I have used Simbrief for many years and always successfully imported the route into my PMDG738 via the PMDG EFB. I have noticed recently, however, that when I import the flightplan via the MCDU after importing the plan via the PMDG EFB, the cruise altitude has jumped by 2000ft. For example, today I’m flying from EDDB to EGLL. Simbrief says that the cruise altitude is FL360, but when imported into the MCDU, it becomes FL380, meaning that my flightplan as filed is incorrect.
Hi, I could be wrong but I do not believe PMDG imports the altitude planned by SimBrief. I think it does its own independent calculation of the optimum altitude based on your planned weight.
Often this will return the same altitude as SimBrief, since both add-ons are using accurate performance data. But there are many reasons why it could occasionally differ (different assumptions, airway or city-pair restrictions, rounding errors, slightly different engine/aircraft configurations, etc).
It should be possible to simply manually enter FL360 into the PERF INIT page if you want to match your flight plan.
Best regards,
Hi, thanks for your response. I would normally accept this explaination, but the issue has only started to happen recently. You’re right about the aircraft choosing it’s optimum level, but surely SimBrief has enough information to calculate that too? I can’t think of any data points that SimBrief is missing to allow it to calculate the optimum FL
Mike
Hi Mike,
Actually the opposite is true, SimBrief would normally have more data points than the FMC to calculate the optimum altitude. SimBrief calculates a complete 4D trajectory for your route, using more granular atmospheric data (both in time and across more altitudes) than the FMC can hold in memory. The FMC’s calculation is a bit narrower in scope as a result, though this usually doesn’t result in a big difference.
You may have just gotten unlucky recently. Consider the possibility that the precise optimum altitude for this flight was FL370, but only FL360 or FL380 are valid for direction of flight. It may simply be that SimBrief rounded down to FL360 while PMDG rounded up to FL380. The 2000 ft difference is misleading, the underlying calculation might have been much closer than that.
In any case, I can confirm that nothing has changed on SimBrief’s end regarding the altitude optimizer or the information PMDG has access to though the API. If the FMC is behaving differently than it used to, I recommend reaching out to PMDG so they can investigate why that might be the case. Perhaps it’s a bug, or maybe they can give a better explanation on how their FMC calculates the optimum altitude.
Best regards,
Thanks for this. I’ll reach out to PMDG and see if they know why this is happening
All the best
Mike