A control unit for an aircraft crew member's breathing mask , said unit comprising: - a holder , - a mode-selection knob mounted with the ability to pivot on the support with respect to an axis of rotation (Z), the axis of rotation (Z) being arranged perpendicular to said support , the said mode-selection knob having a bottom extending perpendicular to the axis of rotation (Z) and a peripheral rim , the said selection knob being able to pivot between at least a first position (EMER), a second position (100%) and a third position (N), - a controller intended to be supplied from a source of breathing gas and adapted to supply a breathing cavity in at least three of the following modes of operation: - when the mode-selection knob is in the first position (EMER), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not higher than a first pressure relative to the ambient pressure, - when the mode-selection knob is in the second position (100%), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not higher than a second pressure relative to the ambient pressure, the first pressure being higher than the second pressure, - when the mode-selection knob is in the third position (N), the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that it comprises at least one locking/unlocking system suitable for locking and unlocking the rotation of the mode-selection knob (20, 200-207) towards the third position (N), the locking/unlocking system comprising an operating member adapted to be pivoted between a position locking the rotation of the mode-selection knob and a position unlocking the rotation of the mode-selection knob. - a holder , - a mode-selection knob mounted with the ability to pivot on the support with respect to an axis of rotation (Z), the axis of rotation (Z) being arranged perpendicular to said support , the said mode-selection knob having a bottom extending perpendicular to the axis of rotation (Z) and a peripheral rim , the said selection knob being able to pivot between at least a first position (EMER), a second position (100%) and a third position (N), - a controller intended to be supplied from a source of breathing gas and adapted to supply a breathing cavity in at least three of the following modes of operation: - when the mode-selection knob is in the first position (EMER), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not higher than a first pressure relative to the ambient pressure, - when the mode-selection knob is in the second position (100%), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not higher than a second pressure relative to the ambient pressure, the first pressure being higher than the second pressure, - when the mode-selection knob is in the third position (N), the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that it comprises at least one locking/unlocking system suitable for locking and unlocking the rotation of the mode-selection knob (20, 200-207) towards the third position (N), the locking/unlocking system comprising an operating member adapted to be pivoted between a position locking the rotation of the mode-selection knob and a position unlocking the rotation of the mode-selection knob. <EOS>