@Estel, this is fixed now when using KP51, it was a problem with the load / unload script for the injection drivers provided by the original author (lxp). Anyway, if you have KP51, you need to use the script from here to use the built-in injection driver modules for KP51. All this, of course, is already part of fAircrack v0.50, which will be released when I finally get time to package it all up. Getting closer now, because of the excellent post from here. So, run the script and it will toggle the load/unload of the KP51 modules and you should be able to use fAircrack's other functionalities.