 The final major derivative was the 707-320, which featured an extended-span wing and JT4A engines, while the 707-420 was the same as the -320, but with Conway turbofan engines.British certification requirements relating to go-arounds with one engine inoperative also forced Boeing to increase the height of the tail fin on all 707 variants, as well as add a ventral fin, which was retrofitted on earlier -120 and -220 aircraft.These modifications also aided in the mitigation of Dutch roll by providing more stability in yaw.