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

Pressure altitude will equal true altitude when standard atmospheric conditions exist. Standard atmospheric conditions reference a specific temperature and pressure as established by the International Standard Atmosphere (ISA), which assumes a temperature of 15 degrees Celsius at sea level and a pressure of 29.92 inches of mercury (inHg).

Under these conditions, both pressure and temperature are in a state of balance where the altimeter, which measures pressure, will reflect the true vertical distance above sea level. When there are deviations from this standard, such as during temperature inversions or when flying over mountainous terrain, the true altitude can differ from the indicated pressure altitude. At sea level, while the pressure altitude can still be recognized, factors such as temperature can alter the true altitude, especially if conditions deviate from the standard. Thus, it's specifically in standard atmospheric conditions that pressure altitude and true altitude align.