What is the primary function of the isobaric range?

• A. To maintain cabin pressure constant despite altitude
• B. To track outside altitude
• C. To limit cabin humidity
• D. To regulate temperature of air supply

Answer: (A)

The isobaric range is all about pressurization control. It defines the altitude span over which the cabin pressure is held essentially constant by the environmental control system, so the cabin altitude stays near the target as the aircraft climbs or descends. The system adjusts the outflow to prevent cabin pressure from following outside pressure changes, protecting occupants and the aircraft structure from large pressure swings. This function is distinct from humidity control or temperature regulation, which are handled by other subsystems. So the primary purpose is to keep cabin pressure constant despite changes in altitude.