The ground proximity warning system (GPWS) is designed to determine what?

• A. Aircraft speed relative to the air mass.
• B. Aircraft altitude above the cloud layer.
• C. Aircraft's clearance above the Earth.
• D. Engine temperature and health.

Answer: (C)

The main idea is that the Ground Proximity Warning System checks how close the aircraft is to the terrain ahead and beneath you, so it can warn you if your clearance above the Earth's surface is getting too small. It does this by using a radar (radio) altimeter to measure height above ground and by monitoring your flight path relative to the terrain to predict potential collision with the ground. This focus on terrain proximity is exactly what GPWS is meant to provide, helping you maintain safe clearance from the surface.

That’s why the option describing the aircraft’s clearance above the Earth is the correct match. The other choices relate to airspeed, altitude above cloud layers, or engine health, which are not what GPWS monitors.