What is the first action required for the LPU-21 before inflation?

The first action required for the LPU-21 before inflation is to manually open the casing. This step is essential because the casing of the life preserver needs to be accessed to ensure that the internal flotation assembly is ready for use. By opening the casing, you allow for access to the inflation mechanism and the actual flotation devices inside. This preparation is crucial for effective inflation of the preserver, ensuring it functions as intended during an emergency.

If the casing is not properly opened, the flotation devices may not inflate correctly, which could lead to ineffective buoyancy in a critical situation. Therefore, opening the casing is a foundational step that precedes further actions, such as inflating the device.