A little summary in between:
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Default aircraft in this sim as far as I know have always used the (world menu) flight planning feature; were a .pln is generated from the in-game flight planner.
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It use to be possible to load/change (or re-load the same) (a) flight plan even after fly now without exiting and starting a new flight.
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When loading a .flt (including a flight plan .pln) or only a .pln (flight plan) the route is not populated in the aircraft’s FMS.
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There is a interface bug? that’s still around since the start that prevents choosing a pln. According to this post; this option was removed by the developers due to sim issues. It is unclear what sim issues and if this goes beyond “just an interface issue”?
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There might be exceptions with 3rd party developers producing default planes and having a different file format for their FMS.
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Previous version had a Flight Planner UI that could be accessed after Fly Now, this sim never had that UI but I think the ability to load flight plan is necessary after fly now too.
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Some 3rd party aircraft use the flight plan from the sim and load it into the FMS and many others use their own system with loading from a file (externally) and this can also be reloaded or changed when desired. These aircraft should not be affected by the issue, described in this thread.
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And another feature many aircraft now have is SimBrief flight plan integration but not exclusively.