How do I fix the ‘Model not defined’ error in FSX?

In FSX and FSX: Steam Edition, a “Model not defined” error means an aircraft’s aircraft.cfg cannot resolve its model folder or the model.cfg inside it. Correct the affected model= entry, restore the matching model folder and .mdl files, or reinstall an incomplete aircraft or repaint package.

Fix the FSX model error step by step

The quickest reliable fix is to trace the affected livery’s model= entry through to the actual model files.

1. Close FSX and identify the affected aircraft. If the message appeared after installing a repaint or aircraft, start with that package. When the culprit is unclear, temporarily move recently added aircraft folders out of SimObjects, then return them in small groups until the error returns.
2. Confirm that you are editing the active installation. Boxed FSX and FSX: Steam Edition use the same aircraft structure, but both may be installed on one computer. Our guide to finding the active FSX aircraft folder helps avoid repairing an unused copy.
3. Back up aircraft.cfg. Copy the file before changing it. If you are unfamiliar with its sections, follow our advice for editing aircraft.cfg without damaging the package.
4. Find the affected [fltsim.x] section. Locate the livery named in the error or the one most recently added, then inspect its model= line. Do not change every livery at once.
5. Match model= to a real folder. An empty model= value uses the folder named model. A value such as model=vc requires a folder named model.vc in the same aircraft directory.
6. Check model.cfg and the .mdl files. Open the selected model folder. It must contain model.cfg, and the file named by its normal= entry must exist as an aircraft model. An interior= entry, when present, must also resolve to the correct virtual-cockpit model.
7. Restore or reinstall missing files. If the model folder is absent, empty or incorrectly nested, extract the original package again. Reinstall the required base aircraft before adding a repaint that depends on it.

Test FSX after each correction. There is normally no reason to delete fsx.cfg or reinstall the whole simulator for this error.

What should the model= entry point to?

The value after model= is a folder suffix, not the name of an .mdl file.

A blank value can therefore be correct. Do not replace it merely because nothing follows the equals sign. If the model= line is missing and the aircraft uses a standard model folder, add model= to that specific [fltsim.x] section.

Do not enter model.cfg, an .mdl filename or a complete Windows path on this line. For a repaint, compare the entry with a working livery of the same aircraft variant; different values may deliberately select another engine, wing or cockpit model.

Why can a repaint cause “Model not defined”?

A repaint can trigger the error when its supplied [fltsim.x] block requests a model variant that is not installed.

• The repaint requires a separate base aircraft or expansion model.
• Its model= value contains a typo or refers to the wrong variant.
• Only the texture folder was installed, while a required model folder was omitted.
• The package was extracted with an extra directory level, leaving the model folder below the aircraft root.

Copy a model= value from another livery only when both liveries are intended for the same model variant. Using an unrelated value may remove the error but produce missing engines, incorrect animations or an invisible virtual cockpit.

What if the model folder already exists?

A present model folder is not enough; FSX must also be able to read its configuration and resolve every required model reference.

• Confirm the file is really named model.cfg, not model.cfg.txt.
• Check that it contains a valid [models] section and a normal= reference.
• Match the referenced filename to the actual .mdl file. Do not borrow an unrelated model file from another aircraft.
• If normal= or interior= uses a relative path to another aircraft, make sure that required base package is installed in the expected location.
• Check whether security software quarantined a file during extraction, then restore the package only from its original trusted archive.

If the error disappears but the aircraft is transparent, lacks a cockpit or shows only some components, continue with our checks for diagnosing an invisible or incomplete aircraft model.

Finally, check the exact wording. If FSX says “Aircraft not defined” rather than “Model not defined”, the problem may instead be a broken aircraft definition or a saved flight referring to a missing title=; that separate error has its own aircraft-definition repair steps.