The ultimate goal is to cut off the fire's fuel supply. Fires need heat, oxygen and fuel, and on the scale of thousands of acres, fuel is the only leg of that triangle firefighters can control. In a Type 1 incident like the High Park Fire — the worst, most complicated kind — crews bring in aerial tankers, helicopters, engines and trucks, and heavy ground-moving equipment. Black Hawks and tankers will drop water and slurry to cool the fire and knock the flames down, literally shortening their height, so ground crews can safely approach. That's a direct-line strategy, explains Greg Poncin, Type 1 incident commander in the Northern Rockies region, based in Kalispell, Mont. But sometimes, as was the case for a while Thursday, helicopters and planes can't even take off because of the smoke.