What is vacuum relief in the context of aircraft pressurization?

Vacuum relief in the context of aircraft pressurization refers to a mechanism that allows outside air to enter the cabin to prevent a situation where the cabin pressure is lower than the ambient pressure outside the aircraft. This is crucial during flight because maintaining appropriate pressure levels is essential for passenger safety and comfort.

When an aircraft climbs to high altitudes, the external air pressure decreases significantly. If the cabin pressure were to drop too low relative to the outside air, it could potentially lead to structural damage to the aircraft. The vacuum relief system acts as a safety feature, ensuring that there is always a balance between internal and external pressures. This balance is critical to avoid issues such as implosion or failure of cabin integrity.

In contrast, the other options do not accurately describe vacuum relief. Increasing cabin pressure typically refers to pressurization systems working to maintain a stable environment, while cabin overpressure prevention mechanisms are designed to release excess pressure, rather than counter low-pressure scenarios. Flushing cabin air involves different systems related to air quality, not directly associated with vacuum relief.