An aircraft air supply system may include a primary duct to supply a primary air flow to a flight deck of an aircraft. A nitrogen generating system may be configured for generating nitrogen enriched air and oxygen enriched air. A secondary duct may be provided for channeling the oxygen enriched air from the nitrogen generating system to the primary duct. The flow of the oxygen enriched air into the primary duct and to the flight deck may be controlled to reduce an effective altitude of the flight deck.