A crash attenuation system for an aircraft, the system having an airbag carried by the aircraft and inflatable generally adjacent an exterior of the aircraft. The airbag has at least one vent for releasing gas from the interior of the airbag. A gas source is in fluid communication with the interior of the airbag for inflating the airbag with gas generated provided by the first gas source. A vent valve is provided for controlling a flow of gas through each vent, each vent valve being selectively configurable between an open state, in which gas can pass through the associated vent from the interior of the airbag, any number of intermediate states, in which the vent is partially open, and a closed state, in which gas is retained within the interior of the airbag.