The AirportCEO Mod Loader Info - Releasing May 18th


The Airport CEO Mod Loader project is a project that aims to completely change the way code-based modding works, so it is easier to use for everybody. Simply, it consists of moving all modding to the steam workshop and moving from the UMF modding system to a new system. Both code-mods (like Tweaks and Custom Buildables) and the mod loader would be on the workshop. This gives the AirportCEO community full control over its modding system, while also making it simpler to use for users:

There would be a new AirportCEO Mod Loader workshop item. This item would be the equivalent of the UMF system. Then, mods like Airport CEO Custom Buildables and Airport CEO Tweaks would have their workshop items. Upon subscribing to the Mod Loader and a code-mod, the installation would already be complete and you could just play and enjoy.


  1. The biggest benefit of the whole system is its ease of use. The workshop-based system allows for no external downloads to be required, thus making it simpler. For Windows users, no interaction with the system is needed other than reloading the game once. Further, the usage of the workshop means dependencies are very obvious, showing clearly what you need to download. A mod can require Tweaks, which requires the aircraft pack and the mod loader. This is simply and effortlessly communicated to users.
  2. Since the mod loader is used by everyone and can include anything, this allows the community to pretty much add things to the game themselves. For example, performance improvements and minor bug fixes could be added, to help improve the gameplay quality of anyone playing.
  3. This is the least applicable to most players, but BepInEx has a wide array of tools to make developing easier and a great community. This means that development time can be decreased, thus increasing the amount of features that can be added. The mod loader can also contain simple-to-use utilities, which means that new mods don’t have to have their own systems (to deal with save/load, the workshop, etc) they can just quickly use the Mod Loaders.


There are lots of reasons why moving to the mod loader is a good idea, and most of them are directly tied to one of the benefits mentioned above. But here are some of the reasons why:

  • The current system relies on external downloads, which introduce complications for some users
  • The current system is “hidden” from users, it’s not easily findable through Steam
  • Dependencies are an absolute mess using the current system
  • UMF is a closed-source system that is generally not preferred by the code-based modding community - BepInEx provides better support and a better community
  • It’s hard to ship fixes and improvements to everyone as there are multiple different mods with different authors
  • The development of Airport CEO has been largely complete, yet has a community interested in adding more features. Airport CEO is a good game in a good place for modding - and this project allows that to happen better, easier, and faster

But, there are some drawbacks to this plan

  • It requires a major shift in users between two incompatible systems (This can be solved by notifying users of the switch through updating the UMF mods)
  • It does require some work by the game developers (See the How? section for more info)
  • Steam workshop mods can be updated by one person only - this is a Steam limitation. This makes it hard for multiple people to work on something, and creates the risk that unmaintained mods become outdated while people use them (This is fixable in many ways. Contributors can be added to mods that can update their title and description to redirect people, the ACEO mod part of the mod loader can be made so that it can work for any workshop item with certain attributes, everything will be open source)
  • Less support and ease of use on Mac (the new system will still be better than UMF though)


The AirportCEO Mod Loader has already been partially developed, the GitHub is linked. About half the project still needs to be developed, but this is not an issue as it is mostly just fleshing out features. The biggest issue is getting the mod loader to work in the game. BepInEx only needs one thing so it can load - it must be in the games directory where the .exe file is. This is sadly not where workshop mods land normally, so the game itself has to move the mod loader from where it is found in the workshop folder, to the place where the .exe is. The game must do this, as I have not found another way to do it.

So, we need to convince our dear game devs to implement this change! If you think that this should be added to Airport CEO, please voice your opinion below!! Thank you.

Technical Details for Devs (or anybody)

The most complicated part of this is successfully getting the mod loader from the workshop item to the game folder so it can start working. This does present some security risk, but nothing more than someone purposefully modding the game.


As mentioned, the mod loader files need to be moved from one folder to another. There are a couple of ways of doing this. One difference is how the game tells the difference between the mod loaders Steam mods and other mods:

  1. Search Method (Prefered) - At Awake, the game will search through all mods and see if it finds a specific “version” subfolder of a mod. Then it will move files based on that. This has worse performance and slightly worse security, but it means if necessary a new mod loader can be implemented by the community easily.
  2. Steam Workshop Item ID method (Hardcoded) - This means that at Awake, the game will search through all mods available. And then if it finds the ID number of the Mod Loader, it will move the respective files. This has better performance and security, but it means there can only be one mod loader (issue if the person who owns the item leaves the community, and so on)


There are a couple of issues with this entire method in the first place:

  • The Lagging Issue - Since BepInEx only loads when the executable is loaded, so whenever the mod is downloaded/updated, it will make the user A) start the game, the stuff is moved to the new directory, B) quit the game, and C) relaunch the game, and now BepInEx is loaded.
    • Solution: A message (popping up if the move was made) could warn the user that the game must be reloaded on the initial install, this could remove nearly all confusion. If its an update, when it is applied does not matter, the lag is okay
  • Update Complications - Since the game would only update mods at game load, updates would be very delayed - Up to two restarts late. But the bigger issue is how to tell if an update has happened
    • Solution: Have some file with a version number in it. If workshop one is higher than game one, move, otherwise don’t. Easy solution.

Thank you for reading. Devs, I really hope you see the immense value in this system and are willing to implement it!
– Humoresque


We need that so much! The current umf is not flexible, and i think @Olof @Fredrik @Alexander you guys love how we got futher with some ideas and new aircrafts? :wink:

So i guess the whole community, not only we modders, love that idea right?

  • We need it
  • There is no other choice
0 voters

I’m so glad this is on its way to becoming a reality!


Please devs! This would be SO much more convenient than using UFM!


The Game needs definitely Further Development. It has a Lot of Potential !


made a little meme for the modloader :rofl:


Hoping it would be successful

1 Like

Completely agree. Since Airport CEO development is basically over and devs are concentrated on making a new game, it would be amazing to give community more freedom to continue working on the project.

BepInEx is known already and used widely in KSP and other Unity-based games, so there is a lot of documentation available and would decrease the entry limit for modders.

Dev pls


really hoping the devs are willing to cooperate with you guys to get this fully implemented!

Just a quick note on this for the general player base. The Mod Loader development is continuing very well, and most things are done and ready for release. Currently, I’m working on converting mods to use the Mod Loader and its features. This process is largely complete with all mods converted, except for Tweaks, which I still have to work through. If you look at my GitHub you can see the evidence.

The current plan will be to release the Mod Loader experimentally fairly soon. This would mean the Mod Loader and associated systems will be fully functional, but as the devs have not implemented the change they have too, there will still be one external download for installation. The installation process will be:

  1. Download the ML external part and unzip into the games directory
  2. Subscribe to the ML Steam Workshop part
  3. Any and all mods are now just one click away on the Steam Workshop

If they devs were to implement their part, then step 1 would be removed :wink:
Non Steam players will download a separate version of the Mod Loader, and they will just have to manually drag and drop files of a mods GitHub or google drive into a folder. They will still be able to use mods fully, just without auto-update and a slightly more complicated install proccess