The Australian government expects its plans to field a sixth Boeing C-17 will double the number of operational airlifters available for overseas duty to four from two.

Australia had long been planning to commit to a sixth C-17, but Defense Minister Stephen Smith formalized those plans on March 19, noting the aircraft will arrive next year. In announcing the procurement plan, the Australian defense department notes “a sixth C-17 will give the government increased options to support a wider range of contingencies that might require heavy-lift aircraft and will extend the life of the C-17 fleet by reducing the use of each aircraft.”

It is the second top-up order Boeing has secured for the C-17 this year, with the U.K. also adding another airlifter. The aircraft maker is trying to build up enough additional orders to keep the C-17 production line open in the absence of additional U.S. purchases. To ease that process, Boeing has reduced annual production to 10 aircraft per year, so the line can remain open until 2014.