FAA Ground School Practice Test

Under which condition will pressure altitude be equal to true altitude?

1. When at sea level

2. When in low pressure

3. When standard atmospheric conditions exist

4. When in high altitude

The correct answer is: When standard atmospheric conditions exist

Pressure altitude will be equal to true altitude when standard atmospheric conditions exist because standard atmosphere defines a specific relationship between pressure, temperature, and altitude. In these conditions, the atmosphere is assumed to have a uniform temperature lapse rate and specific pressure levels defined by the International Standard Atmosphere (ISA). At sea level, the pressure is typically 29.92 inches of mercury (Hg) and is at a temperature of 15 degrees Celsius. When an aircraft is at standard conditions, the altimeter setting accurately reflects the height above sea level, meaning pressure altitude and true altitude align perfectly. In low pressure or high altitude situations, the pressure altitude may not match true altitude due to deviations in atmospheric pressure or temperature from the standard values. Thus, standard atmospheric conditions create a framework in which these two measurements coincide.