Hello,
so this is a longer one, but bear with me...
Since I got the A350, I observed a strange behavior when it comes to Fuel Burn with different CRZ FL. Usually I plan my flights as close to the OPT FL as possible and even wrote down most of the OPT FL from the FMS since Simbrief is often choosing lower FL than the OPT FL would be. With this, I usually arrive with ~1t-1.5t of fuel more than what Simbrief calculated, so less fuel consumption.
Today, however, I was flying NTAA-YPAD with a friend. He was flying the 350-900 and I used the 350-1000. Due to strong headwinds enroute, we figured it actually makes sense to fly at FL340 all the way which would've saved ~15min and around 400kg of fuel according to the calculations. Simbrief suggested 360/380/400, with my OPT FL it would've been 380/400. ZFW 186.2t in the -1000. After T/C my FOB was already 1.5t lower than calculated and I was 4min behind schedule. Then I went into my crew rest and when I got back 6hrs later, my FOB was a whopping 6.2t lower than calculated and 11min behind schedule! I decided to climb to FL400 then to save some fuel.
At the same time, the cruising speed did not match the Simbrief profile. After reaching T/C I was flying at Mach .85 (Cost index 30) which matched Simbrief, however, after I came back from my crew rest, I should've been cruising at Mach .83 according to Simbrief but I was actually going Mach .80. My friend was going as slow as me and had lost 2.5t in the -900. We both use the standard Fuel Bias, so 0.0 for the -1000 and +3.0 for the -900. The weather can be ruled out as the enroute winds/temps matched the forecast pretty well.
So, to summarize, in my opinion something is very off with fuel consumption and speed at different levels and is disproportionately diverging from Simbrief the higher or lower you go. Below FL350 you burn a lot more than what Simbrief calculated and above FL370 it is significantly less. I think the problem lies with the 350 rather than the Simbrief profile as the profile makes sense in my eyes.
I'm using version 1.0.11 in MSFS2024. I would appreciate if you revisited the Fuel flow and cruising speeds for further refinement! 🙂
Thanks for reading my ted talk.