What does "transitional altitude" refer to in aviation?

Transitional altitude refers to the specific altitude at which altimeter settings change from QNH (the altimeter setting that is adjusted for the local air pressure and represents altitude above sea level) to QNE (standard pressure setting of 29.92 inches of mercury, which references flight levels above sea level). This transition is important because it helps ensure that aircraft maintain the correct separation in the airspace and can safely operate within the different altimeter references.

The transitional altitude is crucial for pilots and air traffic controllers as it delineates the change in altimeter settings necessary for maintaining vertical separation among aircraft. Understanding this concept enhances safety and efficient airspace management, particularly when transitioning from the lower levels of airspace, where local altimeter settings are in use, to higher flight levels, where standard pressure settings are employed.