Create an FS2004 airport flatten in SBuilder, match the AFCAD elevation, compile the BGL, and fix cliffs, mesh conflicts, trees and sunk runways.
To create a flatten for a custom airport in FS2004, draw an LWM flatten polygon around the site in SBuilder, assign the airport’s exact reference elevation, compile it as an FS2004 BGL, and place that BGL in an active scenery folder. The flatten elevation and AFCAD airport elevation must match.
Which FS2004 flatten method should I use?
For a distributable custom airport, use an LWM flatten BGL rather than relying on an AFCAD altitude or a local scenery.cfg entry.
| Method | Choose it when | Limitation |
|---|---|---|
| LWM flatten BGL | Building or releasing airport scenery | Requires a terrain design tool and compiler |
scenery.cfg flatten entry | Testing privately or maintaining older scenery | Installation-specific and awkward to package |
| AFCAD elevation only | Editing airport facility data | Does not change the visible terrain mesh |
Use the FS2004 SBuilder scenery tool to draw the polygon. The official FS2004 SDK toolset and documentation provide the required scenery-development components. Set the airport facility elevation separately by using AFCAD 2.21 to edit the airport record.
How do I make the flatten polygon?
The reliable workflow is to choose one airport elevation, use it in both terrain and facility data, then test the polygon boundary from the ground and air.
- Create a dedicated scenery project. Keep the editable SBuilder source files outside the live simulator folders. Plan to place the finished BGL in a folder such as
Addon Scenery\MyAirport\scenery. - Choose the airport elevation. For a modified stock airport that is not being moved vertically, use its existing reference elevation. For a new airport, choose the intended level field elevation before drawing runways or parking.
- Check the units. FS2004 terrain tools commonly use metres, while airport references and editors may show feet. Read the unit shown by each tool and convert where necessary: one foot equals 0.3048 metres. Entering a feet value into a metre field produces an enormous plateau.
- Match the AFCAD altitude. Set the airport reference elevation to the same physical height as the flatten. Do not merely copy the same number if the two tools use different units.
- Draw one simple boundary polygon. Enclose the runway, taxiways, aprons, AI parking positions and a modest strip of surrounding ground. Avoid crossing edges, duplicate points and unnecessarily intricate outlines.
- Assign the terrain property. Set the polygon to the FS2004 LWM flatten type and enter the chosen elevation. Where SBuilder exposes per-vertex heights, give every vertex the same value for a level airport.
- Compile for FS2004. Configure SBuilder to use the required compiler, select the FS2004 target and confirm that it produces a
.bglfile. Project or source files alone have no effect in the simulator. - Install and activate the BGL. Put it in the scenery subfolder, activate the parent scenery area in the FS2004 Scenery Library and position that area above the airport scenery it replaces.
- Reload and inspect the site. Restarting FS2004 is the safest way to refresh terrain BGLs. Check every side of the airport, then load at several parking positions to confirm that aircraft and AI traffic sit on the surface.
How large should an airport flatten be?
Make the flatten large enough to support every operational surface, but no larger than the terrain requires. Drawing it exactly along a runway edge risks leaving a corner or parking stand on the original mesh; extending it across an entire valley can create an obvious mesa.
A basic level flatten has a hard boundary rather than a feathered transition. On uneven ground, shape the perimeter so it meets the surrounding mesh where that mesh is reasonably close to airport elevation. Fewer clean vertices usually produce a more predictable result than a highly detailed outline.
Why is my FS2004 flatten not working?
Most failed airport flattens come from mismatched elevations, incorrect units, an inactive BGL or another terrain polygon occupying the same area.
| Symptom | Likely cause | Correction |
|---|---|---|
| No terrain change | The scenery area is inactive, compilation failed, or the polygon was not assigned as an LWM flatten | Confirm that a compiled .bgl exists, activate its scenery area and check the polygon type |
| Runway or AI aircraft float or sink | The AFCAD and flatten elevations differ | Make the two physical elevations identical after converting units |
| Large cliff or deep trench | The flatten edge meets terrain at a very different height, or feet were entered as metres | Verify the altitude, then contract or reshape the boundary |
| Only part of the airport is level | The polygon misses part of the site, crosses itself or is competing with another flatten | Redraw a simple enclosing polygon and check other active airport or terrain packages |
| Two plateaus or stepped patches appear | Overlapping flatten polygons use different elevations | Remove the duplicate add-on flatten or use the correct terrain exclusion; do not delete stock FS2004 BGLs |
| Trees or buildings remain | A flatten changes height only | Add the appropriate autogen or object exclusion separately |
Does a flatten remove default scenery and autogen?
No. An FS2004 flatten changes terrain elevation; it does not automatically remove trees, buildings, airport facilities or ground textures. If the custom airport replaces a stock field, use the separate process for removing unwanted default airport elements, and add the required autogen or terrain exclusion in SBuilder.
An object exclusion and a terrain exclusion are not interchangeable. Removing a stock terminal may leave the original airport flatten in place, while excluding terrain shaping will not remove the terminal model.
Can an FS2004 flatten make a sloped runway?
Not with one ordinary airport flatten: it creates a level plane at a fixed elevation. Complex surrounding terrain can be built from additional terrain polygons, but FS2004 airport facilities and AI ground operations work most reliably when the runway, taxiways and parking area share one elevation.
Trying to approximate a runway slope with several narrow, differently elevated flattens commonly produces steps, wheel contact problems and visible seams. For a conventional FS2004 airport, keep the operational surface level and grade only the terrain outside it.