6/17/21 

AIM 

Section 3.  En Route Procedures 

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1.  ARTCC Communications 

a.  Direct Communications, Controllers and 

Pilots. 

1. 

ARTCCs are capable of direct communica-

tions with IFR air traffic on certain frequencies. 
Maximum communications coverage is possible 
through the use of Remote Center Air/Ground 
(RCAG) sites comprised of both VHF and UHF 
transmitters and receivers. These sites are located 
throughout the U.S. Although they may be several 
hundred miles away from the ARTCC, they are 
remoted to the various ARTCCs by land lines or 
microwave links. Since IFR operations are expedited 
through the use of direct communications, pilots are 
requested to use these frequencies strictly for 
communications pertinent to the control of IFR 
aircraft. Flight plan filing, en route weather, weather 
forecasts, and similar data should be requested 
through FSSs, company radio, or appropriate military 
facilities capable of performing these services. 

2. 

An ARTCC is divided into sectors. Each 

sector is handled by one or a team of controllers and 
has its own sector discrete frequency. As a flight 
progresses from one sector to another, the pilot is 
requested to change to the appropriate sector discrete 
frequency. 

3. 

Controller Pilot Data Link Communications 

(CPDLC) is a system that supplements air/ground 
voice communications. The CPDLC’s principal 
operating criteria are: 

(a) 

Voice remains the primary and controlling 

air/ground communications means. 

(b) 

Participating aircraft will need to have the 

appropriate CPDLC avionics equipment in order to 
receive uplink or transmit downlink messages. 

(c) 

En Route CPDLC Initial Services offer the 

following services: Altimeter Setting (AS), Transfer 
of Communications (TOC), Initial Contact (IC), and 
limited route assignments, including airborne 
reroutes (ABRR), limited altitude assignments, and 
emergency messages. 

(1) 

Altimeter settings will be uplinked 

automatically when appropriate after a Monitor TOC. 

Altimeter settings will also be uplinked automatically 
when an aircraft receives an uplinked altitude 
assignment below FL 180. A controller may also 
manually send an altimeter setting message. 

NOTE

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When conducting instrument approach procedures, pilots 
are responsible to obtain and use the appropriate altimeter 
setting in accordance with 14 CFR Section 97.20. CPDLC 
issued altimeter settings are excluded for this purpose. 

(2) 

Initial contact is a safety validation 

transaction that compares a pilot’s initiated altitude 
downlink message with an aircraft’s stored altitude in 
the ATC automation system. When an IC mismatch or 
Confirm Assigned Altitude (CAA) downlink time

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out 

indicator is displayed in the Full Data Block (FDB) 
and Aircraft List (ACL), the controller who has track 
control of the aircraft must use voice communication 
to verify the assigned altitude of the aircraft, and 
acknowledge the IC mismatch/time

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out indicator. 

(3) 

Transfer of communications automati-

cally establishes data link contact with a succeeding 
sector. 

(4) 

Menu text transmissions are scripted 

nontrajectory altering uplink messages. 

(5) 

The CPDLC Message Elements for the 

Initial Capabilities rollout are contained in 
TBL 5

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1 through TBL 5

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19, CPDLC Message 

Elements, below. 

NOTE

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The FAA is not implementing ATN B1; the ATN B1 column 
in the tables is there for informational purposes only. 

b.  ATC Frequency Change Procedures. 

1. 

The following phraseology will be used by 

controllers to effect a frequency change: 

EXAMPLE

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(Aircraft identification) contact (facility name or location 
name and terminal function) (frequency) at (time, fix, or 
altitude). 

NOTE

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Pilots are expected to maintain a listening watch on the 
transferring controller’s frequency until the time, fix, or 
altitude specified. ATC will omit frequency change 
restrictions whenever pilot compliance is expected upon 
receipt. 

En Route Procedures 

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