Differentiate pressure altitude and absolute altitude.

• A. Pressure altitude is the altitude above the sea level; absolute altitude is the height above the standard datum plane.
• B. Pressure altitude is the height above the Earth's surface; absolute altitude is the altitude above sea level.
• C. Pressure altitude is the altitude above the standard datum plane when the atmospheric pressure is 29.92 inHg; absolute altitude is the height above the Earth's surface (above ground level).
• D. Pressure altitude is the altitude above sea level; absolute altitude is the altitude above the Earth's surface.

Answer: (C)

Pressure altitude and absolute altitude are defined using different reference surfaces. Pressure altitude is the altitude above the standard datum plane—the theoretical surface where the pressure equals 29.92 inHg in the International Standard Atmosphere. In other words, it’s the height you would have if the atmospheric pressure were 29.92 inHg, regardless of actual terrain. Absolute altitude is the actual vertical distance from the aircraft to the Earth's surface (in this context, height above the ground).

So, the statement that pressure altitude is the altitude above the standard datum plane when the atmospheric pressure is 29.92 inHg is correct, and absolute altitude is the height above the Earth's surface (above ground). The other descriptions mix up the reference surfaces (sea level vs datum plane) or imply the wrong reference for absolute altitude.