What controls the cabin pressure in an aircraft?

The control of cabin pressure in an aircraft is primarily managed through the outflow valves. These valves play a critical role in regulating the air pressure within the cabin by allowing excess air to escape. During flight, as the aircraft climbs to higher altitudes, the external air pressure decreases, and it is crucial to maintain a comfortable and safe pressure level inside the cabin for the safety and comfort of passengers and crew.

As the aircraft ascends, the cabin pressurization system operates by sealing the cabin and then utilizing the outflow valves to release just the right amount of air to achieve and maintain the desired cabin pressure. If the cabin pressure were to increase too much, the outflow valves open to let some of the air out, thus preventing over-pressurization. Conversely, during descent, the outflow valves may close or open less to allow for controlled cabin pressure changes in response to the increasing outside pressure.

This finely tuned mechanism is essential for ensuring that passengers experience a stable environment throughout the flight, making the outflow valves critical in cabin pressure management. The other options, while related to the cabin air system, do not primarily control the cabin pressure in the same direct and crucial manner.