Under what conditions does pressure altitude equal density altitude?

Pressure altitude equals density altitude specifically at standard temperature, which is defined as 15 degrees Celsius (59 degrees Fahrenheit) at sea level. Under these conditions, the standard atmosphere model holds true, meaning both altitudes align due to the predictable behavior of air density with temperature and pressure.

When the atmosphere is at standard temperature, the variables of temperature and pressure interact in a consistent manner, allowing the equations governing density to maintain their proportionality to altitude without external anomalies. This allows for accurate flight planning and performance calculations, as pilots can rely on both altitudes being the same, simplifying the process of understanding aircraft performance in various flight scenarios.

In contrast, higher temperatures result in lower density (which would create a difference between pressure and density altitudes), and low altitudes do not inherently affect the relationship between pressure and density altitude without considering temperature. Similarly, different pressure settings can produce varying results depending on the ambient temperature, meaning pressure altitude would not necessarily equal density altitude at those times. Thus, standard temperature is the key condition for their equivalence.