the recesses 33 in the second plate 31 b are compressed by the component 67 a by means of this translational movement which also involves the component 67 b the at least one locking pin 63 is released from the recess 33 in the plate 31 b in which it was engaged before translation the cushion 2 attached to the first rotary shaft 61 is thus free to pivot the pressure on said cushion continuing to compress the spring means 62 when the cushion 2 has reached a desired position the sideways pressure on said cushion is released the spring means 62 expand pushing back the first rotary shaft 61 and the revolving components 67 a 67 b until the component 67 b comes into contact with the plate 31 b at its outer surface 67 b the at least one locking pin 63 enters one of the aligned recesses 33 in the plate 31 b automatically locking the cushion 2 in position the new position becoming stable advantageously the rotational axis 69 of the first rotary shaft 61 is positioned substantially at a level of the hollow of the nape of a pilot of average size and preferably in an upper part of the nape that is to say substantially above the hollow of the.