<p>They crash randomly like that in all open world games where aircraft can freely spawn in the sky.
</p><p>This also happens in GTA. The large GTA aircraft spawn too high to hit anything, but GTA San Andreas spawns small planes at lower altitude. Just like in JC2, the planes do see what's in front of them, but only for a short distance, so they usually can't react to high vertical walls.
</p><p>This isn't really a glitch. It's just the way the game is made. In the mountains, the ground raises gradually and you can see how they react at the last moment and usually manage to get away.
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