The aircrack-ng package has been removed from the official package tree. Installing aircrack-ng on Ubuntu 12.04 is however straightforward.
First checkout the latest aircrack-ng source code via SVN:svn co http://trac.aircrack-ng.org/svn/trunk/ aircrack-ng
Then, within the “aircrack-ng” directory, run the following commands:makesudo make install