The British concept of a light aircraft mixing all the roles that required extensive communication with land forces: reconnaissance, liaison, artillery spotting, aerial supply, and, last but not least, occasional strikes on the battlefield. The concept was similar to front-line aircraft used in the World War I, which was called the CL class in the German Empire. Eventually the RAF's experience showed types such as Westland Lysander to be unacceptably vulnerable and it was replaced by faster fighter types for photoreconnaissance, and light aircraft for artillery spotting.