What causes the CKPT PX light to illuminate?

The CKPT PX light illuminates when the cockpit pressure differential reaches 4.0 psi or higher. This warning is crucial for flight safety, as it indicates that the difference between the pressure inside the cockpit and the ambient pressure outside has become significant, which could suggest a potential cabin decompression or malfunction in the environmental control system.

Maintaining proper cockpit pressure is essential for the safety and comfort of the pilots, particularly at high altitudes where outside air pressure is much lower. When the CKPT PX light is activated, it serves as an alert for the pilots to take necessary actions, including evaluating the cockpit environment and the status of the aircraft systems, ensuring that there is no immediate danger related to cabin pressure.