Part 1: Chapter 2 —… | FAA Instrument Ch. 2

The Air Traffic

Control System

Chapter 2

Introduction

This chapter covers 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).

Figure 2-2. Audio panel.

Figure 2-1. Typical navigation/communication installation.

Communication Equipment

Navigation/Communication 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). If ATC assigns a frequency that cannot

be selected, ask for an alternative frequency.

Figure 2-1 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

flight service stations (FSS) 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.

An audio panel allows a pilot to adjust the volume of the

selected receiver(s) and to select the desired transmitter.

[Figure 2-2] 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. The

microphone should be positioned close to the lips to reduce

Figure 2-3. Boom microphone, headset, and push-to-talk switch.

Figure 2-4. Combination GPS-com unit.

Headset

Push-to-talk switch

Boom microphone

the possibility of ambient flight deck noise interfering with

transmissions to the controller. Headphones deliver the

received signal directly to the ears; therefore, ambient noise

does not interfere with the pilot’s ability to understand the

transmission. [Figure 2-3]

Switching the transmitter selector between COM1 and

COM2 changes both transmitter and receiver frequencies.

It is necessary only when a pilot wants to monitor one

frequency while transmitting 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 2-4 illustrates an increasingly popular form of

navigation/communication radio; it contains a global

positioning system (GPS) receiver and a communications

transceiver. Using its navigational capability, this unit can

determine when a 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.

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 a transponder, a

coded reply is sent to the ground station where it is displayed

on the controller’s scope. A reply light on the transponder

panel flickers every time it receives and replies to a radar

interrogation. Transponder codes are assigned by ATC.

When a controller asks a pilot to “ident” and the ident button

is pushed, the return on the controller’s scope is intensified for

precise identification of a flight. When requested, briefly push

the ident button to activate this feature. It is good practice

for pilots to verbally confirm that they 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, when installed, 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 a

30-mile circle surrounding the primary airport in Class B

airspace. Altitude reporting should also be ON at all times.

Character Phonic

(Pronunciation) Morse Code Telephony

Figure 9-5. Phonetic Pronunciation Guide. Figure 2-5. Phonetic pronunciation guide.

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 2-5]

ATC 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, to include the FSS, airport traffic control tower

(ATCT), terminal radar approach control (TRACON), and

air route traffic control center (ARTCC).

Flight Service Stations (FSS)

A pilot’s first contact with ATC is usually through FSS,

either by radio or telephone. FSSs provide pilot briefings,

receive and process flight plans, relay ATC clearances,

originate Notices to Airmen (NOTAMs), and broadcast

aviation weather. Some facilities provide En Route Flight

Advisory Service (EFAS), take weather observations,

and advise United States Customs and Immigration of

international flights.

Telephone contact with Flight Service can be obtained

by dialing 1-800-WX-BRIEF. This number can be used

anywhere in the United States and connects to the nearest

FSS based on the area code from which the call originates.

There are a variety of methods of making radio contact:

direct transmission, remote communication 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.

Altitude—37,000 feet Destination—Minneapolis

Departure point—San DiegoCall sign—Northwest 196

Figure 9-6. Flight Strip. Figure 2-6. Flight strip.

The briefer sends a flight plan to the host computer at the

ARTCC (Center). After processing the flight plan, the

computer sends flight strips to the tower, to the radar facility

that handles the departure route, and to the Center controller

whose sector the flight first enters. Figure 2-6 shows a typical

strip. These strips are delivered approximately 30 minutes

prior to the proposed departure time. Strips are delivered to

en route facilities 30 minutes before the flight is expected to

enter their airspace. If a flight plan is not opened, it will “time

out” 2 hours after the proposed departure time.

When departing an airport in Class G airspace, a pilot receives

an IFR clearance from the FSS by radio or telephone. It

contains either a clearance void time, in which case an aircraft

must be airborne prior to that time, or a release time. Pilots

should not take off prior to the release time. Pilots can help

the controller by stating how soon they expect to be airborne.

If the void time is, for example, 10 minutes past the hour and

an aircraft is airborne at exactly 10 minutes past the hour,

the clearance is void—a pilot must take off prior to the void

time. A specific void time may be requested when filing a

flight plan.

ATC Towers

Several controllers in the tower cab are involved in handling

an instrument flight. Where there is a dedicated clearance

delivery position, that frequency is 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 performs 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 pilots read their IFR clearance back to

the clearance delivery controller. Instrument clearances can

be overwhelming when attempting to copy them verbatim,

but they follow a format that allows a pilot to be prepared

when responding “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, a pilot knows most of these items before

engine start. One technique for clearance copying is writing

C-R-A-F-T.

Assume an IFR flight plan has been filed from Seattle,

Washington to Sacramento, California via V-23 at 7,000

feet. Traffic is taking off to the north from Seattle-Tacoma

(Sea-Tac) airport and, by monitoring the clearance delivery

frequency, a pilot can determine the departure procedure

being assigned to southbound flights. The clearance limit

is the destination airport, so write “SAC” after the letter C.

Write “SEATTLE TWO – V23” after R for Route because

departure control issued this departure to other flights. Write

“70” after the A, the departure control frequency printed on

the approach charts for Sea-Tac after F, and leave the space

after the letter T blank—the transponder code is generated by

computer and can seldom be determined in advance. Then,

call clearance delivery and report “Ready to copy.”

As the controller reads the clearance, check it against what

is already written down; if there is a change, draw a line

through that item and write in the changed item. Chances

are the changes are minimal, and most of the clearance is

copied before keying the microphone. Still, it is worthwhile

to develop clearance shorthand to decrease the verbiage that

must be copied (see Appendix 1).

Pilots are required to have either the text of a departure

procedure (DP) or a graphic representation (if one is

available), and should review it before accepting a 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 are not included in the clearance.

The last clearance received supersedes all previous clearances.

For example, if the DP says “Climb and maintain 2,000 feet,

expect higher in 6 miles,” but upon contacting the departure

controller a new clearance is received: “Climb and maintain

8,000 feet,” the 2,000 feet restriction has been canceled. This

rule applies in both terminal and Center airspace.

Figure 9-7. Combined Radar and Beacon Antenna. Figure 2-7. Combined radar and beacon antenna.

013 348

Figure 9-8. Minimum Vectoring Altitude Chart.

Figure 2-8. Minimum vectoring altitude (MVA) chart.

When reporting “ready to copy” an IFR clearance before

the strip has been received from the Center computer, pilots

are advised “clearance on request.” The controller initiates

contact when it has been received. This time can be used for

taxi and pre-takeoff checks.

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 a flight is vectored over an airport at an altitude that

appears to enter the tower controller’s airspace, there is no

need 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 (e.g.,

“Contact departure on 120.4.”).

Terminal radar antennas are located on or adjacent to the

airport. Figure 2-7 shows a typical configuration. Terminal

controllers can assign altitudes lower than published

procedural altitudes called minimum vectoring altitudes

(MVAs). These altitudes are not published or accessible

to pilots, but are displayed at the controller’s position.

[Figure 2-8] However, when pilots are assigned an altitude

that seems to be too low, they should query the controller

before descending.

When a pilot accepts a clearance and reports ready for takeoff,

a controller in the tower contacts the TRACON for a release.

An aircraft is not cleared for takeoff until the departure

controller can fit the flight into the departure flow. A pilot may

have to hold for release. When takeoff clearance is received,

the departure controller is aware of the flight and is waiting

for a call. All of the information the controller needs is on

the departure strip or the computer screen; there is no need to

repeat any portion of the clearance to that controller. Simply

establish contact with the facility when instructed to do so

by the tower controller. The terminal facility computer picks

up the transponder and initiates tracking as soon as it detects

the assigned code. For this reason, the transponder should

remain on standby until takeoff clearance has been received.

The aircraft appears on the controller’s radar display as a

target with an associated data block that moves as the aircraft

moves through the airspace. The data block includes aircraft

identification, aircraft type, altitude, and airspeed.

A TRACON controller uses Airport Surveillance Radar

(ASR) to detect primary targets and Automated Radar

Terminal Systems (ARTS) to receive transponder signals;

the two are combined on the controller’s scope. [Figure 2-9]

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. Light precipitation does not require avoidance

tactics but precipitation levels of moderate, heavy, or

extreme should cause pilots to plan accordingly. Along

with precipitation, the pilot must additionally consider the

temperature, which if between –20° and +5 °C causes icing

even during light precipitation. The returns from higher levels

of intensity may obscure aircraft data blocks, and controllers

may select the higher levels only on pilot request. When

uncertainty exists 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 tower en route control

(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 2-10]

A valuable service provided by the automated radar

equipment at terminal radar facilities is the Minimum Safe

Altitude Warnings (MSAW). This equipment predicts an

aircraft’s position in 2 minutes based on present path of

flight—the controller issues a safety alert if the projected

path encounters terrain or an obstruction. An unusually

rapid descent rate on a nonprecision approach can trigger

such an alert.

Air Route Traffic Control Center (ARTCC)

ARTCC facilities are responsible for maintaining separation

between IFR flights in the en route structure. Center radars

(Air Route Surveillance Radar (ARSR)) acquire and track

transponder returns using the same basic technology as

terminal radars. [Figure 2-11]

Earlier Center radars display weather as an area of slashes

(light precipitation) and Hs (moderate rainfall), as illustrated

in Figure 2-12. 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. All Center frequencies can be found

in the back of the A/FD in the format shown in Figure 2-13;

they are also found on en route charts.

Each ARTCC’s area of responsibility covers several states;

when flying from the vicinity of one remote communication

site toward another, expect to hear the same controller 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, pilots communicate directly with the

Center controller. Departing from a tower-controlled airport,

the tower controller provides instructions for contacting the

appropriate Center controller. When departing an airport

without an operating control tower, the clearance includes

instructions such as “Upon entering controlled airspace,

contact Houston Center on 126.5.” Pilots are responsible

for terrain clearance until reaching the controller’s MVA.

Simply hearing “Radar contact” does not relieve a pilot of

this responsibility.

If obstacles in the departure path require a steeper-

than-standard climb gradient (200 feet per nautical mile

(FPNM)), then the controller advises the pilot. However,

it is the pilot’s responsibility to check the departure airport

listing in the A/FD to determine if there are trees or wires

in the departure path. When in doubt, ask the controller for

the required climb gradient.

Academy Planned View Display

° ° ° °

0013 31 29 89 A

ILS 28R OP

LVL LO 03 04

12 49

2AAL12

7AAL13

3DAL191

5NWA194

4N1212T

SD050

ILLEGAL ENTRY

MHD

B AAL629 1104 A

D DAL629 1102 M

C N41463 1124 B

F N44125 RDR

A UAL246 1103 C

AAL728

030 20

UAL132

040 22

RDR 15 S

ARTS III.

System data area

Untracked target

Nonselect code with mode C

Trackball position symbol

(inhibited when not in use)

Targets in suspend status

Coast/suspend tabular list

Readout area

Preview area

Arrival/departure tabular list

Tracked target center

Controlled partial data block

Partial data as seen by S

if worked by N

Tracked primary target

Untracked target select

code with mode C

Figure 2-9. The top image is a display as seen by controllers in an air traffic facility. It is an ARTS III (Automated Radar Terminal

System). The display shown provides an explanation of the symbols in the graphic. The lower figure is an example of the Digital Bright

Radar Indicator Tower Equipment (DBRITE) screen as seen by tower personnel. It provides tower controllers with a visual display

of the airport surveillance radar, beacon signals, and data received from ARTS III. The display shown provides an explanation of the

symbols in the graphic.

Outer marker

Radar

Indicates downwind alignment

guide for 100L and 28R

Delta 4210

Aircraft number: N1388V

Type aircraft: Cessna 421

Alt: 4,000’ Descending, 180 knots

Obstacles

VOR location

Airport center

Runway 16