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Chapter 9 - The Air Taffic Control System

Chapter 9 - The Air Taffic Control System


This chapter will cover the communication equipment, communication procedures, and air traffic control (ATC) facilities and services available for a flight under instrument flight rules (IFR) in the National Airspace System (NAS).

Communication Equipment

Navigation/Communication (NAV/COM) Equipment

Civilian pilots communicate with ATC on frequencies in the very high frequency (VHF) range between 118.000 and

136.975 MHz. To derive full benefit from the ATC system, radios capable of 25 kHz spacing are required (e.g., 134.500, 134.575, 134.600, etc.). If ATC assigns a frequency that cannot be selected on your radio, ask for an alternative frequency.

figure 9-1 illustrates a typical radio panel installation, consisting of a communications transceiver on the left and a navigational receiver on the right. Many radios allow the pilot to have one or more frequencies stored in memory and one frequency active for transmitting and receiving (called simplex operation). It is possible to communicate with some automated flight service stations (AFSS) by transmitting on 122.1 MHz (selected on the communication radio) and receiving on a VHF omnidirectional range (VOR) frequency (selected on the navigation radio). This is called duplex operation.

Simplex: Transmitting and Duplex: Transmitting on one frereceiving on the same frequency. quency and receiving on a separate


An audio panel allows you to adjust the volume of the selected receiver(s) and to select the desired transmitter. [figure 9-2] The audio panel has two positions for receiver selection, cabin speaker and headphone (some units might have a center “off” position). Use of a hand-held microphone and the cabin speaker introduces the distraction of reaching for and hanging up the microphone. A headset with a boom microphone is recommended for clear communications. Position the microphone close to your lips because ambient cockpit noise may interfere with the controller understanding your transmissions. Headphones will deliver the received signal directly to your ears; therefore, ambient noise does not interfere with your ability to understand the transmission. [figure 9-3]

Switching the transmitter selector between COM1 and COM2 changes both the transmitter and receiver frequencies. It should be necessary to select a receiver, communication or navigation, only when you want to monitor one communications frequency while communicating on another. One example is listening to automatic terminal information service (ATIS) on one receiver while communicating with ATC on the other. Monitoring a navigation receiver to check for proper identification is another reason to use the switch panel.

Most audio switch panels also include a marker beacon receiver; all marker beacons transmit on 75 MHz, so there is no frequency selector.

figure 9-4 illustrates an increasingly popular form of NAV/ COM radio; it contains a global positioning system (GPS) receiver and a communications transceiver. Using its navigational capability, this unit can determine when your flight crosses an airspace boundary or fix and can automatically select the appropriate communications frequency for that location in the communications radio.

Radar and Transponders

ATC radars have a limited ability to display primary returns, which is energy reflected from an aircraft’s metallic structure. Their ability to display secondary returns (transponder replies to ground interrogation signals) makes possible the many advantages of automation.

NAV/COM: Combined Transponder code: One of 4,096 communication and navigation radio. four-digit discrete codes ATC will assign to distinguish between aircraft.

Radar: Radio Detection And Ranging.

A transponder is a radar beacon transmitter/receiver installed in the instrument panel. ATC beacon transmitters send out interrogation signals continuously as the radar antenna rotates. When an interrogation is received by your transponder, a coded reply is sent to the ground station where it is displayed on the controller’s scope. A “Reply” light on your transponder panel flickers every time you receive and reply to a radar interrogation. Transponder codes are assigned by ATC.

When a controller asks you to “ident” and you push the ident button, your return on the controller’s scope is intensified for precise identification of your flight. When requested, briefly push the ident button to activate this feature. It is good practice to verbally confirm you have changed codes or pushed the ident button.

Mode C (Altitude Reporting)

Primary radar returns indicate only range and bearing from the radar antenna to the target; secondary radar returns can display altitude Mode C on the control scope if the aircraft is equipped with an encoding altimeter or blind encoder. In either case, when the transponder’s function switch is in the ALT position the aircraft’s pressure altitude is sent to the controller. Adjusting the altimeter’s Kollsman window has no effect on the altitude read by the controller.

Transponders must be ON at all times when operating in controlled airspace; altitude reporting is required by regulation in Class B and Class C airspace and inside of a 30-mile circle surrounding the primary airport in Class B airspace. Altitude reporting should also be ON at all times.

Ident: Push the button on the Pressure altitude: Altitude above transponder to identify your return the standard 29.92" Hg plane. on the controller’s scope.

Kollsman window: Adjustment for Mode C: Altitude reporting altimeter setting. transponder mode.

Communication Procedures

Clarity in communication is essential for a safe instrument flight. This requires pilots and controllers to use terms that are understood by both — the Pilot/Controller Glossary in the Aeronautical Information Manual (AIM) is the best source of terms and definitions. The AIM is revised twice a year and new definitions are added, so the Glossary should be reviewed frequently. Because clearances and instructions are comprised largely of letters and numbers, a phonetic pronunciation guide has been developed for both. [figure 9-5]

Air traffic controllers must follow the guidance of the Air Traffic Control Manual when communicating with pilots. The manual presents the controller with different situations and prescribes precise terminology that must be used. This is advantageous for pilots, because once they have recognized a pattern or format they can expect future controller transmissions to follow that format. Controllers are faced with a wide variety of communication styles based on pilot experience, proficiency, and professionalism.

Pilots should study the examples in the AIM, listen to other pilots communicate, and apply the lessons learned to their own communications with ATC. Pilots should ask for clarification of a clearance or instruction. If necessary, use plain English to ensure understanding, and expect the controller to reply in the same way. A safe instrument flight is the result of cooperation between controller and pilot.

Communication Facilities

The controller’s primary responsibility is separation of aircraft operating under IFR. This is accomplished with ATC facilities which include the AFSS, airport traffic control tower (ATCT), terminal radar approach control (TRACON), and air route traffic control center (ARTCC).

Automated Flight Service Stations (AFSS)

Your first contact with ATC will probably be through AFSS, either by radio or telephone. AFSS’s provide pilot briefings, receives and processes flight plans, relays ATC clearances, originates Notices to Airmen (NOTAMs), and broadcasts aviation weather. Some facilities provide En Route Flight Advisory Service (EFAS), take weather observations, and advise United States (U.S.) Customs and Immigration of international flights.

En Route Flight Advisory Service (EFAS): An en route weather-only AFSS service.

Telephone contact with Flight Service can be obtained by dialing 1-800-WX-BRIEF anywhere in the United States— you will be connected to the nearest AFSS based on the area code from which you are calling. There are a variety of methods of making radio contact: direct transmission, remote communications outlets (RCOs), ground communication outlets (GCOs), and by using duplex transmissions, through navigational aids (NAVAIDs). The best source of information on frequency usage is the Airport/Facility Directory (A/FD), and the legend panel on sectional charts also contains contact information.

The briefer will send your flight plan to the host computer at the ARTCC (Center). After processing your flight plan, the computer will send flight strips to the tower, to the radar facility that will handle your departure route, and to the Center controller whose sector you will first enter. figure 9-6 shows a typical strip. These strips will be delivered approximately 30 minutes prior to your proposed departure time. Strips will be delivered to en route facilities 30 minutes before you are expected to enter their airspace. If you fail to open your flight plan, it will “time out” 2 hours after your proposed departure time.

When departing an airport in Class G airspace, you will receive your IFR clearance from the AFSS by radio or telephone. It will contain either a “clearance void” time, in which case you must be airborne prior to that time, or a “release” time—you should not be airborne prior to release time. You can help the controller by stating how soon you expect to be airborne. If your void time is, for example, 10 minutes past the hour and you are airborne at exactly 10 minutes past the hour, your clearance is void—you must be airborne prior to the void time. You may ask for a specific void time when filing your flight plan.

Air Traffic Control Towers

Several controllers in the tower cab will be involved in handling your instrument flight. Where there is a dedicated clearance delivery position, that frequency will be found in the A/FD and on the instrument approach chart for the departure airport. Where there is no clearance delivery position, the ground controller will perform this function. At the busiest airports, pre-taxi clearance is required; the frequency for pre-taxi clearance can be found in the A/FD. Taxi clearance should be requested not more than 10 minutes before proposed taxi time.

It is recommended that you read your IFR clearance back to the clearance delivery controller. Instrument clearances can be overwhelming if you try to copy them verbatim, but they follow a format that allows you to be prepared when you say “Ready to copy.” The format is: Clearance limit (usually the destination airport); Route, including any departure procedure; initial Altitude; Frequency (for departure control); and Transponder code. With the exception of the transponder code, you will know most of these items before engine start. One technique for clearance copying is writing C-R-A-F-T.

Clearance void time: Used by ATC to advise an aircraft that the departure clearance is automatically canceled if takeoff is not made prior to a specified time. The pilot must obtain a new clearance or cancel the IFR flight plan if not off by the specified time.

Memory aid for IFR clearance format:

C learance limit R oute (including DP, if any) A ltitude F requency T ransponder code

Assume you have filed an IFR flight plan from Seattle, Washington to Sacramento, California via V-23 at 7,000 feet. You note traffic is taking off to the north from Seattle-Tacoma (Sea-Tac) airport and, by monitoring the clearance delivery frequency, you note the departure procedure being assigned to southbound flights. Your clearance limit will be the destination airport, so you can write “SAC” after the letter C. Write “SEATTLE TWO – V23” after R for Route, because you heard departure control issue this departure to other flights (you could also call the tower on the telephone to ask what departure is in use). Write “7” after the A, the departure control frequency printed on the approach charts for Sea-Tac after F, and leave the space after T blank—the transponder code is generated by computer and can seldom be determined in advance. Now call clearance delivery and report ready to copy.

As the controller reads the clearance, check it against what you have already written down; if there is a change, draw a line through that item and write in the changed item. Chances are the changes will be minimal, and you will have “copied” most of your clearance before keying the microphone. Still, it is worthwhile to develop your own clearance shorthand to cut down on the verbiage that must be copied (see appendix 1).

You are required to have either the text or a graphic representation of a departure procedure (DP) (if one is available), and should review it before you accept your clearance. This is another reason to find out ahead of time which DP is in use. If the DP includes an altitude or a departure control frequency, those items will not be included in the clearance delivered to you from the tower cab.

The last clearance received supersedes all previous clearances. For instance, if the DP says “Climb and maintain 2,000 feet, expect higher in 6 miles” and upon contacting the departure controller you hear “Climb and maintain 8,000 feet,” the 2,000-foot restriction has been canceled. This rule applies in both terminal and Center airspace.

If you report ready to copy your IFR clearance before the strip has been received from the Center computer, you will be advised “clearance on request” and the controller will call you when it has been received. Use this time for taxi and pretakeoff checks.

Clearance on request: Clearance Vectoring: Navigational guidance by has not been received. assigning headings.

Class D airspace: Airspace con-Minimum vectoring altitude

trolled by an operating control tower. (MVA): An IFR altitude, lower than

the minimum en route altitude

(MEA), that provides terrain and

obstacle clearance.

The “local” controller is responsible for operations in the Class D airspace and on the active runways. At some towers, designated as IFR towers, the local controller has vectoring authority. At visual flight rules (VFR) towers, the local controller accepts inbound IFR flights from the terminal radar facility and cannot provide vectors. The local controller also coordinates flights in the local area with radar controllers. Although Class D airspace normally extends 2,500 feet above field elevation, towers frequently release the top 500 feet to the radar controllers to facilitate overflights. Accordingly, when your flight is vectored over an airport at an altitude that appears to enter the tower controller’s airspace, there is no need for you to contact the tower controller— all coordination is handled by ATC.

The departure radar controller may be in the same building as the control tower, but it is more likely that the departure radar position is remotely located. The tower controller will not issue a takeoff clearance until the departure controller issues a release.

Terminal Radar Approach Control (TRACON)

TRACONs are considered terminal facilities because they provide the link between the departure airport and the en route structure of the NAS. Terminal airspace normally extends 30 nautical miles (NM) from the facility, with a vertical extent of 10,000 feet; however, dimensions vary widely. Class B and Class C airspace dimensions are provided on aeronautical charts. At terminal radar facilities the airspace is divided into sectors, each with one or more controllers, and each sector is assigned a discrete radio frequency. All terminal facilities are approach controls, and should be addressed as “Approach” except when directed to do otherwise (“Contact departure on 120.4”).

Terminal radar antennas are located on or adjacent to the airport. figure 9-7 shows a typical configuration. Terminal controllers can assign altitudes lower than published procedural altitudes called minimum vectoring altitudes (MVAs). These altitudes are not published and accessible to pilots, but are displayed at the controller’s position, as shown in figure 9-8. However, if you are assigned an altitude that seems to be too low, query the controller before descending.

Terrain Clearance Responsibility

Terrain clearance responsibility is yours as pilot in command, until you reach the controller’s MVA; but even then, if the altitude assigned seems too low, query the controller before descending.

When you receive and accept your clearance and report ready for takeoff, a controller in the tower contacts the TRACON for a release—you will not be released until the departure A TRACON controller uses Airport Surveillance Radar controller can fit your flight into the departure flow. You may (ASR) to detect primary targets and Automated Radar have to hold for release. When you receive takeoff clearance, Terminal Systems (ARTS) to receive transponder signals; the the departure controller is aware of your flight and is waiting two are combined on the controller’s scope. [figure 9-9] for your call. All of the information the controller needs is on the departure strip or the computer screen, so you need not repeat any portion of your clearance to that controller; simply establish contact with the facility when instructed to do so by the tower controller. The terminal facility computer will pick up your transponder and initiate tracking as soon as it detects the assigned code; for this reason, the transponder should remain on standby until takeoff clearance has been received.

Your aircraft will appear on the controller’s radar as a target with an associated data block that moves as your aircraft moves through the airspace. The data block includes aircraft identification, aircraft type, altitude, and airspeed.

At facilities with ASR-3 equipment, radar returns from precipitation are not displayed as varying levels of intensity, and controllers must rely on pilot reports and experience to provide weather avoidance information. With ASR-9 equipment, the controller can select up to six levels of intensity. Level 1 precipitation does not require avoidance tactics, but the presence of levels 2 or 3 should cause pilots to investigate further. The returns from higher levels of intensity may obscure aircraft data blocks, and controllers may select the higher levels only on pilot request. When you are uncertain about the weather ahead, ask the controller if the facility can display intensity levels— pilots of small aircraft should avoid intensity levels 3 or higher.

Tower En Route Control (TEC)

At many locations, instrument flights can be conducted entirely in terminal airspace. These TEC routes are generally for aircraft operating below 10,000 feet, and they can be found in the A/FD. Pilots desiring to use TEC should include that designation in the remarks section of the flight plan.

Pilots are not limited to the major airports at the city pairs listed in the A/FD. For example, a tower en route flight from an airport in New York (NYC) airspace could terminate at any airport within approximately 30 miles of Bradley International (BDL) airspace, such as Hartford (HFD). [figure 9-10]

A valuable service provided by the automated radar equipment at terminal radar facilities is the Minimum Safe Altitude Warnings (MSAW). This equipment predicts your aircraft’s position in 2 minutes based on present path of flight — the controller will issue a safety alert if the projected path will encounter terrain or an obstruction. An unusually rapid descent rate on a nonprecision approach can trigger such an alert.

Air Route Traffic Control Centers (ARTCC)

Air route traffic control center facilities are responsible for maintaining separation between IFR flights in the en route structure. Center radars (Air Route Surveillance Radar) acquire and track transponder returns using the same basic technology as terminal radars. [figure 9-11]

Nonprecision approach: An instrument approach without vertical guidance.

Earlier Center radars display weather as an area of slashes (light precipitation) and H’s (moderate rainfall), as illustrated in figure 9-12. Because the controller cannot detect higher levels of precipitation, pilots should be wary of areas showing moderate rainfall. Newer radar displays show weather as three levels of blue. Controllers can select the level of weather to be displayed. Weather displays of higher levels of intensity can make it difficult for controllers to see aircraft data blocks, so pilots should not expect ATC to keep weather displayed continuously.

Center airspace is divided into sectors in the same manner as terminal airspace; additionally, most Center airspace is divided by altitudes into high and low sectors. Each sector has a dedicated team of controllers and a selection of radio frequencies, because each Center has a network of remote transmitter/receiver sites. You will find all Center frequencies in the back of the A/FD in the format shown in figure 9-13; they are also found on en route charts.

Each ARTCC’s area of responsibility covers several states; as you fly from the vicinity of one remote communication site toward another, expect the same controller to talk to you on different frequencies.

Center Approach/Departure Control

The majority of airports with instrument approaches do not lie within terminal radar airspace, and when operating to or from these airports you will communicate directly with the Center controller. If you are departing a tower-controlled airport, the tower controller will provide instructions for contacting the appropriate Center controller. When you depart an airport without an operating control tower, your clearance will include instructions such as “Upon entering controlled airspace, contact Houston Center on 126.5.” You are responsible for terrain clearance until you reach the controller’s MVA. Simply hearing “Radar contact” is not sufficient to relieve you of this responsibility.

If obstacles in the departure path require a steeper-thanstandard climb gradient (200 feet per NM), you should be so advised by the controller. However, you should check the departure airport listing in the A/FD to determine if there are trees or wires in the departure path just to be sure; when in doubt, ask the controller for the required climb gradient.

A common clearance in these situations is “When able, proceed direct to the Astoria VOR…” The words “when able” mean to proceed when you can do so while maintaining terrain and obstruction clearance—they do not mean to proceed as soon as a signal suitable for navigation is received from the NAVAID. Using the standard climb gradient, you will be 2 miles from the departure end of the runway before it is safe to turn (400 feet above ground level (AGL)). When a Center controller issues a heading, a direct route, or says “direct when able,” the controller becomes responsible for terrain and obstruction clearance.

Another common Center clearance is “Leaving (altitude) fly (heading) or proceed direct when able.” This keeps the terrain/ obstruction clearance responsibility in the cockpit until above the minimum IFR altitude. A controller cannot issue an IFR clearance until you are above the minimum IFR altitude unless you are able to climb in VFR conditions.

On a Center controller’s scope, 1 NM is about 1/28 of an inch; when a Center controller is providing Approach/Departure control services at an airport many miles from the radar antenna, estimating headings and distances is very difficult. Controllers providing vectors to final must set the range on their scopes to not more than 125 NM; this is to provide the greatest possible accuracy for intercept headings. Accordingly, at locations more distant from a Center radar antenna, pilots should expect a minimum of vectoring.

Control Sequence

The IFR system is flexible and accommodating if you have done your homework, have as many frequencies as possible written down before they are needed, and have an alternate in mind if your flight cannot be completed as planned. Familiarize yourself with all the facilities and services available on your route of flight. [figure 9-14] Always know where the nearest VFR conditions can be found, and be prepared to head in that direction if your situation deteriorates.

A typical IFR flight, with departure and arrival at airports with control towers, would use the ATC facilities and services in the following sequence:

    AFSS: Obtain a weather briefing for your departure, destination and alternate airports, and en route conditions, then file your flight plan by calling 1-800-WXBRIEF.

    ATIS: Preflight complete, listen for present conditions and the approach in use.

    Clearance Delivery: Prior to taxiing, obtain your departure clearance.

    Ground Control: Noting that you are IFR, receive taxi instructions.

    Tower: Pretakeoff checks complete, receive clearance to takeoff.

    Departure Control: Once your transponder “tags up” with the ARTS, the tower controller will instruct you to contact Departure to establish radar contact.

    ARTCC: After departing the departure controller’s airspace, you will be handed off to Center who will coordinate your flight while en route. You may be in contact with multiple ARTCC facilities; they will coordinate the hand-offs.

    EFAS/HIWAS: Coordinate with ATC before leaving their frequency to obtain inflight weather information.

    ATIS: Coordinate with ATC before leaving their frequency to obtain ATIS information.

  1. Approach Control: Center will hand you off to approach control where you will receive additional information and clearances.

11. Tower: Once cleared for the approach, you will be instructed to contact tower control; your flight plan will be canceled by the tower controller upon landing.

A typical IFR flight, with departure and arrival at airports without operating control towers, would use the ATC facilities and services in the following sequence:

  1. AFSS: Obtain a weather briefing for your departure, destination and alternate airports, and en route conditions, then file your flight plan by calling 1-800-WX-BRIEF. Provide the latitude/longitude description for small airports to ensure that Center is able to locate your departure and arrival locations.
  2. AFSS or UNICOM: ATC clearances can be filed and received on the UNICOM frequency if the licensee has made arrangements with the controlling ARTCC; otherwise, you need to file with AFSS via telephone. Be sure your preflight preparations are complete before filing. Your clearance will include a clearance void time. You must be airborne prior to the void time.
  3. ARTCC: After takeoff, establish contact with Center. You may be in contact with multiple ARTCC facilities; they will coordinate the hand-offs.
  4. EFAS/HIWAS: Coordinate with ATC before leaving their frequency to obtain in-flight weather information.
  5. Approach Control: Center will hand you off to approach control where you will receive additional information and clearances. If you are able to land under visual meteorological conditions (VMC), you may cancel your IFR clearance before landing.

Letters of Agreement (LOA)

The ATC system is indeed a system and very little happens by chance. As your flight progresses, controllers in adjoining sectors or adjoining Centers coordinate its handling by telephone or by computer. Where there is a boundary between the airspace controlled by different facilities, the location and altitude at which you will be handed off is determined by Letters of Agreement (LOA) negotiated between the two facility managers. This information is not available to you in any Federal Aviation Administration (FAA) publication. For this reason, it is good practice to note on your en route chart the points at which hand-offs occur as you fly over them. Each time you are handed off to a different facility, the controller knows your altitude and where you are—this was part of the hand-off procedure.

figure 9-14. ATC facilities, services, and radio call signs.