In this undated file photo, China's Y-20 heavy transport aircraft takes part in an airborne and air delivery training. The Chinese Air Force announced a roadmap for building a stronger modern air force in three steps. The building of a stronger modern air force is in line with the overall goal of building national defense and the armed forces, Lieutenant General Xu Anxiang, deputy commander of Chinese Air Force, said at a press conference on celebrating the 69th anniversary of the establishment of Chinese Air Force held in Zhuhai, south China's Guangdong Province, Nov. 11, 2018. (Xinhua)