What is defined as the transition altitude?

The transition altitude is defined as the altitude at which pilots switch to local QNH, which is the altimeter setting that reflects local atmospheric pressure at sea level. Above this altitude, pilots are required to use the standard pressure setting of 29.92 inches of mercury (Hg) or 1013.25 hPa for their altimeters. This ensures uniformity and consistency in altitude measurement among aircraft flying at high altitudes and helps in maintaining safe vertical separation between aircraft.

Understanding the transition altitude is crucial for pilots, as it facilitates the safe management of altitude assignments and the implementation of altimeter settings based on local atmospheric conditions. It is critical for pilots to transition from standard pressure back to regional altimeter settings when descending below this altitude to ensure accurate altitude reporting and navigation with respect to the terrain and local air traffic control instructions.