There is normally nothing very difficult involved in takeoff, climb, cruise from point to point, and descent solely by reference to instruments. The complications arise when you must execute these maneuvers at precise times, at specified altitudes, over designated routes and geographic positions, and in an orderly sequence with other aircraft. An understanding of the Air Traffic Control system will impress upon you the importance of the training necessary for you to apply the proficiency you have acquired in basic instrument flying and radio navigation techniques.

 Federal regulation of civil aviation began with the Air Commerce Act of 1926 and the creation of the Aeronautics Branch in the U.S. Department of Commerce. The Department was concerned with the promotion of air safety, licensing of pilots, development of air navigation facilities, and issuing of flight information. Until the volume of air traffic increased, there was no need for air traffic control, since the likelihood of aircraft colliding in flight was remote.

 The need for controlling air traffic was recognized in the 1930's as the aviation industry produced bigger, faster, and safer aircraft, and air transportation became an accepted mode of public travel. A number of large cities, concerned with regulating the increasing air traffic at their airports, built control towers and inaugurated a control service on, and in the immediate vicinity of, the airports. Airline companies, eager to expand and improve their operations, established control centers at Cleveland, Chicago and Newark to provide their pilots with position and estimated time of arrival information during instrument flights between those cities.

 In 1936, the Federal Government assumed the responsibility for operation of the centers, employing eight controllers. As aviation has grown, so have the Federal Government functions and the agency charged with the promotion and safety of civil aviation. By the late 1970's, approximately 25,000 ATC personnel provided direction and assistance to over 100 million flights annually.

 The number of active aircraft has increased from 29,000, all flying at relatively slow and uniform speeds, to more than 200,000 aircraft operating at various speeds ranging to more than 1,000 miles per hour. The aerial highways have expanded from a few intercity routes to more than 250,000 miles of very high frequency routes utilizing approximately 1,000 VOR and VORTAC stations. A continued increase in traffic volume is expected during the coming decade.

 The difficulties associated with mixed IFR and VFR traffic and with diverse pilot training and varying aircraft capabilities, the trend toward automated electronic equipment, and other aspects of control will have a profound effect on flight operations, under both Visual and Instrument Flight Rules.

 As an instrument pilot normally operating under the jurisdiction of Air Traffic Control, your understanding of the present system and its operation will better enable you to make full use of ATC services.

Structure and Functions of Air Traffic Service

 Air Traffic Service of FAA is responsible for three major general functions: developing plans, establishing standards, and implementing systems for control of air traffic. The two specific functions of immediate concern to the instrument pilot are:

  1. Providing preflight and inflight service to all pilots.
  2. Keeping aircraft safely separated while operating in controlled airspace.

 The preflight and inflight services to pilots are the responsibility of the Flight Service Stations (FSS). An extensive teletype and interphone system permits relay of information from many sources. Many of the services provided by tower and flight service personnel are familiar to the VFR pilot. Aircraft separation is the primary responsibility of both Airport Traffic Control Towers and Air Route Traffic Control Centers (ARTCC). Knowledge of the physical setup and services provided by each type of facility enables the instrument pilot to get information and assistance and to communicate with the appropriate controllers with confidence and efficiency.

Airport Traffic Control Towers

 Jurisdiction. The ATC tower is responsible for control of aircraft on and in the immediate vicinity of airports. Terminals handling a large traffic volume employ specialized personnel for operations; they use light signals, radio, and ASDE radar (Airport Surface Detection Equipment) for control of surface traffic. Less congested airports have fewer controllers to handle the workload with less specialization.

 Organization of the tower operations falls into the following units:

  Local Control is concerned mainly with VFR traffic in and around the traffic pattern and with ground traffic. The local controller works with the other IFR controllers to integrate VFR and IFR flights into a smooth, safe traffic flow in and out of the airport.

  Ground Control directs the movement of aircraft on the airport surface, working closely with other tower positions. The controller relays clearances from ARTCC to departing IFR flights unless a special position is assigned that function.
  Clearance Delivery is accomplished by a separate controller at most busy terminals where heavy ground traffic and frequent IFR departures require division of the workload.

  Departure Control originates departure clearances and instructions to provide separation between departing and arriving IFR flights. Although the physical location of this controller varies at different terminals - in the tower at some locations, in separate radar installations at others - there is a close coordination with the Approach Controller and Local Controller.

  Approach Control formulates and issues approach clearances and instructions to provide separation between arriving IFR aircraft, using radar if available.

Tower Services Provide:
 1. Control of aircraft on, and in the vicinity of the airport.
 2. Coordination with pilots and Air Route Traffic Control Centers for IFR clearances.
 3. Air traffic advisories to pilots concerning observed, reported, and estimated positions of aircraft that might present a hazard to a particular flight.
 4. Flight assistance, including transmission of pilot reports and requests, and weather advisories.

Approach/departure Control Services Provide:
 1. Navigation assistance by radar vector for departing and arriving aircraft.
 2. DF assistance to lost aircraft, and cooperation with other facilities in Search and Rescue operations.
 Some of the larger towers have more than one of the above described positions of operation, each position having its area of responsibility on, or in the vicinity of, the airport. Also, the APPROACH CONTROL function at some highly complex multi-airport metropolitan areas has been established separate from and serving several major or less busy airports.

Air Route Traffic Control Centers

 Jurisdiction. The primary function of an ARTCC is to provide air traffic control service to aircraft operating on IFR flight plans within controlled airspace, principally during the enroute phase of flight. When equipment capability and controller workload permit, certain advisory/assistance services may be provided to VFR aircraft.

 Organization. The ARTCC facilities are located throughout the United Sates at central points in areas over which they exercise control. Each Center controls IFR traffic within its own area and coordinates with adjacent Centers for the orderly flow of traffic from area to area. Each Center's control area is divided into Sectors, based upon traffic flow patterns and controller workload.

 Each Sector Controller normally has a sector discrete frequency for direct communications with IFR flights within the sector. As an IFR flight departs, the affected Sector Controller follows the progress of the flight, maintaining a continuing record of route, altitude, and time, and monitors the flight with long-range radar equipment when available. Each Sector Controller has a sector discrete frequency for direct communication with IFR flights within the sector. As an IFR flight progresses to adjacent sectors and centers, and finally to the destination terminal facility, the IFR pilot is requested to change to appropriate frequencies.

 The chief functions of the Air Route Traffic Control Centers are to: (1) control aircraft operating under IFR in controlled airspace, (2) provide air traffic advisories to aircraft concerning potential hazards to flight, anticipated delays, and any other data of importance to the pilot for the safe conduct of the flight, (3) provide navigation assistance by radar vectors for detouring thunderstorms and expediting routing, (4) transmit pilot reports and weather advisories to enroute aircraft, and (5) provide flight assistance to aircraft in distress.

Air Traffic Control Automation

NAS Enroute Stage A

 This term refers to ATC's automation of the routine tasks of enroute traffic control through the use of computers. The automation is being accomplished in two phases, the flight data processing phase and the radar data processing phase. In the flight data processing phase, the computer takes over the task of processing, printing, distributing, and updating flight plans. In the past, controllers have performed these clerical chores.

 The radar data processing phase of enroute automation involves bringing vital information about each flight to the radar scope. The controller formerly transferred control information from the flight progress strips to plastic markers called "shrimp boats." These "shrimp boats" were manually kept in association with the displayed radar targets. The controller had to remember which flight progress strip applied to each radar target and "shrimp boat" on the scope - a process which called for heavy concentration and constant verification. Through an electronic process, the computer converts digital information into letters and numbers and projects a bright tag containing all necessary flight data on the radar scope. This alphanumeric display includes flight identification, assigned altitude, Mode C altitude, computer identification, and a special field that includes control data. Whether an aircraft is climbing or descending is also graphically shown by an arrow pointing up or down. The data tag is attached to the radar target of each aircraft being controlled. A controller "hands of" an aircraft to another controller by making a keyboard entry to the computer, causing the data tag to transfer to the new controller's scope. The tag blinks until the new controller acknowledges the handoff. The Mode C altitude readout is accomplished through the aircraft's transponder and encoding altimeter.


 This automated system provides controllers at medium and high activity terminal facilities information similar to NAS Enroute Stage A. This includes the display of aircraft flight data and automated handoff capability within the terminal facility with other ARTS-III facilities and with NAS Enroute Stage A.

 New programs have been added to the system such as Minimum Safe Altitude Warning (MSAW) and Conflict Alert. The former alerts the controller when a controlled aircraft is below or is predicted to descend below a minimum designated altitude or minimum descent altitude (MDA) if past the final approach fix. The latter alerts the controller to unsafe or potentially unsafe proximity between controlled aircraft. These alerts are in the form of a warning message displayed blinking on the controller's radar display accompanied by an aural alarm.