At what altitude does the transition layer for NAS occur?

The transition layer for National Airspace System (NAS) operations typically occurs at FL180. This altitude marks the boundary between controlled airspace and the lower flight levels where aircraft operate under Visual Flight Rules (VFR) or below Class A airspace (which begins at FL180). At FL180, air traffic control transitions from providing services to primarily instrument flight rules (IFR) traffic.

Understanding this altitude is crucial for pilots, as it influences route planning and altitude selection during ascent or descent. Knowing the transition layer helps in maintaining proper separation from IFR and VFR traffic and ensures compliance with air traffic regulations designed to enhance safety within the NAS.

The other altitudes listed do not correspond to the defined transition layer and would not provide the correct context for airspace operations.