with ATC rather than an ATC clearance is required for VFR operations. Other controlled airspace is designated as Class E, this includes a large part of the lower airspace. Class E airspace exists in many forms. It can serve as a surface-based extension to Class D airspace to accommodate IFR approach/departure procedure areas. Class E airspace can be designated to have a floor of 700' AGL or 1,200' AGL, or a customized floor of any other altitude. Class E airspace exists above Class G surface areas from 14,500' MSL to 18,000 MSL. Federal airways from 1,200 AGL to 18,000 MSL within of the centerline of the airway is designated Class E airspace. Airspace at any altitude over FL600 (60,000 MSL) (the ceiling of Class A airspace) is designated Class E airspace. The U.S. does not use ICAO Class F. Class G (uncontrolled) airspace is mostly used for a small layer of airspace near the ground, but there are larger areas of Class G airspace in remote regions. Airspace classes and VFR Authorities use the ICAO definitions to derive additional rules for VFR cloud clearance, visibility, and equipment requirements. For example, consider Class E airspace. It is possible that an aircraft operating under VFR is not in communication with ATC, so it is imperative that its pilot be able to see and avoid other aircraft (and vice versa). That includes IFR flights emerging from a cloud, so the VFR flight must keep a designated distance from the edges of clouds above, below, and laterally, and must maintain at least a designated visibility, to give the two aircraft time to observe and avoid each other. The low-level speed limit of 250 knots does not apply above , so the visibility requirements are higher. On the other hand, in Class B and Class C airspaces, separation is provided by ATC to all aircraft. In these situations the VFR pilot only needs to see where his/her own aircraft is going, so visibility requirements are less stringent, and there is no designated minimum distance from clouds. Similar considerations determine whether a VFR aircraft must use a two-way radio and/or a transponder. Special use airspace Each national authority designates areas of special use airspace (SUA), primarily for reasons of national security. This is not a separate classification from the ATC-based classes; each piece of SUA is contained in one or more zones of letter-classed airspace. SUAs