civilian traffic are advised but not obliged to do the same (although they must respect the ATZ contained within the MATZ). A MATZ usually consists of a circular zone with a radius of 5 nm, extending from the surface to AAL (above aerodrome level). Stubs 4 nm wide and 5 nm in length, orientated with the aerodrome's main runway, extend on opposite sides of the central circle, extending from to AAL. United States The U.S. adopted a slightly modified version of the ICAO system on September 16, 1993, when regions of airspace designated according to older classifications were converted entirely. The exceptions are some terminal radar service areas (TRSA), which have special rules and still exist in a few places. With some exceptions, Class A airspace is applied to all airspace between and flight level (FL) 600 (approximately 60,000 ft). Above FL600, the airspace reverts to Class E. The transition altitude is also consistently everywhere. All operations in US Class A airspace must be conducted under IFR. SVFR flight in Class A airspace is prohibited. Class B airspace is used to control the flow of traffic around major airports. The airspace is charted on a VFR sectional chart with a series of blue lines. Within these blue lines, the floor and the ceiling of the Class B airspace is defined. The lateral boundaries of Class B airspace are individually tailored to facilitate arriving and departing traffic operating under IFR. Class B airspace extends from the surface to generally MSL. In Denver, Colorado and Salt Lake City, Utah, the ceiling is at MSL, while in Phoenix, Arizona, the ceiling is at MSL. Charts must be consulted for the most current floor and ceiling information. Aircraft must establish two-way radio communication with ATC and obtain a clearance to enter Class B airspace. All aircraft operating inside or within 30 NM of the primary Class B airport(s) are required to have a transponder with Mode C. The 30 NM Mode C Veil is denoted on VFR charts by a thin magenta line. VFR traffic must remain clear of clouds and maintain 3 SM of visibility while operating within Class B airspace. Class C airspace is used around airports with a moderate traffic level. Class D is used for smaller airports that have a control tower. The U.S. uses a modified version of the ICAO class C and D airspace, where only radio contact