What must a ground proximity warning system convey under FAR Part 135?

A ground proximity warning system (GPWS) is designed to provide warnings to pilots regarding potential collisions with the ground, particularly during critical phases of flight such as approach and landing. Under FAR Part 135, which governs commuter and on-demand operations, it is essential for the safety of the flight that this system effectively alerts the crew to deviations from the intended flight path.

The requirement for the system to provide warnings of deviation below glide slope is crucial. Glide slope refers to the optimal descent path for landing, and being below this path can lead to terrain impacts if not corrected in time. Thus, the system is mandated to convey alerts that ensure pilots are aware of their altitude in relation to the glide slope to avoid controlled flight into terrain (CFIT) accidents.

In contrast, while cabin pressure changes, fuel imbalances, and engine failures are important issues during flight operations, they fall under different monitoring systems and are not the primary focus of a GPWS, which specifically addresses ground proximity and flight path deviations.