What exactly is lost by flashing the FS?

[Beep88]

one thing that bothers me all the time is being asked if I want to start the phone in flight or normal mode. Can I put the default option to disable in my custom pack, is that possible? Where can I learn to do these things?

Within the file customize.xml found inside the custpack (try to read and understand the various variables, most of them are self-explanatory):

<mmi>
<settings>
<pictures-view-mode>3x3-thumbnails</pictures-view-mode>
<camera-pictures-view-mode>3x3-thumbnails</camera-pictures-view-mode>
<video-thumbnails-view-mode>3x3-thumbnails</video-thumbnails-view-mode>
<flight-mode-option-at-startup>false</flight-mode-option-at-startup>
<sleep-mode-clock>true</sleep-mode-clock>
<enable-national-characters>true</enable-national-characters>
</settings>
</mmi>