For the route and weights below, according to Simbrief I should use about 4000kg more fuel with cost index 400 (compared to the CI 45 I did the original flight plan with). So trip fuel should be about 4-5ts more than on the screenshot. However, neither the fuel predictions nor the actual usage (!) increased during the flight with cost index 400. In other words: I was able to fly with cost index 400 and use about the same amount of fuel compared to cost index 45.

EBBR/25L ROUSY2P ROUSY DCT GTQ DCT POGOL DCT LASAT DCT MOROK DCT GILIR DCT GEVEA DCT KOGAS DCT MEDAM DCT VEVAR DCT BARSO DCT KOLON DCT RARQU DCT NESUL DCT ETOIL DCT CIRTA UA605 DJA UB730 DIR UG862 BUN DCT ASEKA DCT MUBAK DCT EGREK EGREK1B HRYR/10

I can reproduce this on every flight. The highest (!) fuel usage actually comes with cost index 0. So there's something fundamentally wrong.

Edited June 24Jun 24 by Fiorentoni

I’ve also noticed that EFOB at destination increases as the CI is increased. Flight time does reduce, but so does the fuel usage, which is not the way it should be.

I did a test at different speeds and CIs and found the following:

M.780 ~ 1300 kg/hr (per engine)

CI 10 ~ 1320kg/hr

CI 160 ~ 1340 kg/hr

M.840 ~ 1390 kg/hr

I did my best to let the engines stabilize before noting the values. I think that the fuel flow spread across the speed range is way too small and should be looked at. This also affects fuel predictions, as in this case it seems that the gained time outweighs the higher FF when increasing the speed. Here, a bigger increase in FF with speed (engine output) would be required to restore normal FF ~ speed ~ flight time relationships.

Would it be possible to look into this issue?