Did you get past this point? cause i'am having the same problem, cargo plane is in full health
until it turns right up the canyon then it crashes! I have played it dozens of times for exactly the same result. Sometimes I have the sky clear of planes then as soon as the cargo plane turns up the canyon enemy planes come from
every where and there is no way you can shoot them all down before the cargo plane crashes.
I am starting to think there is some sort of glitch in my game (PS3).
You might have to shoot down the planes quick enough, I use the DO 335, try using the increased damage cheat