Hey TH,
Not sure if I understand you OK... So I'll give you an example of how I'd do it...
Place a repeating trigger and call it DEADZONE
Make the condition, EAST/WEST/RES PRESENT, depending on the side the heli and tank belong to.
In the ON ACT field put the following;
"_x setdamage 1" foreach unit (list DEADZONE)
Think this should work, but not certain the syntax is 100%... Hope this helps...