Eighteen test vehicles, more than 5,000 sorties, and 10,000 flight test hours are just a few ways to sum up the testing planned for the System Development and Demonstration, or SDD, phase of the F-35 program. Thirteen flyable aircraft and five ground test airframes will be flown, pushed, poked, and prodded during this phase. The flyable aircraft fall into two basic categories: flight sciences and mission systems.