If you approach the cliff with a full load of rockect ammo, the Apache never appears. Seems the sequence triggers off the pickup of the rocket launcher in the room with the turrets.
It’s designed to show up after you pick up the RPG. That’s when the boss battle starts.
who hacked themself an RPG 
It’s my second time through some select chapters. I did impulse 101 which is probably the issue here. It was just odd that I had to deplete a rocket, go back in and touch the launcher to get the sequence going.