A control unit (1, 100, 101, 102, 103, 104, 105, 106, 107) for an aircraft crew member's breathing mask, said unit comprising: - a holder, - a mode-selection knob (20, 200, 201, 202, 203, 204, 205, 206, 207) mounted with the ability to pivot on the support with respect to an axis of rotation, the axis of rotation being arranged perpendicular to said support, the said mode-selection knob (20, 200, 201, 202, 203, 204, 205, 206, 207) having a bottom extending perpendicular to the axis of rotation 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, - 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 (20, 200, 201, 202, 203, 204, 205, 206, 207) 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 (20, 200, 201, 202, 203, 204, 205, 206, 207) 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 (20,200, 201, 202, 203, 204, 205, 206, 207) is in the third position, the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that it comprises at least one locking/unlocking system (30, 300, 301, 302, 303, 304, 305) suitable for locking and unlocking the rotation of the mode-selection knob (20, 200-207) towards the third position, the locking/unlocking system (30, 300, 301, 302, 303, 304, 305) comprising an operating member (31, 310, 311, 312, 313) 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 (20, 200, 201, 202, 203, 204, 205, 206, 207) mounted with the ability to pivot on the support with respect to an axis of rotation, the axis of rotation being arranged perpendicular to said support, the said mode-selection knob (20, 200, 201, 202, 203, 204, 205, 206, 207) having a bottom extending perpendicular to the axis of rotation 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, - 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 (20, 200, 201, 202, 203, 204, 205, 206, 207) 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 (20, 200, 201, 202, 203, 204, 205, 206, 207) 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 (20,200, 201, 202, 203, 204, 205, 206, 207) is in the third position, the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that it comprises at least one locking/unlocking system (30, 300, 301, 302, 303, 304, 305) suitable for locking and unlocking the rotation of the mode-selection knob (20, 200-207) towards the third position, the locking/unlocking system (30, 300, 301, 302, 303, 304, 305) comprising an operating member (31, 310, 311, 312, 313) 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.