aircraft now struggling to gain speed and altitude to make even a single head-on pass against the high-speed/high-altitude IJNAF bombing runs, and even more vulnerable to the massed-firepower of those heavy bomber formations. Also by mid-1940, the Japanese had employed new tactics to stave off attacks from defending Chinese fighters; the Chinese air raid early warning network would alert the Chinese fighter squadrons to scramble their fighters towards approaching Japanese bomber formations (ideally with ample time to attain sufficient altitude ahead of the raids); however, they were being monitored by fast IJNAF C5M (IJAAF designation Ki-15) scouting-attack planes at high altitudes which would radio instructions to the bombers to circle out of range as the Chinese fighters rose to meet the bombers; once the Chinese fighters had run low on fuel and returned to base, the scouts would then redirect the bombers to attack the Chinese fighters on the ground refueling. While the Chinese soon cracked the code used by the C5M crews relaying messages to the bomber formations, some of the countermeasures used by the Chinese pilots were to launch a pair of I-16s that were almost fast enough to intercept the C5M in pursuit, but enough to force the scouts away, while a second flight of fighters were launched against returning Japanese bombers as the first flight of Chinese interceptors returned to disperse in smaller auxiliary airbases to refuel, although successful interceptions were still limited against the IJNAF bombers as the Polikarpovs struggled in particular against the new Model 22/23 G3M bombers. A C5M and three G3Ms were shot down over eastern Chongqing on 20 May 1940 by the 24th PS, the 4th PG equipped with the few (10 in total) 20mm ShVAK cannon-armed and more-powerful M-25V engined I-16 Type 17 fighters that the Chinese Air Force had; unfortunately, two of the I-16 Type 17s were put out of action following the battle, having been shot-up by the defensive fire from the Japanese raiders and making forced landings. The Japanese air raids against Chongqing had become increasingly intense and destructive as the "joint air strike force" of the Imperial Japanese Army and Navy bomber formations began to comprise upwards of 150–200 bombers per raid, under the new codename "Operation 101" (101号作战), and massed defensive machine gun and cannon fire from the bomber formations were very effective against the slow Chinese fighter aircraft further handicapped with burning low-grade fuel,