How come I can't cross Kyrat using the buzzer?

I was flying around kyrat so I decided to go through the mountains to the fortresses not yet conquered using the buzzer. But every time I try to do that, he whistles and starts to fall. Why does it happen? Is this why I have not yet unlocked the map part or another reason?

Here is an image of the buzzer in case someone doesn't know what is it by the name.