Getting Started with the FSX SDK

The Microsoft Flight Simulator X Software Development Kit (FSX SDK) remains one of the most comprehensive toolsets for creating custom aircraft, scenery, and other add-ons. Although the simulator was released over a decade ago, the SDK is still widely used by the flight simulation community, with many developers continuing to produce high-quality content. Whether you are a novice wanting to create your first virtual aircraft or an experienced programmer looking to build detailed scenery, understanding how to install, navigate, and utilize the FSX SDK is essential.

Begin by installing the FSX SDK. It is included with the Deluxe or Gold Edition of FSX, or available as a separate download from Microsoft’s official archive. After installation, locate the SDK folder (typically C:\Program Files (x86)\Microsoft Games\Microsoft Flight Simulator X SDK). Inside you will find subdirectories for core components such as the Core Utilities, Environment Kit, Mission Creation Kit, SimObject Creation Kit, and more. The SDK also includes extensive documentation in both HTML and PDF formats, which you should bookmark for quick reference.

Understanding the FSX SDK Components

The FSX SDK is modular, with each kit targeting a specific type of development. Below are the primary components you will use when building aircraft and scenery.

Documentation and Sample Projects

The SDK Documentation folder contains API references, configuration file specifications, and tutorials. Pay particular attention to the SimObject Creation Kit Documentation for aircraft and the Environment Kit Documentation for scenery. Sample projects – including a simple aircraft, a helicopter, and a basic scenery object – are provided to demonstrate proper file structure and compilation processes. Use these samples as templates rather than starting from scratch.

Core Tools

Key executables in the SDK include:

  • XtoMdl.exe – Converts 3D model files (in .x format) to the MDL format used by FSX.
  • BGLComp.exe – Compiles XML scenery definition files into BGL files.
  • Terrain SDK tools – For creating and editing terrain mesh, landclass, and vector data.
  • SimProp Browser – A tool for viewing and editing simulation variables (simvars) and their properties.
  • Panel and Gauge tools – Utilities for testing gauge XML scripts and creating 2D panels.

Creating a Custom Aircraft for FSX

Developing an aircraft from scratch involves several disciplines: 3D modeling, texture creation, configuration file editing, and testing. The workflow splits into modeling and configuration steps.

Modeling in 3D Software

FSX expects models in a specific format (MDL) that is derived from a DirectX .x file. The two most common modeling tools are GMax (a free, stripped-down version of 3ds Max) and Blender with the FSX plugin. Both require you to export to the .x format.

Setting Up the Development Environment

If you use GMax, ensure you install the FSX game pack that comes with the SDK. For Blender, install the Blender2FSX add-on (available from the community). Set your units to meters and align the model with FSX’s coordinate system: X is longitudinal (forward positive), Y is vertical (up positive), Z is lateral (right positive).

Basic Modeling Considerations

Keep polygon counts manageable – FSX is not as performant as modern simulators, so aim for under 50,000 triangles for a complete exterior. Use a single material per object to reduce draw calls. Name your parts logically (e.g., Propeller1, LandingLight) because these names will be referenced in the aircraft configuration files for animations and effects.

Configuring Aircraft Parameters

Once the model is exported, you need to configure how FSX interprets it. The three main files are:

  • aircraft.cfg – Defines general flight dynamics properties, engine parameters, weight, and contact points.
  • panel.cfg – Specifies which 2D gauges are displayed in the virtual cockpit.
  • model.cfg – Points to the MDL file and defines LOD (level of detail) models.

The aircraft.cfg file is the most critical. Use the sample from the SDK and modify sections such as [General] (title, category), [pitot_static] (air data), and [contact_points] (gear locations). Validate your .cfg with the SDK’s CfgEdit tool to catch syntax errors.

Applying Textures and Materials

Textures must be in DDS format (DXT1 for opaque surfaces, DXT5 for transparency). Prepare your diffuse, specular (bump), and light map textures using a paint program like GIMP or Photoshop with the NVIDIA DDS plugin. In GMax or Blender, assign a DirectX 9 material and ensure the texture names match those in your model’s .x file. After compilation, place the DDS files in a texture subfolder alongside your model folder.

Animations and Attachments

Animations such as propeller rotation, landing gear retraction, and control surface deflections are defined in the model using keyframe animation. FSX uses named animation tags (e.g., prop_anim, gear_retract). You must also add attachment points for lights, exhaust, and cargo using dummy objects with specific naming conventions (e.g., attachpt_light_nav_red). Refer to the Animation and Attachment section of the SimObject Creation Kit documentation for a full list of supported names.

Compiling with XtoMdl

Export your 3D model as a DirectX .x file. Open a command prompt, navigate to the SDK’s \SDK\Core Utilities\XtoMdl folder, and run: XtoMdl.exe -XtoMdl:keep -XtoMdl:decompile myaircraft.x The -keep flag retains intermediate files for debugging. The output is an MDL file that you place in the model folder of your aircraft package. If you have multiple LODs, compile each separately and list them in model.cfg.

Testing in the Simulator

Place the complete aircraft folder into SimObjects\Airplanes inside your FSX installation. Launch FSX, select your aircraft from the menu, and perform a quick flight. Use the FSX SimConnect client or the built-in debugging tools (e.g., SHIFT+Z to show frame rate and debug text) to verify that animations, sounds, and gauges function correctly. If the model does not appear, check the Event Viewer logs or run the SDK’s MDL Diagnose utility.

Developing Custom Scenery for FSX

Scenery development involves modifying terrain, placing 3D objects, and altering landclass to create realistic environments. The FSX Environment Kit provides all the necessary tools.

Terrain Design with the Terrain SDK

For elevation changes, create a DEM (Digital Elevation Model) file. Use the Resample tool to convert raw height data (e.g., SRTM) into a BGL file. You must supply an INF file that defines the source and output parameters. Example INF for a 1-degree tile: 

[Source]
Type = Elevation
Layer = Elevation
SourceDir = "."
SourceFile = "srtm_30m.tif"
After resampling, place the resulting BGL in Scenery\World\Scenery.

Object Placement Using BGLComp

To add buildings, trees, or airport structures, use the Object Placement SDK. Write an XML file that references library objects (either default or custom). For example: 

<?xml version="1.0" encoding="Windows-1252"?>
<FSData version="9.0">
  <SceneryObject lat="47.123" lon="-122.456" altitude="0" pitch="0" bank="0" heading="0">
    <LibraryObject name="bush_01" />
  </SceneryObject>
</FSData>
Compile the XML with BGLComp.exe to produce a BGL file. Place it in the scenery area folder, e.g., Addon Scenery\scenery.

Creating Landclass and Vectors

Landclass textures control what ground cover appears (forest, urban, water). Use the LandClass Editor tool (part of the Terrain SDK) to paint polygons on a background grid. Vector data (roads, rivers, shorelines) can be added through the VTP (Vector Terrain Polygon) system. Both landclass and vector files must be compiled with the appropriate SDK tools and placed in the scenery area.

Packaging Scenery into BGL Files

All scenery you create – whether terrain, objects, or landclass – is packaged as BGL files. Organize your add-on in a folder with a scenery subfolder for BGLs and a texture subfolder for DDS textures (if any custom textures are used). Add the area to FSX via the Scenery Library to activate it.

Debugging and Optimization

Common problems include missing textures, CTDs (crashes to desktop), and performance drops. Use the SDK’s Logbook tool to track errors during loading. Check that all DDS files are in the correct folder and that your .cfg files use consistent GUIDs and titles. For performance, limit the number of nearby scenery objects and use LODs effectively. The Frame Rate Tester in the SDK can help you measure the impact of your add-ons.

Distributing Your Add-ons

When your project is ready, package it as a zip or installer. Include a readme with installation instructions, credits, and licensing terms (e.g., freeware, open-source, or payware). Many developers host their files on Flightsim.com, AVSIM, or FlightSim.Com. Ensure you respect intellectual property – do not include default textures without modification.

Community Resources and Additional Help

No developer works in isolation. The following resources will accelerate your learning:

With patience and practice, the FSX SDK allows you to bring virtually any aircraft or location to life. Use the official documentation as your foundation, and leverage the community to overcome challenges. Happy flying and developing!