As a regional airline first officer, when should you put on your oxygen mask if your captain leaves the cockpit?

The correct answer emphasizes the importance of maintaining safety and adhering to regulations regarding oxygen use in the cockpit. At altitudes above Flight Level 150 (FL150), it is mandatory to use supplemental oxygen due to the decrease in cabin pressure and the associated risk of hypoxia.

In the event a captain vacates the cockpit, the first officer is responsible for ensuring that the flight operates safely. Since the regional airline operates at altitudes where the risks associated with oxygen deprivation become significant, it is prudent to don the oxygen mask to mitigate any potential risks.

Other choices may imply different scenarios that do not align with established protocols for oxygen use in aviation. For instance, relying solely on emergencies (when oxygen is typically expected to be utilized) overlooks the preventative measures that should be taken at critical flight levels. The option suggesting wearing the mask only upon the captain's departure does not consider the physiological effects of altitude. Lastly, waiting for direction from ATC concerning oxygen use would not be a proactive safety measure; pilots are trained to manage their oxygen requirements independently based on altitude and flight safety protocols.

Thus, the proper action is to wear the oxygen mask at altitudes above FL150, which aligns with aviation safety standards and practices.