Some years ago I composed from different manuals, regulations, books a list with distances a plane will be visible to a pilot of the other plane. Of course it is the very averaged values because very much depends on the pilot sight, weather, the observation angle and so on but anyway. Later I lost that list somewhere, and now decided to recollect it. Here is what I can only remember a little:

Piston engine fighter – 4000 km
Jet fighter 5000 km
Piston engine bomber – 12000 km
Jet bomber – 16000 km

Does anyone any idea about it?