The law defines airports by categories of airport activities,  including commercial service, primary, cargo service,  reliever, and general aviation airports, as shown below:  Towered Airport  A towered airport has an operating control tower. Air traffic  control (ATC) is responsible for providing the safe, orderly,  and expeditious flow of air traffic at airports where the  type of operations and/or volume of traffic requires such a  service. Pilots operating from a towered airport are required  to maintain two-way radio communication with ATC and to  acknowledge and comply with their instructions. Pilots must  advise ATC if they cannot comply with the instructions issued  and request amended instructions. A pilot may deviate from  an air traffic instruction in an emergency, but must advise  ATC of the deviation as soon as possible.  A nontowered airport does not have an operating control  tower. Two-way radio communications are not required,  although it is a good operating practice for pilots to transmit  their intentions on the specified frequency for the benefit  of other traffic in the area. The key to communicating at an  airport without an operating control tower is selection of the  correct common frequency. The acronym CTAF, which stands  for Common Traffic Advisory Frequency, is synonymous  with this program. A CTAF is a frequency designated for  the purpose of carrying out airport advisory practices while  operating to or from an airport without an operating control  tower. The CTAF may be a Universal Integrated Community  (UNICOM), MULTICOM, Flight Service Station (FSS), or  tower frequency and is identified in appropriate aeronautical  publications. UNICOM is a nongovernment air/ground radio  communication station that may provide airport information  at public use airports where there is no tower or FSS. On pilot  request, UNICOM stations may provide pilots with weather  information, wind direction, the recommended runway, or  other necessary information. If the UNICOM frequency  is designated as the CTAF, it is identified in appropriate  aeronautical publications.  Figure 14-1  lists recommended  communication procedures. More information regarding  radio communications is provided later in this chapter.  Nontowered airport traffic patterns are always entered at  pattern altitude. How you enter the pattern depends upon the  direction of arrival. The preferred method for entering from  the downwind side of the pattern is to approach the pattern  on a course 45 degrees to the downwind leg and join the  pattern at midfield.  There are several ways to enter the pattern if you’re coming  from the upwind leg side of the airport. One method of entry  from the opposite side of the pattern is to announce your  intentions and cross over midfield at least 500 feet above  There are two types of airports—towered and nontowered.  These types can be further subdivided to:  Nontowered Airport  Types of Airports  • Civil Airports—airports that are open to the general  public.  • Military/Federal Government airports—airports  operated by the military, National Aeronautics and  Space Administration (NASA), or other agencies of  the Federal Government.  • Private Airports—airports designated for private or  restricted use only, not open to the general public.  • General Aviation Airports — the remaining airports  are commonly described as General Aviation Airports.  This airport type is the largest single group of airports  in the U.S. system. The category also includes privately  owned, public use airports that enplane 2500 or more  passengers annually and receive scheduled airline  service.  • Reliever Airports—airports designated by the FAA  to relieve congestion at Commercial Service Airports  and to provide improved general aviation access to the  overall community. These may be publicly or privately- owned.  • Cargo Service Airports—airports that, in addition to any  other air transportation services that may be available,  are served by aircraft providing air transportation of  only cargo with a total annual landed weight of more  than 100 million pounds. “Landed weight” means the  weight of aircraft transporting only cargo in intrastate,  interstate, and foreign air transportation. An airport  may be both a commercial service and a cargo service  airport.  • Commercial Service Airports—publicly owned  airports that have at least 2,500 passenger boardings  each calendar year and receive scheduled passenger  service. Passenger boardings refer to revenue passenger  boardings on an aircraft in service in air commerce  whether or not in scheduled service. The definition  also includes passengers who continue on an aircraft  in international flight that stops at an airport in any of  the 50 States for a non-traffic purpose, such as refueling  or aircraft maintenance rather than passenger activity.  Passenger boardings at airports that receive scheduled  passenger service are also referred to as Enplanements.  14-2 