European defence production is being asked to rise faster than at any point since the end of the Cold War, and the constraints most often counted are the visible ones: machine tools, forging and casting capacity, single-crystal castings, titanium and nickel, skilled labour, working capital. There is a further constraint that rarely appears in capacity assessments because it produces no photographs and occupies little floorspace. Before a part enters an engine it has to be accepted, and for a growing share of aero-engine hardware acceptance depends on establishing what is inside the part rather than what can be seen on its surface. Powder-metallurgy disks, additively manufactured hardware, three-dimensionally woven composite structures and cast hollow blades all share this property: the defects that matter are internal, and surface inspection cannot find them.