Please instruct me on how to light myself on fire [duplicate]

Luftrausers is a quick, light, nigh-monochrome game of flying around in a configurable plane, shooting enemies, and dying quickly. During the game, there are missions you need to achieve: kill any 10 enemies, kill enemies while firing nonstop, kill specific enemies, score X points (by killing enemies), etc. Something enemy something kill something.

Then there's the current mission I'm on: kill 50 enemies while on fire. How does being on fire work? I usually manage to rack up a few of these every game, but I haven't figured out what "being on fire" means. I've tried getting to a low amount of health and then killing enemies, but that doesn't work — or at least not consistently.

So how do I set myself on fire? How long do I remain on fire, and is there something I'm doing that's putting me out?