Read this: In order to enlist Stalin's support, Zhou and a Chinese delegation left for Moscow on 8 October, arriving there on 10 October at which point they flew to Stalin's home at the Black Sea. There they conferred with the top Soviet leadership which included Joseph Stalin as well as Vyacheslav Molotov, Lavrentiy Beria and Georgi Malenkov. Stalin initially agreed to send military equipment and ammunition, but warned Zhou that the Soviet Union's air force would need two or three months to prepare any operations. In a subsequent meeting, Stalin told Zhou that he would only provide China with equipment on a credit basis, and that the Soviet air force would only operate over Chinese airspace, and only after an undisclosed period of time. Stalin did not agree to send either military equipment or air support until March 1951. Mao did not find Soviet air support especially useful, as the fighting was going to take place on the south side of the Yalu. Soviet shipments of matériel, when they did arrive, were limited to small quantities of trucks, grenades, machine guns, and the like.
Now answer this question, if there is an answer (If it cannot be answered, return "unanswerable"): Where did the fighting take place that rendered Soviet air support useless to the Chinese?
the south side of the Yalu