I did small “thought experiment” with very much of simplification.
The comparison of fuel consumption between two cases at descent from FL370 to 3000 ft.
Case (1) start descent at the TOD calculated by FMS using speed-brake to decelerate on lower altitude.
Case (2) start descent 5 NM earlier and decelerate on level flight at 3000 ft.
At cruise: case (1) stays at FL370 for extra 5 NM.
It will take 39 seconds. (assuming Mach.8 TAS 459 kt)
Therefore burn +109LBS (assuming fuel flow of 5000LBS/HR for one engine).
At descent: case (1) need a little longer to reach 3000 ft because of slowed airspeed in the later portion.
I put extra 10 second for this reason. so +6 LBS of fuel. (Fuel flow 1000 LBS/HR for one engine)
At 3000 ft: case (2) will fly level at idel thrust while deceleration at average speed of TAS 214 kt. (deceleration from CAS 240 kt to CAS 170 kt)
So it will take 84 seconds, consequentry burn extra 47 LBS. (assuming FF 1000 LBS/HR per one engine)
Summarizing the above, we can save 68 LBS (case (1) 109+6 LBS versus case (2) 47 LBS ) by starting descent earlier than FMS' TOD by 5 NM and avoiding use of speed-brake.
However, who can tell whether 5 NM is appropriate?
If it is too long, we have to burn some more fuel in the dense air mass of low altitude. It will cost a lot.
Furthermore, we need to think about noise, terrain, traffic flow and etc.
I welcome any comment!
Last edited by firstmrj; 22nd Jun 2020 at 16:24.