“This concurrent testing is due to the Joint Program Office working to achieve Milestone C, which means the F135 will be finished with its SDD phase and move on to production.”
Source: Mike Dent, deputy director of the Aeropropulsion Combined Test Force at AEDC, commenting on the surge in testing at the Arnold Engineering Development Complex Engine Test Facility at Arnold AFB in southeastern Tennessee, as the System Development and Demonstration Phase for the F135, engine for the F-35 Lightning II Joint Strike Fighter, comes to an end.