When does the CKPT PX annunciator illuminate?

The CKPT PX annunciator illuminates when the differential pressure within the cockpit exceeds the thresholds set by the aircraft's systems, which is at 3.9 to 4.0 psi. When the pressure differential reaches this level, it indicates that the cockpit is not adequately pressurized, which can occur due to a failure in the pressurization system or a loss of cabin pressure. The illumination of this annunciator is a critical safety feature, alerting the pilot to potential issues that could lead to hypoxia or an unsafe flying environment.

This pressure range is specifically designed to trigger the warning, as it reflects a state where corrective action may be necessary. Maintaining proper cockpit pressure is essential for the safety and performance of flight operations, and this warning plays a vital role in ensuring that pilots are aware of any anomalies in cockpit conditions. In contrast, the other options represent pressure levels that are either too low or too high to trigger this specific annunciation.