There seems to be a bit of a discrepancy here. For example, a flight planned at FL320 was planned by Simbrief at CI 30 and at M.77 , while the Zibo FMS provided .75 for that CI.
Per the Zibo developer, they use real-world data. My understanding is that Simbrief does as well. Could this please be corrected, split off into a Zibo specific correct profile, or a cruise speed offset option added similar to the altitude option we currently have?
Thanks!
Discussed on the Zibo discord and came to this table explaining the discrepancy
61T F380
CI 6 simbrief .76 FMC .756
CI15 simbrief .78 FMC .767
CI25 simbrief .79 FMC .776
CI35 simbrief .79 FMC .781
CI50 simbrief .80 FMC .787
61T F320
CI 6 simbrief .72 FMC .698
CI15 simbrief .74 FMC .721
CI25 simbrief .76 FMC .742
CI35 simbrief .77 FMC .757
CI50 simbrief .78 FMC .773
As another Zibo user in XP12 I have been part of this discussion ‘in house’. However, there are more questions beyond the difference between SimBrief target cruise speeds and Zibos. The Zibo FMC settles on one CRZ target speed and this is given to each and every waypoint along the route between TOC and TOD. SimBrief OFP navdata gives a different speed for each waypoint…and these also differ (rounding. I think) between different OFP layouts (e.g. LIDO vs KLM where one has two and the other three decimal places). It would be good if any answer to the OP from Navigraph/SimBrief explains a little more about how these speeds are calculated and how accurately this reflects real world dispatch. When it comes to the dispatcher vs the FMC, who is meant to calculate what?
SimBrief is correct here, the Mach number should absolutely differ from waypoint to waypoint:
The Mach number for a given cost index varies depending on weight, in general higher weights give higher Mach numbers. During flight, you are burning fuel as you progress from waypoint to waypoint, so it’s expected that your Mach number should reduce the farther along your route you go.
The Mach number also varies depending on altitude (higher altitudes give higher Mach numbers), so if you have any step climbs planned, the Mach number for waypoints before and after the climb will likely be different.
Finally, the Mach number can vary depending on atmospheric conditions. ISA deviation, and more importantly, headwind/tailwind components can result in different Mach numbers. So as the temperature and winds change along your route, the Mach number can fluctuate a bit from waypoint to waypoint.
The end result, with all of these factors combining into each other, is that the Mach number can seem to fluctuate randomly along your route. Possibly the Zibo (any maybe even real-world) FMS doesn’t show these fluctuations in the preflight stage, but presumably it should be applying them once in flight based on the actual conditions the aircraft encounters.
You’re correct, some layouts simply round the Mach number to 2 decimal places, while others show 3 decimal places. Fluctuations in the third decimal place are likely due to atmospheric changes between waypoints, while the second decimal place will change more gradually as the flight progress (for a long flight, it might be 0.01 or 0.02 lower between TOC and TOD, assuming the wind component stays the same).
While the goal is for the OFP to have Mach numbers which are as accurate as possible, there can be differences between the OFP and FMS due to the variety of factors I explained above.
Small differences (+/- 0.01 Mach) don’t really matter much though. The time and fuel impact will be negligible (for example, for a 4 hour flight, if SimBrief plans Mach 0.77 while the FMS plans Mach 0.78, that only results in a difference of 2 minutes and 100 kg. And most flights will see differences of even less than that).
In the end, the OFP is just a planning exercise. Small deviations between planned and actual conditions are unavoidable, and the Mach number for a given cost index is just one of many factors which can vary. This is equally true in the real world. As the pilot, I wouldn’t worry too much about the OFP Mach number. Just follow what your FMS calculates and trust that in the end, the OFP’s fuel and time estimates should be close enough.
With all that said, it’s safe to assume that real-world multi-million dollar flight planning software will have less of a discrepancy between the OFP and the FMS mach numbers. Both because they have access to proprietary high quality data, and also because the airline will use data that is tailored for their specific fleet (engine type, aircraft variants, and even FMS software revisions can give different Mach numbers for a given cost index).
I have reviewed SimBrief’s data based on our sources and it seems correct (or at least close enough, within +/- 0.01 Mach across most conditions). That doesn’t mean that Zibo hasn’t used real data though, it’s possible their source is for a different aircraft variant, engine rating, or FMS revision. Honestly there are too many factors that can explain the discrepancy, to tell for sure where it comes from.
Adding a specific Zibo-tailored cost index database is currently not possible. A cruise speed offset may be possible in a future update, but is unlikely to be added soon.
Best regards,
Wow! Thank you for that extremely thorough answer!
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