Well that's exactly the point...the helicopter was evacuating casualties and was completely unprepared to get shot by at.
And in this case the helicopter is going after them anticipating all the possible ways in which they can be engaged.
And for Mi-17s you already have the option to put an impressive amount of applique armour
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As for terrain and maneuvering, this is what humans look like on any decent FLIR from even kilometres away
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You can engage them from quite a distance