What is the first action to take when you receive a "CABIN ALTITUDE HI" message?

The appropriate first action upon receiving a "CABIN ALTITUDE HI" message is to don the oxygen mask at 100%. This immediate step is crucial because a "CABIN ALTITUDE HI" warning indicates that the cabin pressure has dropped to an unsafe level, which can lead to hypoxia or other serious physiological effects for both crew and passengers.

By putting on the oxygen mask, you ensure that you can maintain cognitive function and situational awareness, allowing you to effectively manage the emergency situation. This step takes priority over other actions because it directly addresses the risk to your health and safety in the context of cabin pressure loss.

While notifying the cabin crew, descending immediately, or communicating with ATC are also important actions that will follow, they require first ensuring that you are adequately protected against the immediate effects of reduced cabin pressure. After securing your own safety, you can then inform the cabin crew, initiate a descent to a safe altitude, and communicate with air traffic control as needed.