Air Traffic Control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of traffic, and provide information and other support for pilots. To prevent collisions, ATC enforces traffic separation standard (i.e. vertical, longitudinal, lateral and RADAR), which ensure each aircraft maintains a minimum separation standard at all times. Many aircraft also have collision avoidance systems, which provide additional safety by warning pilots when other aircraft get too close. ATC provides services to all flights including private, military, and commercial aircraft operating within its airspace. Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to obey, or advisories (known as flight information) that pilots may, at their discretion, disregard. Generally the pilot in command is the final authority for the safe operation of the aircraft and may, in an emergency, deviate from ATC instructions to the extent required to maintain safe operation of their aircraft. Airport Control The primary method of controlling the immediate airport environment is visual observation from the aerodrome control tower (TWR). Aerodrome or Tower controllers are responsible for the separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport itself, and aircraft in the vicinity of the airport, generally 5 to 10 nm depending on the airport procedures. Surveillance displays are also available to controllers at certain airports to assist with controlling air traffic. Air Traffic Controllers may use a radar system called Secondary Surveillance Radar for approaching and departing aircraft. In adverse weather conditions the tower controllers may also use Surface Movement Radar (SMR), Surface Movement Guidance and Control Systems (SMGCS) or Advanced SMGCS to control traffic on the manoeuvring area (taxiways and runway). The areas of responsibility for Air Traffic Control Tower fall into three general operational disciplines; Local Control or Air Control, Ground Control, and Flight Data/Clearance Delivery—other categories, such as Apron Control or Ground Movement Planner, may exist at extremely busy airports. Ground Movement Control is responsible for the airport movement areas This generally includes all taxiways, inactive runways, holding areas, and some transitional aprons or intersections where aircraft arrive, having vacated the runway or departure gate. Exact areas and control responsibilities are clearly defined in local documents (Aeronautical Information Publication) and SOP at each airport. Any aircraft, vehicle, or person walking or working in these areas is required to have clearance from Ground Control. This is normally done via VHF/UHF radio, but there may be special cases where other procedures are used. Aircraft or vehicles without radios must respond to ATC instructions via aviation light signals or else be led by vehicles with radios. People working on the airport surface normally have a communications link through which they can communicate with Ground Control, commonly either by handheld radio or even cell phone. Ground Control is vital to the smooth operation of the airport, because this position impacts the sequencing of departure aircraft, affecting the safety and efficiency of the airports operation. Tower/Aerodrome Control is responsible for the active runway surfaces to clears aircraft for takeoff or landing, ensuring that prescribed runway separation will exist at all times. If Local Control detects any unsafe condition, a landing aircraft may be told to go-around and be re-sequenced into the landing pattern by the approach or terminal area controller. A highly disciplined communications process (Radio Telephony) between Tower Control and Ground Control is an absolute necessity. Ground Control must request and gain approval from Tower Control to cross any active runway with any aircraft or vehicle. Likewise, Local Control must ensure that Ground Control is aware of any operations that will impact the taxiways, and work with the approach radar controllers to create holes or gaps in the arrival traffic to allow taxiing traffic to cross runways and to allow departing aircraft to take off. Flight data / clearance delivery is the position that issues route clearances to aircraft, typically before they commence taxiing. These contain details of the airways/route that the aircraft is expected to fly after departure. Clearance Delivery will, if necessary, coordinate with the En route (Area) Control Centre to obtain releases for aircraft. Often, however, such releases are given automatically or are controlled by local agreements allowing free-flow departures. The primary responsibility of Clearance Delivery is to ensure that the aircraft have the proper route and slot time. This information is also coordinated with the en route (area) centre and Ground Control in order to ensure that the aircraft reaches the runway in time to meet the slot time provided by the command centre. At some airports, Clearance Delivery also plans aircraft push-backs and engine starts. Flight Data (which is routinely combined with Clearance Delivery) is the position that is responsible for ensuring that both controllers and pilots have the most current information: pertinent weather changes, airport ground delays/ground stops, runway closures, etc. Flight Data may inform the pilots using a recorded continuous loop on a specific frequency known as the Automatic Terminal Information Service (ATIS). Many airports have Approach Radar control facility that is associated with the airport. It handles aircraft in a 30 to 50 nm radius from the airport. Where there are many busy airports close together, one consolidated Approach Control may service all the airports. The airspace boundaries and altitudes assigned to Approach Control vary widely from airport to airport, are based on factors such as traffic flows, neighboring airports and terrain. Approach controllers are responsible for providing all ATC services within their airspace. Traffic flow is broadly divided into departures, arrivals, and overflights. As aircraft move in and out of the terminal airspace, they are handed off to the next appropriate control facility (a control tower, an en-route (area) control facility, or a bordering terminal or approach control). Terminal control is responsible for ensuring that aircraft are at an appropriate altitude when they are handed off, and that aircraft arrive at a suitable rate for landing. Not all airports have a radar approach or approach control available. In this case, the en-route (area) control centre or a neighboring terminal or approach control may co-ordinate directly with the tower on the airport and vector inbound aircraft to a position from where they can land visually. At some of these airports, the tower may provide a non-radar procedural approach service to arriving aircraft handed over from a radar unit before they are visual to land. Some units also have a dedicated approach unit which can provide the procedural approach service either all the time or for any periods of radar outage for any reason. En route (area) control Centre ATC provides services to aircraft in flight between airports as well. Pilots fly under one of two sets of rules for separation: Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Air traffic controllers have different responsibilities to aircraft operating under the different sets of rules. En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots are required to comply with these instructions. En-route controllers also provide air traffic control services to many smaller airports around the country, including clearance off of the ground and clearance for approach to an airport. Controllers adhere to a set of separation standards that define the minimum distance allowed between aircraft. These distances vary depending on the equipment and procedures used in providing ATC services. Each ATCC is responsible for many thousands of square miles of airspace (known as a Flight Information Region, as determined by ICAO) and for the airports within that airspace. ATCC controllers are responsible for climbing the aircraft to their requested altitude while, at the same time, ensuring that the aircraft is properly separated from all other aircraft in the immediate area. Additionally, the aircraft must be placed in a flow consistent with the aircrafts route of flight. This effort is complicated by crossing traffic, severe weather, special missions that require large airspace allocations, and traffic density. When the aircraft approaches its destination, the center is responsible for meeting altitude restrictions by specific points, as well as providing many destination airports with a traffic flow, which prohibits all of the arrivals being bunched together. These flow restrictions often begin in the middle of the route, as controllers will position aircraft landing in the same destination so that when the aircraft are close to their destination they are sequenced. As an aircraft reaches the boundary of area (enroute) control centre (also known as Air Traffic Control Centre-ATCC), it is handed off or handed over to the next ATCC. The hand-off process involves a transfer of identification and details between controllers so that air traffic control services can be provided in a seamless manner; in some cases local agreements may allow silent handovers such that the receiving ATCC does not require any co-ordination if traffic is presented in an agreed manner. After the hand-off, the aircraft is given a frequency change and begins talking to the next controller. This process continues until the aircraft is handed off to a terminal controller (approach).
Posted on: Wed, 12 Mar 2014 15:33:59 +0000
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