Learn what an FSX .AIR file stores, how aircraft.cfg points to it, where it belongs, and how to fix a missing or mismatched flight-dynamics file.
An .AIR file in Microsoft Flight Simulator X (FSX and FSX: Steam Edition) is a binary flight-dynamics file. It stores aerodynamic and performance data that FSX reads alongside aircraft.cfg to determine how an aircraft flies. It does not contain the visual model, cockpit panel, sounds or textures.
What data does an FSX .AIR file contain?
An FSX .AIR file holds lookup tables and coefficients used by the simulator's flight-dynamics engine. These can cover lift, drag, stability, control response and other aerodynamic or performance behaviour.
FSX combines this information with values from aircraft.cfg. The exact division varies between aircraft, and parameters in one file can affect how values in the other behave.
| Component | Primary role |
|---|---|
.AIR file | Binary aerodynamic and performance tables |
aircraft.cfg | Text-based aircraft definitions such as weight, geometry, fuel, contact points and system settings |
model folder | Exterior and virtual-cockpit model files |
texture, panel and sound folders | Paintwork, instruments and audio |
The two flight-dynamics files together are often called the aircraft's FDE, or flight-dynamics environment. An .AIR file is therefore only one component of an FSX aircraft, not a complete flyable add-on by itself.
Where is the .AIR file, and how does FSX find it?
The .AIR file normally sits in the aircraft's main folder, such as SimObjects\Airplanes\AircraftFolder\. If the installation root is unclear, our guide can help you find the correct FSX aircraft directory.
FSX selects the file through the sim= entry in each relevant [fltsim.x] section of aircraft.cfg. For example, sim=MyAircraft tells FSX to load MyAircraft.air from that aircraft folder. The extension is omitted from the sim= line.
The .AIR filename does not have to match the folder name or the aircraft's displayed title=. It must match the sim= value. This works the same way in boxed FSX and FSX: Steam Edition; only the parent installation location differs.
Does every FSX repaint need its own .AIR file?
No; multiple liveries normally share the base aircraft's .AIR file by using the same sim= value. A repaint-only download may contain textures and an aircraft.cfg entry but no .AIR file at all.
A complete standalone aircraft does need the .AIR file referenced by its configuration. Our explanation of how a complete FSX aircraft package is installed shows how that file fits with the model, panel, sound and texture folders. A mistake we see regularly is treating a repaint as a full aircraft or overwriting the base .AIR file because the repaint archive did not include one.
Can you edit an FSX .AIR file?
Yes, but an .AIR file is binary and should not be edited in Notepad or another ordinary text editor. Use a compatible flight-model utility and back up both the .AIR file and aircraft.cfg before changing anything.
A purpose-built FSX aircraft configuration and flight-model editor is more suitable for examining or tuning aircraft behaviour. Change one area at a time: an apparently small lift, drag or stability adjustment can interact with configuration values and produce unrealistic handling.
How do you fix a missing or mismatched .AIR file?
Match the .AIR file's base name to the aircraft's sim= entry, then restore the original package if the required file is absent.
- Show filename extensions: This exposes mistakes such as
MyAircraft.air.air. - Open aircraft.cfg: Find the affected
[fltsim.x]section and read itssim=value. - Check the main aircraft folder: Confirm that a file with that base name and the
.airextension exists there, not inside a texture or model subfolder. - Identify repaint-only packages: If the download is only a livery, it is expected to use the base aircraft's existing .AIR file.
- Restore rather than substitute: Reinstall the original aircraft or recover its backup instead of copying an unrelated .AIR file. A mismatched file can make the aircraft's performance and handling completely wrong.
When the reference is broken, the aircraft may be omitted from the selector or fail to load. When it points to the wrong but valid .AIR file, the aircraft can load yet fly with incorrect acceleration, control response, stall behaviour or performance.