Alright. This is pretty simple.
The zombies will pretty much always swarm the helicopter when evac arrives but there is a simple fix to this.
While not directly stated, flare grenades are incredibly useful. they act as hoard bait, inviting zeds over to them.
You can use this to your advantage by throwing some away(but still in the area of) the helicopter, giving you enough time to escape.

give me steam points.