Aeronautical Information Manual (AIM), page 323

Index   322 -- Page 323 -- 324

Section 3. En Route Procedures

5-3-1. ARTCC Communications

a. Direct Communications, Controllers and
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

3. Controller Pilot Data Link Communications
(CPDLC) is a system that supplements air/ground
voice communications. As a result, it expands
two-way air traffic control air/ground communica-
tions capabilities. Consequently, the air traffic
system's operational capacity is increased and any
associated air traffic delays become minimized. A
related safety benefit is that pilot/controller read-
back and hear-back errors will be significantly
reduced. The CPDLC's principal operating criteria

(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) CPDLC Build 1 offers four ATC data link
services. These are altimeter setting (AS), transfer of

communications (TC), initial contact (IC), and menu
text messages (MT).

(1) Altimeter settings are usually trans-
mitted automatically when a CPDLC session and
eligibility has been established with an aircraft. A
controller may also manually send an altimeter
setting message.
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 ATC host

computer stored altitude. If an altitude mismatch is

detected, the controller will verbally provide

corrective action.

(3) Transfer of communications automati-
cally establishes data link contact with a succeeding

(4) Menu text transmissions are scripted

nontrajectory altering uplink messages.

Initial use of CPDLC will be at the Miami Air Route Traffic
Control Center (ARTCC). Air carriers will be the first
users. Subsequently, CPDLC will be made available to all
NAS users. Later versions will include trajectory altering
services and expanded clearance and advisory message

b. ATC Frequency Change Procedures.
1. The following phraseology will be used by
controllers to effect a frequency change:

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

En Route Procedures 5-3-1

Page 323 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   322 -- Page 323 -- 324