In the 1990s, when the Chinese air force began introducing hundred of modern jet fighters, they had to decide which air divisions would get them. They decided to assign the new Russian (J-11, which are Russian Su-27s) and Chinese (J-8) aircraft to those divisions that had the best performance and maintenance records. So the air divisions getting the new fighters also have the best officers, and performance. Because each air regiment has about the same number of maintenance personnel (150-200), and the new aircraft required more effort to keep them flying, the regiments with new planes have fewer of them. That means Su-27/Su-30 regiments have two squadrons (eight aircraft each), while J-8 regiments have three squadrons, and regiments with older aircraft (usually J-7, a MiG-21 clone) have four squadrons. Each regiment has some spare aircraft (three for J-11 regiments, four for J-8 regiments and only two for J-7 regiments.) Each regiment also has four two seat trainer versions of their aircraft, plus up to a dozen miscellaneous older aircraft (station hacks) for transportation and training. These include two or more CJ-6 turboprop transports and some older MiGs.