How far away can you hear a helicopter?

[Legbreaker]

Of course in a T2K type scenario where there are few to no vehicles moving about (even if they are available, fuel might restrict them to just an hour or so of operation a day), the sound of a helicopter will attract a lot of attention. It's something so out of the ordinary that even the slightest indication of their presence will cause people to sit up and take notice, even if just on a subconscious level.

All in all though, it comes back to the individual situation. There's just too many factors to account for.