What is pressure altitude when the altimeter is set to 29.92 inches?

Pressure altitude is defined as the height above the standard datum plane when the altimeter is set to 29.92 inches of mercury (inHg). This setting is based on the International Standard Atmosphere (ISA), which assumes a sea level pressure of 29.92 inHg and a specific temperature lapse rate.

When the altimeter is calibrated at this standard pressure, it measures the vertical distance from the standard datum plane, which is an imaginary plane that corresponds to the standard atmospheric pressure at sea level. This means that the altimeter reading represents the altitude at which an aircraft would be if the atmospheric pressure were 29.92 inHg throughout the entire atmosphere.

This understanding is crucial for pilots because it helps in maintaining accurate altitude awareness and ensures safety during flight operations. It is particularly important when flying under various atmospheric conditions that can affect pressure readings and, consequently, altitude calculations.