Which of the following is NOT an operating range of the pressurization system?

• A. Unpressurized
• B. Isobaric range
• C. Differential pressure range
• D. Positive pressure range

Answer: (D)

When analyzing how a pressurization system operates, you look at how cabin pressure relates to outside air. There are three recognized operating states: unpressurized (no cabin pressurization), isobaric (the cabin pressure is kept at a constant target as altitude changes), and differential pressure range (the system controls the difference between cabin pressure and outside air, within safe limits for the aircraft structure).

The term positive pressure range isn’t a standard operating range. In flight, the cabin is brought to a higher pressure than outside, which is a result of maintaining a differential pressure, but that state is handled within the differential pressure control. It isn’t described as its own separate range.

So, the not-an-operating-range option is the one that isn’t used as a distinct range: positive pressure range.