once you have enough experience on the 20 and as and when your company sees it fit for you to upgrade , you undergo a CCQ course on the 30 / 40 3 days ground school , exam is the same as a normal type !!!! 3 days FFS plus one check , after that its Low Viz + ZZFT go back as per your company requirements plus LOFT and one line check later voila . . . . you can fly the slugs
