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A non-towered airport is an airport with no operating tower, or air traffic control unit. The vast majority of the world's airports are non-towered, and even airports with control towers may operate as non-towered during off-hours, typically during the night.
At non-towered airports, instead of receiving instructions from a tower controller, pilots follow recommended procedures. The exact procedures vary from country to country, but they often involve standard arrival and departure patterns, and they may also include radio calls over a common frequency, such as a Common Traffic Advisory Frequency in the United States, Canada, New Zealand and Australia.
When the traffic volume at an airport gets too high for safe and efficient operations, or when the mix of aircraft types and speeds becomes too large, an airport may be considered for a tower. However, it is also necessary to find the money to construct a building and pay the controllers' salaries; in some cases aviation regulations or local opposition may prevent establishment of the unit.
For special events such as fly-ins, temporary towers may operate for only several days each year at fields that are otherwise non-towered. Temporary towers may operate out of an existing airport building, an RV, or even simply a chair (with a portable transmitter and binoculars).
Non-towered airports may lie inside or underneath controlled airspace. In that case, some or all aircraft arriving and departing require clearances from a remote air traffic control unit, such as terminal or center control, even though there is no control tower managing landings and takeoffs. Pilots may be able to obtain those clearances by radio, by phone, or through a company dispatcher or local Flight Service Station; in some cases, departing aircraft (IFR or VFR) take off and level out below the floor of controlled airspace, then radio for a clearance before climbing further. Some countries establish low-altitude VFR corridors for non-towered airports in large urban areas so that VFR arrivals and departures can avoid controlled airspace altogether.
Even though they do not have control towers, many non-towered airports have radio operations such as UNICOM to assist aircraft arriving, departing, or maneuvering on the ground. These radio operators have no authority to give aircraft clearances or instructions, but they can issue advisories to let them know about weather conditions, runway conditions, traffic, and other concerns.
Hazards are created by failure to use radios to report positions and intentions when operating within the airspace, which can lead to collisions between aircraft unaware of each other. Some pilots fail to use the correct runway at non-towered airports.
Some countries, such as Canada and Norway, use mandatory frequency airports (MF) or mandatory traffic advisory airports (MTAF), which operate like towered airports in some ways: the radio operators (typically a Flight Service Station) still issue only advisories, but aircraft are required to make radio contact with the ground station before operating in the airport's Control Zone.