Fix the FSX “Aircraft not defined” error by checking saved-flight references, SimObjects paths, aircraft.cfg, .air files and model folders.
In FSX and FSX: Steam Edition, “Aircraft not defined” means the simulator cannot resolve the aircraft variant requested by a saved flight, mission or add-on. Fix the missing reference, put the aircraft in the correct SimObjects folder, or repair its aircraft.cfg, .air file and model references.
Why does FSX say “Aircraft not defined”?
FSX identifies each aircraft variant through a numbered [fltsim.x] section in aircraft.cfg, not simply by the aircraft folder’s name. Its title= value is the unique identifier used by saved flights, missions and other simulator components.
A saved .FLT file normally records that identifier on the Sim= line under [Sim.0]. If the corresponding title= entry has been removed, renamed or never loaded, FSX reports that the aircraft is not defined. A damaged definition can also fail later when sim= does not resolve to an available .air file or model= points to a missing model folder.
| When the error appears | Likely cause | Best first action |
|---|---|---|
| When FSX starts | The default flight references a removed or renamed aircraft | Select an installed aircraft and save a new default flight |
| When loading one saved flight | Its Sim= reference no longer matches an installed title= | Reinstall the original aircraft or change the saved flight to a valid title |
| After installing an add-on | Wrong destination, an extra nested folder or an incomplete package | Check the package under the active FSX installation |
| With one repaint only | Broken [fltsim.x] entry or missing base aircraft | Repair the repaint entry and install its required base package |
| With stock aircraft as well | Default files have been altered or removed | Repair or verify the FSX installation |
How do I find which aircraft reference is broken?
The quickest diagnosis is to test a known stock aircraft in a new Free Flight session. If that loads, the simulator itself is working and the fault is limited to a particular saved flight, mission, repaint or add-on package.
- Note when the message appears. Identify the flight, mission or newly installed aircraft that triggers it. Avoid changing several packages at once.
- Test a stock aircraft. Start a fresh flight rather than reopening the affected saved flight. If the stock aircraft works, concentrate on the requested add-on.
- Inspect the saved flight. Make a backup of its
.FLTfile, open the copy in a plain-text editor and find[Sim.0]followed bySim=. Compare that value exactly with the add-on’stitle=line. - Isolate a suspect package. Move its complete folder outside every FSX
SimObjectspath, then restart the simulator. Merely renaming it to something such as “backup” while leaving it underSimObjects\Airplanesmay not stop FSX scanning it.
If only an ordinary saved flight is affected, the safest fix is usually to select a working aircraft and save the flight again. Editing Sim= to match an installed title= can preserve the flight’s position and weather, but keep the original file in case the replacement aircraft behaves differently.
How do I repair aircraft.cfg and the add-on folder?
A valid aircraft package must preserve its intended folder structure and contain a usable aircraft definition. For a conventional aeroplane, aircraft.cfg should sit directly inside its package folder under SimObjects\Airplanes, unless the developer explicitly uses another registered SimObjectPaths location.
- Remove extra folder levels: avoid layouts such as
SimObjects\Airplanes\Addon\Addon\aircraft.cfg. Our FSX aircraft installation and folder-structure checks cover this common extraction mistake. - Confirm the file name: Windows may hide known extensions, leaving a file named
aircraft.cfg.txtthat looks likeaircraft.cfg. - Check each variant: every
[fltsim.x]block needs its own number and a non-empty, uniquetitle=. When adding a repaint, use the next unused section number. - Check the flight model: the value after
sim=must match the base name of the package’s.airfile. For example,sim=ExampleexpectsExample.air. - Check the model folder: a blank
model=normally uses the folder namedmodel;model=vcusesmodel.vc. That folder must contain a valid model configuration and model file. - Check repaint dependencies: a texture-only repaint is not a complete aircraft. Install it inside the required base aircraft package and add the supplied
[fltsim.x]entry as instructed.
Old installers can silently target boxed FSX even when FSX: Steam Edition is the simulator being used. Confirm the destination rather than trusting automatic detection; our explanation of FSX and Steam Edition add-on paths covers the relevant installation differences.
If the aircraft appears in the selection menu but its exterior is invisible or incomplete, the definition has loaded and the fault has moved to the model or texture stage. Use these model-reference and incomplete-aircraft checks instead of repeatedly editing the saved flight.
What if a mission or default aircraft causes the error?
A mission may require an exact aircraft title rather than any aircraft of the same type. Reinstall the aircraft or mission dependency with its original title=; substituting another model can upset cockpit instructions, camera positions, payload assumptions or mission triggers.
If only the default flight fails, choose a working aircraft, save a new flight and mark it as the default when FSX offers that option. If stock aircraft are genuinely missing or their definitions have been edited, use Steam’s file-verification facility for FSX: Steam Edition or the repair option from the boxed FSX installer. Back up customised stock files and add-ons first, because repair operations may restore original files.