When does a red CKPT PX annunciator illuminate?

The red CKPT PX annunciator illuminates when cockpit differential pressure exceeds 3.9 to 4.0 psi. This threshold indicates a significant pressure imbalance within the cockpit environment, which could pose a risk to the safety of the crew and the operation of the aircraft. When the differential pressure reaches this level, it serves as a warning that corrective action may need to be taken to ensure the safety and integrity of the cockpit, as continued operation under these conditions could lead to structural or safety issues. Monitoring cockpit pressure levels is vital for maintaining a safe flight environment, and understanding the specific parameters associated with different annunciators helps pilots respond appropriately to potential issues.