Since the exact configuration of the tubes is not specified this rule is intended to limit the size of object which can intrude into the driver’s cell.
AF6.1 An impactor is defined as a circular disk with diameter of 254 mm (10 inches). The thickness is not relevant, but will generally be around 2mm (0.080 inches) for the inspection process.
AF6.2 The primary structure between the front bulkhead and main roll hoop must not allow the impactor to enter the primary structure.
AF6.3 Anywhere on the structure where the impactor is attempted to be passed through the impactor must contact the structure in at least three points. This is not a projection requirement but a full 3-dimensional requirement