I don’t know what AI OFFLINE (an option on the data settings/traffic menu) has to do with the LEGACY flight model setting (an option on the flight model menu). Totally separate settings. Totally unrelated to AI.
The sim doesn’t currently have SDK support for a lot of AI things people want to do but that wasn’t intended to be the purpose of this thread. Discussions of custom AI aircraft would be much better suited for the Third Party or SDK section of the forum.
I will say I continue to see new threads on multiple forums where people have inadvertently set Legacy flight model or that setting has changed itself somehow, resulting in broken behavior of aircraft. So this is still very much an issue.
Statement from Carenado:
CARENADO FLIGHT DYNAMICS STATEMENT
All of our MSFS2020 aircraft are designed for the new MSFS2020 (MODERN) flight dynamics system.
Therefore, all our aircraft must be used in the MODERN option of the FLIGHT MODEL menu.
This applies to both those already released and future ones.
If they are used with the old dynamics system (LEGACY) the behavior is indeterminate.
Apparently still a lot of confusion caused, and people still select the legacy ‘realistic’ mode because they think it’s better.
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I’ve read these many accounts, and I have to admit I found it perplexing. Mainly because I know what the word “Legacy” means, especially when used in this context, and that a new flight model is likely better than an old one.
According to Automatic detection of Modern vs Legacy flight modes - #3 by Bishop398 this has been implemented – however as far as I know the option still exists in the UI, confusing people and often producing wrong results. Is this considered “fixed” by Asobo or would removing the user-visible option be a further improvement that can be made?
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I can not find how it is implemented. I have studied the SDK documentation and don’t find a parameter in a .cfg file to programmatically say this model is to be run “legacy” mode. Google searching turns up nothing. I will create a post on fsdeveloper.com.
@ Bishop398
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Have a look in the “flight_model.cfg” file for:
modern_fm_only=1
https://docs.flightsimulator.com/html/Content_Configuration/SimObjects/Aircraft_SimO/flight_model_cfg.htm?rhhlterm=modern_fm_only&rhsearch=modern_fm_only
A few years later and I’m still unsure why some people think Legacy is the way to go over Modern.
IIRC the menu screen for “flight model” also includes some “realism” settings, and only allows setting things to “realistic” if you pick the “legacy” flight model. If you pick “modern” these settings are disabled.
Thus, I think people are thinking “Oh I want realism! I guess I have to set this to get it.”
(I suspect that these settings only apply to the “legacy” flight model and are simply not affecting “modern” which is why they are disabled. But I can see how it would be confusing.)
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For me it was Modern > Legacy.
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Yeah it should be clear from the name… 
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My need to convert an FSX model to an MSFS model no longer exists. A person will be creating a real MSFS version of the plane I am after. So the “legacy” issue is now dead.
A little background: The person who was helping me with the conversion process said I needed to run MSFS in legacy mode. Following up on that, and long story short, I ended up chasing what apparently doesn’t actually exist – a way to set a plane to legacy mode in the configuration files of a model. I’m not “for” legacy mode, I was just chasing a rabbit down a rabbit hole as it turns out. Thanks to those who were trying to help me in this pursuit. Case closed.
That one may have been a new addition. I already knew there was an option to force the new flight model, but I didn’t know there was a similar option for the legacy flight model. I can’t imagine that gets much use.
