What feature is indicated by a light brown hatched boundary on an En Route Low Altitude Chart?

A light brown hatched boundary on an En Route Low Altitude Chart indicates an Alert Area. This designation serves to inform pilots that the area may contain a high volume of pilot training activities or unusual aerial activity, which could pose a potential hazard to other aircraft. Pilots are advised to exercise caution when flying through these areas due to the possibility of conflicting air traffic or maneuvers that may not be expected in normal airspace.

In contrast, the other options—Warning Area, Restricted Area, and Prohibited Area—each possess distinct characteristics and are depicted with different symbols or markings on the charts. A Warning Area is generally depicted with a wavy boundary line and extends from three nautical miles out from the coastline, indicating potential hazards to non-participating aircraft. A Restricted Area is typically marked with solid blue lines and may contain activities that are hazardous to aircraft, but access is restricted under certain conditions. A Prohibited Area is marked with a solid red line, indicating that flight is strictly prohibited in that airspace unless specific permissions are granted.

Thus, understanding the specific meaning of the light brown hatched boundary is essential for pilots to ensure safe navigation and awareness of potential hazards in areas of increased activity.