FAA Chart Users Guide - VFR Terms and Symbols 18 If the ceiling is less than 18,000' MSL, the value (preceded by the word "ceiling") is shown along the limits of the controlled  airspace. These limits are shown with the same symbol indicated above. Special Use Airspace (SUA)  confines certain flight activities and restricts entry, or cautions other aircraft operating within  specific boundaries. Except for Controlled Firing Areas, SUA areas are depicted on VFR Charts. Controlled Firing Areas  are not charted because their activities are suspended immediately when spotter aircraft, radar, or ground lookout posi- tions indicate an aircraft might be approaching the area. Nonparticipating aircraft are not required to change their flight  paths. SUA areas are shown in their entirety (within the limits of the chart), even when they overlap, adjoin, or when an  area is designated within another area. The areas are identified by type and identifying name/number, and are positioned  either within or immediately adjacent to the area. * Alert Areas do not extend into Class A, B, C and D airspace, or Class E airport surface areas.  Mode C Required Airspace  (from the surface to 10,000' MSL) within a 30 NM radius of the primary airport(s) for which a  Class B is designated, is depicted by a solid magenta line. Mode C is required, but not depicted for operations within and above all Class C up to 10,000' MSL.  Enroute Mode C requirements (at and above 10,000' MSL except in airspace at and below 2500' AGL) are not depicted.  See FAR 91.215 and the AIM. FAR 91  Airports where fixed wing special visual flight rules operations are prohibited (FAR 91) are shown  with the type "NO SVFR" above the airport name. National Security Areas  indicated with a broken magenta line   and Special Flight Rules Areas (SFRAs) in- dicated with the following symbol:  , consist of airspace with defined vertical and lateral dimensions estab - lished at locations where there is a requirement for increased security and safety of ground facilities. Pilots should avoid  flying through these depicted areas. When necessary, flight may be temporarily prohibited. The Washington DC Flight Restricted Zone (FRZ)  is related to National Security. It is depicted using the Prohibited/ Restricted/Warning Area symbology   and is located within the SFRA. It is defined as the airspace within  approximately a 13 to 15 NM radius of the DCA VOR-DME. Additional requirements are levied upon aviators requesting  access to operate inside the National Capital Region. OTHER AIRSPACE AREAS FAR 93  Airports and heliports under Federal Aviation Regulation 93 (FAR 93), (Special Air Traffic Rules and Airport Traffic Patterns), are shown by "boxing" the airport name. UNCONTROLLED AIRSPACE SPECIAL USE AIRSPACE Class G Airspace  within the United States extends up to 14,500' Mean Sea Level. At and above this altitude is Class E,  excluding the airspace less than 1500' above the terrain and certain special use airspace areas. and a number indicating the floor. 