at the top of the editor there's a "triggers" button. click it.
now double click on the map. the triggers dialog will appear.
make axis a 100, axis b 3000 and the angle 90. leave it as an ellipse.
leave all the activation stuff.
leave the next bit as well.
at the bottom where it says 'on activation', type
chopper_name setfuel 0
click ok.
now you have a huge trigger. if you click and hold on it, and press the shift key, you can move the mouse around to change its angle to whatever you want if it's not correct.
click and drag the trigger to wherever you need it on the map.
at the top of the editor there's a "groups" button. click it.
click and hold on the trigger. now drag the mouse to the chopper. once the mouse is over the chopper, let go. there should be a blue line connecting the trigger and the chopper, meaning they're grouped.
save and preview. the chopper's fuel will be set to 0 once it enters the trigger.