Commercial, Instrument, Private

The Air Traffic Control System

The Air Traffic Control System

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.

If ATC assigns a frequency that cannot be selected on your radio, ask for an alternative frequency.

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

The audio panel has two positions for receiver selection, cabin speaker and headphone.

A headset with a boom microphone is recommended for clear communications.

Switching the transmitter selector between COM1 and COM2 changes both the transmitter and receiver frequencies.

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

Radar and Transponders

A transponder is a radar beacon transmitter/receiver installed in the instrument panel

Works on continuous interrogation and reply  signal.

Flickers on the transponders is indication  the receipt and sending of these interrogation signals

ATC assigns Transponder Codes

If asked to ident your return signal is intensified on the controllers scope.

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 if the aircraft is equipped with an encoding altimeter or blind encoder

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.

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.

Air traffic controllers must follow the guidance of the Air Traffic Control Manual when communicating with pilots

Controllers are faced with a wide variety of communication styles based on pilot experience, proficiency, and professionalism.

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.

Communication Facilities

The controller’s primary responsibility is separation of aircraft operating under IFR.

This is accomplished with ATC facilities which include:

Automated Flight Service Station (AFSS)

Airport Traffic Control Tower (ATCT)

Terminal Radar Approach Control (TRACON)

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.

Flight Service can be obtained by dialing 1-800-WX-BRIEF anywhere in the United States.

There are a variety of methods of making radio contact:

Direct transmission

Remote communications outlets (RCOs)

Ground communication outlets (GCOs),

Duplex transmissions, through navigational aids (NAVAIDs)

The briefer will send your flight plan to the host computer at the ARTCC (Center).

flight strips  are sent to the tower, to the radar facility that will handle your departure route, and to the Center controller whose sector you will first enter.

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 mist be airborne prior to Clearance Void Time

Air Traffic Control Towers

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

If 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.


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.

Memory aid for IFR clearance format:

Clearance limit

Route (including DP, if any)



Transponder code

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

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.

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.

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.

The “local” controller is responsible for operations in the Class D airspace and on the active runways.

At visual flight rules (VFR) towers, the local controller accepts inbound IFR flights from the terminal radar facility and cannot provide vectors.

The departure radar controller may be in the same building as the control tower. 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 controllers can assign altitudes lower than published procedural altitudes called minimum vectoring altitudes


However, if you are assigned an altitude that seems to be 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 controller can fit your flight into the departure flow.

When you receive takeoff clearance, the departure controller is aware of your flight and is waiting for your call.

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)

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.

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.

Earlier Center radars display weather as an area of slashes (light precipitation) and H’s (moderate rainfall).

Newer radar displays show weather as three levels of blue.

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

You will find all Center frequencies in the back of the A/FD they are also found on en route charts.

Each ARTCC’s area of responsibility covers several states; 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..

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-than standard climb gradient (200 feet per NM), you should be so advised by the controller.

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.

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.

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. 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:

1. AFSS: Obtain a weather briefing file your flight plan by calling 1-800-WXBRIEF.

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

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

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

5. Tower: Pretakeoff checks complete, receive clearance to takeoff.

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

7. 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.

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

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

10. 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.

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  reparations 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)

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.

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.


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