What are the three modes of operation for the mask oxygen system?

The correct answer identifies the three operational modes of the mask oxygen system as Normal, 100%, and Emergency. This classification is important for understanding how the system adapts to different situations that may arise during flight.

In "Normal" mode, the system provides oxygen at a continuous flow rate suitable for average needs during routine situations, such as cruising at altitude where supplemental oxygen might be necessary but not critical. The "100%" mode is designed to deliver a pure oxygen supply, typically used in more severe situations such as during rapid decompression or when crew members are at higher altitudes requiring supplemental oxygen at its maximum efficacy to prevent hypoxia. Lastly, the "Emergency" mode typically delivers oxygen in a way that ensures the best possible concentration of oxygen is available to the person using the mask during critical scenarios. This mode would be activated in life-threatening conditions where quick and effective oxygen delivery is paramount.

Understanding these modes is crucial for flight safety, as they dictate how crew and passengers receive oxygen under various stress conditions that can occur during flights, particularly at high altitudes where the air is thin.