Fg-optional-mp-files.bin

You will typically find fg-optional-mp-files.bin in one of two locations, depending on your operating system and how you installed FlightGear:

It often resides alongside other cache files like nasal-cache.cache, aircraft-index.cache, or in a subfolder named cache or fgcom.

Use Python’s open(), struct.unpack(), and decompression libraries based on identified compression.

If you want, I can:

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If FlightGear crashes during a write operation (e.g., sudden power loss, segfault), fg-optional-mp-files.bin can become corrupt. Corruption manifests as:

On Linux or macOS, improper file ownership can lock the file. You might see Permission denied when FlightGear tries to update the cache, forcing it to rebuild a temporary cache that never persists between sessions.

If you have spent any time navigating the depths of your FlightGear installation directory or its associated cache folders, you may have stumbled upon a peculiar file named fg-optional-mp-files.bin. At first glance, it looks like a binary ghost—neither an executable nor a standard scenery file. For many users, this file becomes a source of confusion, especially when it grows to several gigabytes or appears to cause crashes. You will typically find fg-optional-mp-files

This article provides a deep dive into what fg-optional-mp-files.bin is, why FlightGear creates it, whether you can delete it, and how to troubleshoot common issues related to it.

Over months of multiplayer flying, fg-optional-mp-files.bin can bloat to 2GB, 5GB, or even 10GB. This happens because:

Symptoms: Low disk space warnings, longer launch times (ironically), or failure to write new cache data due to filesystem limits. It often resides alongside other cache files like

Never delete or modify cache files while FlightGear is running.

For the technically curious, you can inspect fg-optional-mp-files.bin using tools like hexdump (Linux/macOS) or a hex editor (HxD on Windows). Most of it will be unintelligible, but you may see plaintext snippets like:

/models/737-800.xml
livery_united.png
fgcom_voice_effects.cache

These fragments confirm that the file is indeed a compiled index of multiplayer assets.

Warning: Do not attempt to manually edit the binary file. Any modification will invalidate its internal checksums, causing FlightGear to ignore the entire cache.