FAA Ground School Practice Test

Under what condition are pressure altitude and density altitude the same value?

1. At low temperature

2. At altitude above 18,000 feet

3. At standard pressure

4. At sea level

The correct answer is: At standard pressure

Pressure altitude and density altitude are the same when the atmospheric conditions align with standard atmospheric conditions, which is defined as a temperature of 15 degrees Celsius (59 degrees Fahrenheit) at sea level and a pressure of 29.92 inches of mercury (Hg). Under these standard conditions, the effects of temperature and pressure on air density are balanced, allowing pressure altitude and density altitude to have the same numerical value. In other scenarios, variations in temperature and pressure alter the air density, leading to discrepancies between the two altitude measurements. For example, at low temperatures, the density altitude would be lower than pressure altitude, while at higher altitudes, the relationship can also be impacted by temperature changes, not necessarily aligning them. Therefore, the condition where pressure altitude and density altitude are the same specifically occurs at standard pressure.