AIM 8/15/19  Section 2. Controlled Airspace 3 − 2 − 1. General a. Controlled Airspace. A generic term that covers the different classification of airspace (Class A, Class B, Class C, Class D, and Class E airspace) and defined dimensions within which air traffic control service is provided to IFR flights and to VFR flights in accordance with the airspace classification. (See FIG 3 − 2 − 1.) b. IFR Requirements. IFR operations in any class of controlled airspace requires that a pilot must file an IFR flight plan and receive an appropriate ATC clearance. c. IFR Separation. Standard IFR separation is provided to all aircraft operating under IFR in controlled airspace. d. VFR Requirements. It is the responsibility of the pilot to ensure that ATC clearance or radio communication requirements are met prior to entry into Class B, Class C, or Class D airspace. The pilot retains this responsibility when receiving ATC radar advisories. (See 14 CFR Part 91.) e. Traffic Advisories. Traffic advisories will be provided to all aircraft as the controller’s work situation permits. f. Safety Alerts. Safety Alerts are mandatory services and are provided to ALL aircraft. There are two types of Safety Alerts: 1. Terrain/Obstruction Alert. A Terrain/ Obstruction Alert is issued when, in the controller’s judgment, an aircraft’s altitude places it in unsafe proximity to terrain and/or obstructions; and 2. Aircraft Conflict/Mode C Intruder Alert.  An Aircraft Conflict/Mode C Intruder Alert is issued if the controller observes another aircraft which places it in an unsafe proximity. When feasible, the controller will offer the pilot an alternative course of action. FIG 3 − 2 − 1 Airspace Classes MSL - mean sea level AGL - above ground level FL - flight level CLASS B CLASS C CLASS E CLASS D CLASS G CLASS G CLASS G Nontowered Airport FL 600 18,000 MSL 14,500 MSL 1,200 AGL 700 AGL CLASS A Controlled Airspace 3 − 2 − 1