What does "Kill 50 enemies while on fire" mean?
Solution 1:
Q: What does "on fire" mean?
A: When your plane is damaged to a certain extent it starts to smoke, this is what's classed as "on fire".
Q: Is the counter reset each round?
A: No the counter is not reset.
What this objective fails to mention is that you have to kill your enemies by ramming into them. This is not at all clear and so leads to confusion.