What does the GPWS warn the crew of regarding terrain closure?

The Ground Proximity Warning System (GPWS) is designed to alert the flight crew of potentially hazardous situations concerning terrain closure, particularly when it detects that the aircraft is on a collision course with the ground. A warning is triggered when the rate of terrain closure is considered excessive — meaning that the aircraft is approaching the ground too rapidly or too close for safety.

This system uses a combination of barometric altitude, radar data, and aircraft flight path information to determine when a warning is necessary. By focusing on excessive terrain closure, the GPWS helps enhance situational awareness for the pilots, allowing them to take corrective action to avoid a controlled flight into terrain, which is one of the significant risks during certain phases of flight, such as approach and landing.

Alternative options do not accurately reflect the intended purpose of the GPWS; for instance, a warning when terrain closure is decreasing does not convey an imminent threat, while a nominal approach would indicate safe conditions. A warning about an incline would not specifically address the critical aspect of ground proximity that the GPWS is designed to monitor, which is the dangerous rate of closure rather than the general angle of descent.