A control assembly for a breathing mask for an aircraft crew member, said assembly comprising: - a support, - a mode selection button capable of being switched between at least a first position (EMER), a second position (100%) and a third position, the second position (100%) being positioned between the first position (EMER) and the third position, - a controller intended to be supplied by a source of breathing gas and adapted for supplying a breathing cavity at least in the following three operating modes: - when the mode selection button is in the first position (EMER), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a first pressure relative to the ambient pressure, - when the mode selection button is in the second position (100%), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a second pressure relative to the ambient pressure, the first pressure being greater than the second pressure, - when the mode selection button is in the third position, the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that the mode selection button includes a base pivotally mounted on the support relative to an axis of rotation, and a cover carried by the base; said axis of rotation being perpendicular to the support and extending in an axial direction; said cover being movable in the axial direction relative to the base between a locking position of the mode selection button and an unlocking position of the mode selection button, the locking position locking the rotation of the mode selection button from the second position (100%) to the third position, the unlocking position authorising the rotation of the mode selection button from the second position (100%) to the third position. - a support, - a mode selection button capable of being switched between at least a first position (EMER), a second position (100%) and a third position, the second position (100%) being positioned between the first position (EMER) and the third position, - a controller intended to be supplied by a source of breathing gas and adapted for supplying a breathing cavity at least in the following three operating modes: - when the mode selection button is in the first position (EMER), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a first pressure relative to the ambient pressure, - when the mode selection button is in the second position (100%), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a second pressure relative to the ambient pressure, the first pressure being greater than the second pressure, - when the mode selection button is in the third position, the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that the mode selection button includes a base pivotally mounted on the support relative to an axis of rotation, and a cover carried by the base; said axis of rotation being perpendicular to the support and extending in an axial direction; said cover being movable in the axial direction relative to the base between a locking position of the mode selection button and an unlocking position of the mode selection button, the locking position locking the rotation of the mode selection button from the second position (100%) to the third position, the unlocking position authorising the rotation of the mode selection button from the second position (100%) to the third position. - when the mode selection button is in the first position (EMER), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a first pressure relative to the ambient pressure, - when the mode selection button is in the second position (100%), the controller supplies the breathing cavity as long as the pressure in the breathing cavity is not greater than a second pressure relative to the ambient pressure, the first pressure being greater than the second pressure, - when the mode selection button is in the third position, the controller supplies the breathing cavity with breathing gas diluted with air, characterised in that the mode selection button includes a base pivotally mounted on the support relative to an axis of rotation, and a cover carried by the base; said axis of rotation being perpendicular to the support and extending in an axial direction; said cover being movable in the axial direction relative to the base between a locking position of the mode selection button and an unlocking position of the mode selection button, the locking position locking the rotation of the mode selection button from the second position (100%) to the third position, the unlocking position authorising the rotation of the mode selection button from the second position (100%) to the third position.