What is indicated by the outer magenta circle around Class B airspace?

The outer magenta circle around Class B airspace is known as the Mode C "veil." This indicates that all aircraft operating within this area, regardless of altitude, are required to have an altitude-reporting transponder equipped with Mode C. The purpose of the veil is to provide air traffic control with the ability to monitor and manage aircraft operating in proximity to the Class B airspace, thereby enhancing safety and separation between different types of air traffic.

The concept of the Mode C veil is important for pilots to understand, as it informs them of the requirements they must adhere to when flying near or approaching Class B airspace. The requirement for altitude reporting helps ensure that air traffic is appropriately managed, especially in busy terminal areas where Class B airspace is typically located.

Overall, the Mode C veil is critical for maintaining situational awareness and promoting safety for both VFR (Visual Flight Rules) and IFR (Instrument Flight Rules) flights in and around controlled airspace.