Private Pilot Private Exam

Under what conditions is pressure altitude and density altitude the same value?

1. At 10,000 feet

2. At sea level under standard conditions

3. At standard temperature

4. At altitude where temperature is below standard

The correct answer is: At standard temperature

Pressure altitude and density altitude are the same value at standard temperature conditions, which is defined as 15 degrees Celsius at sea level. This is because both pressure altitude and density altitude are computed using the same reference points when the atmosphere follows the International Standard Atmosphere (ISA) model. Pressure altitude is the height above the standard datum plane, where the atmospheric pressure is 29.92 inches of mercury (Hg). Density altitude, on the other hand, is a corrected altitude that accounts for non-standard temperature and pressure, affecting aircraft performance in terms of lift, engine power, and propeller efficiency. When the temperature is at standard conditions — meaning it is neither warmer nor colder than what is expected at a given altitude in the ISA model — both pressure and density altitudes converge because the standard atmosphere assumptions hold true. Under these conditions, no adjustments are necessary, resulting in equal values for both types of altitude. In scenarios where the temperature deviates from standard conditions, the values of pressure and density altitude will differ; for instance, warmer than standard conditions usually lead to a higher density altitude compared to pressure altitude, indicating reduced aircraft performance capabilities. Temperature below standard would result in lower density altitude compared to pressure altitude, as cooler air is denser. Thus