Illumination of the MODE SEL FAULT Light indicates what situation?

The illumination of the MODE SEL FAULT Light signifies that there is a fault detected in both automatic pressurization systems of the aircraft. In the A320, there are typically two automatic pressurization controllers that manage the cabin pressure. If both systems encounter a malfunction, the MODE SEL FAULT light will activate to alert the crew of the issue. This indicates a more serious situation than if only one system were to fail, as multiple systems working together are essential for safe and effective pressurization. The crew would need to take appropriate action to ensure passenger safety and manage cabin pressure manually if necessary.

Other scenarios such as a single automatic pressurization controller failure or excess pressurization while airborne do not trigger the MODE SEL FAULT indication, as the light specifically denotes faults involving both systems. Additionally, the airplane being pressurized on the ground does not represent an automatic system failure, rather it is a normal operation. Therefore, the presence of the fault light is a clear indication of a dual system malfunction that requires attention.