest possible fuel flow in holding (below 20,000')for a 500,000 lbs 777-200 is 3%.


For the U-2 the high altitude drops fuel flow to small amounts.

I would say that the U2 engines must be much more efficient at very high altitudes than at the ground, and that probably the U2 flies at relatively low mach numbers.