Commonly used debugging tools

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There are some relatively simple but commonly used debugging tools that might answer questions of for example why your installation of FlightGear will not run properly or at all, why an aircraft might have problems or why flying in a certain area always cause FlightGear crashes.

The more useful of those tools are the console window (also known as the "black box" or confusingly the "command line"), the fgfs.log file and the property browser.

There are of course times when these will not do, and sometimes one would have to compile FlightGear from source and add some additional debugging features to find some bugs.

Console and startup log output

Debug levels

Before we go any further we should mention the debug level setting (also known as log level). When FlightGear runs it will output information on various things. As a default only things that are expected to cause troubles are output, but by setting a higher debug level more information will be shown. At the highest debug level one can expect FlightGear to run slow due to all the debug output.

There are five debug levels:[1]

Will output very frequent messages. Using this setting might slow down FlightGear a lot, but might show problems not otherwise found.
Will output less frequent debug messages.
Will output informatory messages.
Will output messages about possibly impeding problems.
Will output messages about very possibly impeding problems. This is the default level.

Setting the debug level


In FGRun go to the last page click Advanced... then, in the list to the left, click "Debug" and then chose the desired debug level in the list.

Command line
Note  Debug levels must be lowercase. For example debug, not Debug.

Add --log-level=debug to your command line.

The console window

Before FlightGear 2.6 this was always opened in a separate window when starting FlightGear through the FGRun wizard, but these days it is normally hidden and can be shown by ticking a checkbox, "Show command line".


The startup log file contains nearly the same information that is output on the console.

The purpose of the log file is to capture debugging output in a way that is robust for users, in essence that just work, and that capture errors that happen. That is in essence useful when a problem occur since it was already enabled.

The level defaults to info because a lower level would not contain enough detail for debugging, but a higher level, for example debug, the files would be huge for a long FlightGear session. This can of course be changed if people are happy with text files bigger than 100 MB in their $FG_HOME. Or we add a new option to set the file-log-level, but again the intention was for this feature to be as automatic as possible.[2] This feature was not yet implemented as of 2022/11/16 so logging e.g. debug log messages to the fgfs.log file is not yet possible.

Finding fgfs.log

~/Library/Application Support/FlightGear/fgfs.log
Note  Open C:\Users\user name\AppData\Roaming\ by opening the Run app, entering %AppData%, then clicking OK. [3]
C:\Users\user name\AppData\Roaming\\fgfs.log

The property browser

1rightarrow.png See Property browser for the main article about this subject.

The property browser is a tool useful for many purposes. It might for example tell if a property is tied and can not be written to.


Related content

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