Do you have the aircraft pack activated?
Any news on this?
Have the same issue with the latest versions of tweaks and performence mod on the latest game version with helicopters dlc installed.
Where is your airport located?
This is looking to me like the tweaks aircraft pack not being installed or being broken; though it would be odd for that to affect one save and not others. A complete log in text form might get me further though…
Aircraft Pack Update
===Tweaks Aircraft Pack Update===
- several data fixes (scaling / fueling)
- new aircraft types and variants
Pilatus Porter PC6
SAAB340F
DC10 & DC10F
(no specific variants cause of no visible changes between)
A350-900ULR
Mods using them coming soon!
Nice 
(filler filler)
Something to do with that service that is even stored in JSON but not implemented into the game?
Whats that? I’m a bit dumb lol
Is that possibly going to lead to fuel pipes on the stand?
Still have the issue with an error while going into the flight planner. Not everytime, sometimes earlier, sometimes later. Haven’t found out, if something is triggering that beforehand.
Latest version is installed…
2.3.6.359 (Beta 1)
- Fixed the flight planner bug?
- Aircraft selection for routes now accounts for cargo and non cargo aircraft separately
- Aircraft airport restrictions made more permissive
- Updated route generation code to generate a more complete route table sooner after loading a save
- Implemented route caching per-airline to help improbable routes (such as from small countries) generate
- Modified route generation settings to encourage respecting unbalanced feet compositions
- Rescaled ground vehicles to better match aircraft scale
- Halved passenger waste generation to help with waste processing bottlenecks
- Unallocated flights are automatically cleared on loading a save; this is suspected to help with flight planner crashes
The route gen changes were pretty involved and finicky to get working; I’m curious for feedback - It still probably needs a lot of tuning but the foundation is improved…
I’m excited about vehicle resizing - it is done on a per-vehicle-type basis. At the moment I scale things that live adjacent to aircraft (like baggage loaders and push-back tugs) most aggressively, which helps a lot to point out how big aircraft really are. It also helps vehicles fit on a busy stand/aircraft. Even the most aggressively resized vehicles are still 5% oversized with respect to the aircraft. Lots of room for tuning down the line. Feel free to leave feedback on this once your eyes adjust to the change 
I’ve just loaded my game with this beta update and while it has removed all the unallocated flights in the planner it’s also removed most of the allocated ones.
Doe it now happen after each reload or was it may once due to the new logic?
I’ve stuck the stable version back on, but the save I made with all the flights missing was still missing them after restarting. I’ll put the beta version back on and have a play around and report back.
Update
I’ve gone in with the beta version again, and when I went to schedule a new flight, I found that the missing flights are sporadic, they are gone on some days and not others which then made scheduling something of a challenge. The new flight I scheduled didn’t obey the flight separation rules, assigning itself on right up against another flight (I’m set to the default 30 mins) but it has stayed put through a save/quit/relaunch.
Done a bit more testing and new scheduled flights seem to behave against each other regarding flight separation and have stayed put through a save/quit/relaunch as well, so I suspect it may be a case of kicking all exisiting flights into touch and starting again.
I was also testing this a bit. I do think this is a one-time case with flights scheduled before switching to the newest version. Those removed flights might be those which could have caused the flight planner crashes.
I’ve also planned a lot of new flights, let the game pass a day, saved and restarted the game and they are still there.
But if everything is fixed I will see on the weekend when I can let the game run for some hours. 
Thanks to you and the other modders, hiccups aside your giving the game a new lease of life
Sadly still seems to be crashing occasionally when I go into flight planner
A log file on this would be super helpful; as the issue that I had logs of happening is no longer possible.
This is my current theory, but its also possible that there’s a continuing issue with allocated flights not being tagged as allocated. I keep an eye out.