What is the critical pressure threshold for a CKPT PX (red) light activation?

The critical pressure threshold for a CKPT PX (red) light activation is specifically set between 3.9 and 4.0 psi. This range is crucial because it indicates an abnormal condition within the aircraft's pressurization system. When pressure drops below this threshold, it can lead to inadequate cabin pressure, affecting passenger and crew safety.

Understanding this threshold is essential for pilots and crew members as it prompts them to take immediate corrective action to restore proper cabin conditions. Monitoring and recognizing these pressures enable timely responses to ensure safety during flight operations. Other ranges, such as those presented in the other options, do not trigger the CKPT PX (red) light, meaning they are not critical for pressurization warnings.