Abstract: A two-way check valve for a breathing gas supply assembly, comprises a body, a first inlet in the body for receiving a normal breathing gas flow, an outlet for delivering a gas flow from the body, first passage means communicating the first inlet with the outlet, second passage means communicating the second inlet with the outlet, a valve member mounted internally of the body so as to be movable between a first position in which it closes communication between the second inlet and the outlet and a second position in which it closes communication between the first inlet and the outlet, and resilient means biasing the valve member towards the first position.

Abstract: Air-oxygen mixture control apparatus includes a pressure control valve arrangement for substantially equalizing the pressures of oxygen and air supplied to the apparatus at variable pressures. A volume flow control arrangement produces volume flows of oxygen and air appropriate for mixing to provide gas of desired oxygen concentration to a breathing demand regulator for breathing by an aircraft aircrew member. An aneroid acts to occlude the supply of air when the aircrew member is exposed to low aircraft ambient atmospheric pressure so that undiluted oxygen is made available for breathing. The apparatus overcomes the problem of mixing oxygen with air when either one or both is supplied from a variable pressure source, such as is the case when high pressure air from a compressor stage of a gas turbine engine is used for dilution purposes.