The aperture delimiter (4) has a support arm (6) that includes an engagement recess which opens a passage opening. A test arrangement is coupled in the recess with a wing-side coupling bolt remote from the arm. A driving rod (13) is displaceable in the recess in several switching positions. A barrier is arranged in the recess at the free end of the arm and opened to the opening to permit engagement of the rod in the recess and to restrict disengagement of the rod after passing through the barrier.
