I debugged the film and everything was working as expected, it's just a lucky distribution of hits for the guy in the first plane. You hit him with 9 .50s and 21 .30s. No 37mm hit. You smoked his engine and wounded the pilot and came within a sneeze of exploding him. The second plane you hit with 2 .30s, 2 .50s and a 37mm. The 37mm knocked off his tail.