Even though several militaries are flying the F-35 Lightning II in combat or potential combat situations, the stealth aircraft still hasn’t completed part of its Initial Operational Test and Evaluation (IOT&E), typically a necessary step for US jets to enter full production.

One phase of the F-35’s IOT&E remains incomplete, despite the aircraft being approved for full-scale production and more than 400 of the advanced jets rolling off Lockheed Martin’s assembly line: the Joint Simulation Environment (JSE), an advanced simulator that’s lagged behind in development for years.

“The JSE is a man-in-the-loop, F-35 software-in-the-loop mission simulator intended to conduct IOT&E scenarios with modern threat types and densities that are not able to be replicated in open air,” including weather, geography and range, according to a Fiscal Year 2018 handout on the F-35 program by the Director of Operational Testing & Evaluation (DOT&E).