Commercial Pilot License (CPL) Practice Exam

The function of the vacuum relief valve in the cabin air pressure system is primarily to manage cabin pressure and ensure it does not drop below ambient pressure, particularly during abrupt changes in altitude. When the cabin experiences a decrease in pressure, the vacuum relief valve allows outside air to enter the cabin, ensuring that pressure levels remain stable and passengers are not subjected to uncomfortable or potentially dangerous conditions. In this scenario, given the answer choice selected, while it might seem reasonable that allowing outside air to enter serves some function, the primary role of the vacuum relief valve is more about maintaining a balance rather than simply admitting outside air. Therefore, the more accurate understanding is that the vacuum relief valve works to ensure that cabin pressure does not exceed ambient pressure, which is essential for maintaining safety and comfort in the aircraft. This function mitigates the risk of cabin structural failure due to excessive pressure differences. The vacuum relief valve's role as a regulator that prevents excessive pressure build-up highlights its critical importance in aircraft design and safety features.