Rule of thumb equation or graph for sfc for turbofans

[Phil Drake]

Hi folks,

Hard to believe, I know but our company yanked all our fuel consumption tables out of all of our manuals, so on a long flight all we have is our flt plan listing a number of different altitudes. Does any one have any rule of thumb formulas for fuel / sfc consumption on turbojet aircraft working down from your optimum to say 10,000'. I scanned my Aerodynamics for Naval Aviators and Handeling the Big Jets 2nd edition to no avail. Yes I know jets love high rpm and high altitude but I was looking for a formula or two that could be worked from a baseline known quantity.

The reason for the question is: when all goes to sh**t it really can bite you.

Thanx,
Phil

[barit1]

One simple check is to compare eng #1 vs #2 (vs #3 vs #4) at matched EPR (or N1 as the case may be). Keep track of the FF difference from flight to flight - this will require setting up an informal system for sharing data between crews.

Note that the thirstier engine never will decrease FF - if the delta value decreases it will ALWAYS be the "good" engine getting thirstier, "catching up" to its mate.

Also - many years ago the engine makers used to produce slide rule (maybe circular slide rule) computers to represent a nominal engine on wing. I developed one that fit in the F/E's shirt pocket.
They worked pretty well as a crude trend monitoring tool.

But personally, I'd insist that the dunce who pulled the tables ride along on every flight so he understood what he hath wrought.

[ifleeplanes]

There is a great site from NASA that has a free program that calculates SFC etc etc for many different engines for you and if you carry a laptop on board with you it might help.

Its a good program anyway!

http://www.grc.nasa.gov/WWW/K-12/airplane/ngnsim.html

[fcom100]

A340 20 kg/nm at 10000'.
Or double your distance and add 2 to get fuel required plus half an hour.

eg. Dist to ERA 500nm x 2 = 1000 make that 10T add 2 =12T

[enicalyth]

phil

tell me which a/c and i might just have done what you are doing. by the way i have issues with both the nasa prog and jack mattingly's that haven't been fully addressed so take care as always.