The mission of the ARVN central column was to advance down the valley of the Sepon River, a relatively flat area of brush interspersed with patches of jungle and dominated by heights to its north and the river and more mountains to the south. Almost immediately, supporting helicopters began to take fire from the heights, which allowed PAVN gunners to fire down on the aircraft from pre-registered machine gun and mortar positions. Making matters worse for the advance, Route 9 was in poor condition, so poor in fact that only tracked vehicles and jeeps could make the westward journey. This threw the burden of reinforcement and resupply onto the aviation assets. The helicopter units then became the essential mode of logistical support, a role that was made increasingly more dangerous due to low cloud cover and incessant anti-aircraft fire.