What does FIBER_FRAME_TIME_FRACTION do in FSX, and should I change it?
Learn what FIBER_FRAME_TIME_FRACTION does in FSX, when changing 0.33 helps, and how to avoid blurry terrain, lost FPS and ineffective tweaks.
In FSX and FSX: Steam Edition, FIBER_FRAME_TIME_FRACTION controls how much main-thread time FSX gives its background fibre jobs, chiefly terrain and scenery loading, relative to rendering. Leave the default 0.33 alone unless you are diagnosing blurry terrain or deliberately trading texture-loading speed for frame rate; change it only in small, measured steps.
What does the 0.33 value mean?
The default 0.33 tells FSX to provide roughly one unit of fibre-processing time for every three units spent rendering. It does not reserve 33 per cent of the whole processor, assign one-third of every CPU core or alter scenery quality directly.
Reducing the value favours frame rendering. That can improve frame rate when FSX is limited by its main thread, but terrain textures may take longer to sharpen and scenery may appear late. Raising it gives fibre jobs more opportunity to catch up, potentially at the cost of frame rate or smoother frame delivery.
The setting does not increase texture resolution, change the level-of-detail radius or cure every form of stuttering. Our fuller explanation of FSX fibre timing and CFG trade-offs covers how this parameter fits into the simulator’s ageing rendering system.
Should I lower or raise FIBER_FRAME_TIME_FRACTION?
Most users should keep 0.33; change it only when a repeatable test shows that FSX needs a different balance between rendering and scenery loading.
| Setting | When to use it | Likely trade-off |
|---|---|---|
0.33 | Normal starting point and troubleshooting baseline | FSX’s default balance |
Below 0.33, such as 0.25 | FSX is clearly CPU-limited, but terrain already loads promptly | Possible FPS gain, with greater risk of blurred or delayed scenery |
Above 0.33, such as 0.40 | Terrain cannot keep up even though frame rate has headroom | Faster scenery loading, with a possible FPS or smoothness penalty |
A very low value is not a universal performance fix. Fast aircraft, detailed add-on scenery and high terrain settings all need loading time; starving the fibre jobs can leave ground textures blurred long after they enter view. An SSD may reduce storage delays, but it cannot remove the CPU scheduling work.
How do I change the setting safely?
- Close FSX completely. Editing the file while the simulator is running risks having your change overwritten on exit.
- Find the active FSX.cfg. Boxed FSX normally uses
%APPDATA%\Microsoft\FSX\FSX.cfg. Steam Edition may use either that folder or%APPDATA%\Microsoft\FSX-SE\FSX.cfg, depending on whether boxed FSX has coexisted on the same Windows profile. - Make a backup copy. This gives you a known working configuration without forcing FSX to rebuild every setting.
- Edit the existing MAIN section. Add or change one line under
[MAIN], for exampleFIBER_FRAME_TIME_FRACTION=0.25. Do not create a second[MAIN]section or leave duplicate versions of the setting. - Test one value at a time. Use the same aircraft, location, weather, scenery and frame-rate mode. Check both frame rate and how quickly terrain ahead of the aircraft becomes sharp.
- Restore the default if necessary. Set it back to
0.33, or remove the line and allow FSX to use its implicit default.
Do not change LOD_RADIUS, texture settings and several unrelated CFG entries during the same comparison. You will not know which alteration caused the result.
Why does changing it sometimes make no difference?
A change may appear ineffective when fibre scheduling is not the actual bottleneck.
- The wrong configuration file was edited. This is common with Steam Edition installations that have used both
FSXandFSX-SEfolders. The active file’s modified time should update after FSX closes. - The system is GPU-limited. Shifting main-thread time will not fix a graphics-card bottleneck caused by resolution, anti-aliasing, shadows or demanding clouds.
- The frame-rate arrangement changed. Unlimited rendering, FSX’s internal limiter and an external limiter create different scheduling conditions. Test them separately rather than copying a value selected for another setup.
- The test is too static. Sitting at one airport after all scenery has loaded reveals little. Assess texture recovery while moving across detailed terrain.
- Another setting is overwhelming the loader. Excessive scenery complexity, autogen, traffic or terrain detail can consume any benefit gained from this single parameter.
What should I do if FSX terrain remains blurry?
Return FIBER_FRAME_TIME_FRACTION towards 0.33 before applying more tweaks, then reduce the workload through FSX’s normal display and scenery controls. Our settings-first FSX performance checklist explains how to identify CPU and graphics bottlenecks without stacking old CFG recipes.
Keep a changed value only when it produces a visible, repeatable improvement without delayed textures. If 0.33 provides acceptable frame rate and terrain stays sharp, there is no practical reason to alter it.