What is the transition flight level?

The transition flight level is defined as the lowest usable flight level during descent when approaching an area where air traffic operates under controlled airspace and altimeter settings may differ due to local atmospheric conditions. This flight level is particularly important for ensuring separation between aircraft that are transitioning from altitude-based flight in the upper airspace to the lower altitudes where different reference plane settings are used.

As aircraft descend through the transition flight level, pilots switch from using flight levels, which are expressed in terms of altitude in hundreds of feet, to altitudes, which are expressed in feet above mean sea level. This switch occurs because below the transition level, because local atmospheric pressure can cause aircraft to be at different altitudes for the same flight level.

Understanding the transition flight level is crucial for maintaining safe air traffic control and ensuring proper separation between aircraft, thereby preventing potential altitude conflicts. The other options either misrepresent the function of the transition flight level or refer to other phases of flight, making them less relevant in this context.