we done two projects where we've used coolant and (external) radiators ... just like old liquid cooled engines.
As helpful as Will's links are certain to be in giving you information about cooling, I doubt they answer your question (as I read it) ... "when should I use passive cooling as opposed to liquid cooling ?".
First, aircraft and spacecraft are two very different things. Spacecraft are, for the most part, swimming in a huge heat sink; often the question is "how do we stay hot ?". Admittedly there are also environments (like the Shuttle bay, or other things in low earth orbit) that go from +100degC to -100degC and back every 90 minutes. Airplanes are obviously very different. So please don't say "electronics cooling for aircraft and spacecraft" ... unless you're after design approaches for those two different situations. In which case, I'd ask ... "is this for work ?"
another day in paradise, or is paradise one day closer ?