Private Pilot Private Exam

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

1. When flying above 18,000 feet

2. When at sea level

3. When standard atmospheric conditions exist

4. When the altimeter setting is in inches of mercury

The correct answer is: When standard atmospheric conditions exist

Pressure altitude will equal true altitude when standard atmospheric conditions exist. This is based on the assumption that the atmosphere behaves in a predictable manner according to the International Standard Atmosphere (ISA) model, which defines a standard temperature and pressure at various altitudes. Under standard atmospheric conditions, the pressure and temperature are consistent throughout the layers of the atmosphere, allowing for a direct correlation between the altimeter reading (pressure altitude) and the actual altitude above sea level (true altitude). In contrast, when flying above 18,000 feet, variations in temperature and atmospheric pressure can lead to discrepancies between pressure altitude and true altitude. Similarly, at sea level, although pressure altitude is defined, the conditions may not align with standard atmospheric conditions, given that local weather patterns can affect the actual pressure. Lastly, the altimeter setting being in inches of mercury does not guarantee that pressure altitude equals true altitude unless those settings align with the standard atmospheric conditions. Therefore, only when standard atmospheric conditions prevail will pressure altitude match true altitude.