The Australian Defence Force has operated Boeing CH-47 Chinook heavy-lift helicopters for most of the period from 1974. Twelve CH-47C Chinooks were acquired initially, entering service with the Royal Australian Air Force in December 1974. The eleven surviving aircraft were retired in 1989 as a cost-saving measure, but it was found that the Australian Defence Force's other helicopters could not emulate their capabilities. As a result, four of the CH-47Cs were upgraded to CH-47D status, and returned to service in 1995 with the Australian Army. The Army acquired two more CH-47Ds in 2000 and another pair in 2012. The CH-47Ds were replaced with seven new CH-47F aircraft during 2015, and another three were delivered in 2016.