What is meant by transition layer in aviation?

The transition layer in aviation refers to the area of airspace that exists between the transition altitude and the transition flight level (FL). This layer is critical for maintaining safe separation between aircraft flying under different altimeter settings—specifically, aircraft that use altitude measurements based on the Standard Pressure Setting (29.92 inHg) and those using local altimeter settings.

When flying below the transition altitude, pilots use local altimeter settings, adjusting their altimeters according to ground pressure conditions. However, once an aircraft climbs to the transition altitude, it operates on a standard pressure setting, transitioning to the standard flight levels. The transition layer helps prevent any conflict that could arise if an aircraft at one altimeter setting inadvertently intrudes into the airspace of another aircraft that is at a different setting but operating at the same height.

Understanding this layer ensures that pilots maintain appropriate vertical separation, particularly in areas where both types of operations can occur simultaneously.