Figure: CPDLC Contact Message, from Eddie's aircraft.

Eddie Sez:

ADS, CPDLC, ATC COM, ACARS, AOC, Whaaah? Here's how we keep this straight:

  • ADS-B Out — You can think of Automatic Dependent Surveillance - Broadcast Out (ADS-B Out) as a higher tech replacement for your transponder. ADS-B OUT sends your GPS position to air traffic control and other aircraft equipped with ADS-B In. It is much more accurate than a radar blip.
    More about this: International Operations / ADS-B Out.

  • ADS-B In — You can think of Automatic Dependent Surveillance - Broadcast In (ADS-B In) as a more accurate version of your TCAS. While TCAS aircraft positions shown in your cockpit are approximate, ADS-B In positions are exact.

  • ADS-C — You can think of Automatic Dependent Surveillance - Contract (ADS-C) as a replacement for Air Traffic Control radar contact. When you have logged on to an Air Traffic Service Unit with ADS-C, you have agreed to contracts which send information to the ATSU, such as your position, on a regular basis.
    More about this: International Operations / ADS-C.

  • CPDLC — You can think of Controller Pilot Data Link Communications as a replacement for your HF when oceanic and VHF over some domestic areas. It is far superior to your HF and has distinct advantages over VHF.

  • ATC COM, ACARS, AOC — Air Traffic Control Communications (ATC COM) is Honeywell-speak for CPDLC. Aircraft Communication Addressing and Reporting System (ACARS) used to be Airline Communication Addressing and Reporting System and is simply communications through a data service provider and includes your PDC, digital ATIS, and most of your oceanic clearances. Aeronautical Operational Control (AOC) is Gulfstream-speak for ACARS.

This page contains an overview of CPDLC. For more about pilot procedures:

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.


The role CPDLC plays in the Future Air Navigation System is to reduce the time it takes for air traffic control to issue instructions to a pilot and the pilot to acknowledge. In a domestic environment this is rarely a problem. When oceanic, however, it can be a factor. Reducing that time, known as Required Communications Performance (RCP), allows ATC to reduce aircraft spacing.

[AC 120-70C, para;6.1] The concept of RCP relates to the communications component of the communication, navigation, and surveillance/air traffic management (CNS/ATM) framework and complements Required Navigation Performance (RNP) and Required Surveillance Performance (RSP). In general, the requirements for operation in a defined airspace or performance of a defined procedure include elements of CNS functionality and performance, as well as ATM functionality and performance. The guidance provided in this AC regarding RCP is consistent with ICAO Doc 9869, Manual on RCP. RCP is a statement of the performance requirements for operational communication in support of specific ATM functions. The RCP is determined by cognizant authorities in consideration of air traffic operations, target levels of safety, separation assurance, Flight Standards Service (AFS), and functional hazard analysis associated with the airspace, operation, or procedure. Thus, RCP is operationally derived and not based on any specific technology, or combination of technologies, that may be utilized for communications. The performance of a communications is generally accepted as comprising communication transaction time, integrity, continuity, and availability.

[Guidance Material for ATS Data Link Services in NAT Airspace, ¶12.8.1.]

RCP Type Satisfies requirements as
RCP 240 Normal means of communication for application of 30 NM lateral separation and reduced longitudinal separation minima
RCP 400 Alternative means of communication for application of 30 NM lateral separation and reduced longitudinal separation minima
RCP 400 Normal means of communication for application of lateral separation greater than or equal to 50 NM and time-based longitudinal separation

The bottom line on CPDLC, then, if you can reduced the round trip time between the issuing of the message from ATC and the reception of the pilot's reply to 240 seconds or less, you can reduce the lateral and longitudinal spacing.

Domestic CPDLC — A Caveat

What about domestic CPDLC? How is it different than oceanic CPDLC? You obviously have a perfectly good VHF over most domestic regions but there is more to it than that. You have more airplanes and you need to communicate more quickly. The RCP type is understandably much quicker:


If we could add on to the previous table it would look like this:

RCP Type Satisfies requirements as
RCP 150 Normal means of communication for domestic airspace

The CPDLC you use while oceanic might not be good enough for domestic operations. See Oceanic CPDLC versus Domestic CPDLC (That Whole "Protected Mode" Debate), below.

Oceanic CPDLC versus Voice

[ICAO GOLD, ¶5.1.3.]

If you have CPDLC and you are in airspace where CPDLC is being used, you should use CPDLC as primary, voice communications as secondary. The general rule of thumb is: if contacted by CPDLC, respond with CPDLC; if contacted by voice, response with voice. When oceanic you still need to check in with HF, get a good SELCAL check, and maintain a listening watch if SELCAL fails.

Domestic CPDLC versus Voice

[FANS-1/A Operations Manual, ¶9.2.1] Implementation of CPDLC into continental airspace is intended as a supplement to the use of VHF voice and the intent is to build a single communications environment where both voice and CPDLC are considered as being normal Air Traffic Management (ATM) tools.

[FANS-1/A Operations Manual, ¶9.3]

You might not be able log in to CPDLC domestically over Europe, depending on your CPDLC. See Oceanic CPDLC versus Domestic CPDLC (That Whole "Protected Mode" Debate), below.

Actual practice over domestic Europe:

Oceanic CPDLC versus Domestic CPDLC (That Whole "Protected Mode" Debate)

Figure: Different ATSU/Aircraft Interoperable Connectivity, from ICAO GOLD, figure 2-3.


[ICAO GOLD, Table 2-1.]


So, how about all this in English?

  1. Most aircraft certified, equipped, maintain, and operated in the United States will have installed an older version of CPDLC called Future Air Navigation System (FANS 1/A) with Air Traffic Services (ATS) applications, ATC Facilities Notification (AFN), CPDLC and ADS-C.

  2. Many aircraft certified, equipped, maintained, and operated outside the United States recently, as well as many recently certified in the United States, will have installed a newer version of CPDLC that adheres to a newer standard called Aeronautical Telecommunications Network Baseline 1 (ATN B1).

  3. A part of the newer standard that is probably not installed on FANS 1/A aircraft is Very High Frequency Data Link Mode 2 (VDL M2), sometimes called "protected mode VHF Data Link" (PM CPDLC).

  4. You can operate in most (if not all) oceanic and remote airspace with a FANS 1/A aircraft, even if you do not have VDL M2. This capability carries into at least the first domestic ATSU, since they are responsible for the transition to and from oceanic.

  5. You may find yourself without CPDLC coverage domestically if you do not have ATN B1 and VDL M2. Most European countries specifically say "log-on from FANS 1/A or non-PM CPDLC capable aircraft will not be accepted.

  6. You can find rules and regulations that say PM CPDLC is, or will become, mandatory. Some manufacturers, such as Gulfstream, managed to secure exemptions for their entire fleet of aircraft.

Why Europe is Different

[Honeywell White Paper, ¶3.1]

Figure: PM CPDLC/Link 2000+ Implementation Schedule, from Honeywell White Paper.


[Honeywell White Paper, ¶3.2]

  • Feb. 5, 2015 — By this date, all aircraft operating within European airspace above FL 285 must be equipped with a compliant PM-CPDLC datalink system (aircraft built before 1997 and which will be removed from service by December 31, 2017 are exempt from this requirement). Some business aircraft which will remain in service after this date are also permanently exempted.

Protected Mode CPDLC over VHF Data Link (VDL Mode 2) was initially packaged with a program known as Link 2000+. Though the implementation dates tend to slip, if you want to fly about 28,500' in Europe you either need it installed or you need an exemption. You need to contact your aircraft manufacturer to find out where you stand.

The Gulfstream world used to be pretty cut and dried. The GIV and GV do indeed have lifetime exemptions. The G450 and G550 were said to have also been given lifetime exemptions based on installation of equipment needed to be FANS 1/A qualified. That might have changed depending on how you read Gulfstream literature. The latest "Avionics Update," available at, says the G450 is qualified "holding release ASC 092" and the G550 "holding release ASC 115." These are both described as "Protected Mode CPDLC Installation." As of October 2016, neither ASC is available and Gulfstream has both on hold. (They were announced in 2014.) Reading between the lines I would say we are exempt until those ASC's are released, and then we have to install them.

Flight Crew Response Times

[ICAO GOLD, ¶5.3.2.]

It is said that if they don't hear back from you in sixty seconds, they will consider the communications lost and if a clearance was involved, that clearance is cancelled. I've not seen that in writing but the threat is clear: respond within a minute, use "STANDBY" if you must. When dealing with clearances, I usually hit the print button, send the acknowledge, and then read the contents. I figure I can always respond a second time if I have to.

Multiple CPDLC Contacts

Figure: CPDLC CQZX Active, EGGX Next, from Eddie's aircraft.

[ICAO GOLD, ¶] An aircraft can have a maximum of two CPDLC connections established concurrently, each with a different ATSU. Only one CPDLC connection can be active at any given time; any second connection is inactive.

[ICAO GOLD, ¶] An ATSU system may request multiple simultaneous ADS contracts to a single aircraft, including one periodic and one event contract, which may be supplemented by any number of demand contracts. Up to five separate ground systems may request ADS contracts with a single aircraft.

You can only have one active CPDLC connection, think of it as your actual air traffic control contact. But you can have two connections, usually the active connection and the "next."

Position Reports at FIR Boundaries

Figure: Gander vs. Fukuoka Position Reporting Requirements under CPDLC, from ICAO Gold, Appendix E.

[ICAO GOLD, ¶] When notified that a new active CPDLC connection has been established, and if entering an FIR that requires the crew to send a CPDLC position report to confirm current data authority status (refer Appendix E, paragraph E.2.2), the flight crew should send a CPDLC position report without delay unless advised through a CONTACT or MONITOR instruction of a specific transfer point.

This is often taught this way: "Send a position report whenever entering oceanic airspace, except in the North Atlantic," meaning the North Atlantic is the only exception. In fact, the rule seems to be: "Send a position report crossing an FIR boundary if Appendix E tells you to."

Gulfstream Notes

G-450 Limitations

[G450 Airplane Flight Manual, §1-03-10 ¶10.]

AFN, ADS-C, and CPDLC Operations

  1. Aircraft complies with the interoperability requirements of DO-258A as expressed in the AFM, Section 01-34-30, Flight Management System (FMS).

  2. Operation must be authorized by civil aviation authorities via Letter of Authorization or Ops Specs.
  3. Operation must be conducted in accordance with the guidance and limitations expressed in Section 01-34-30, Flight Management System (FMS).

  4. Other requirements and operational guidance are found in AC 120-70X and ICAO's GOLD (Global Data Link Document) Manual.

[G450 Airplane Flight Manual, §1-34-30 ¶6.]

AFN, ADS-C, and CPDLC Operations: Airplanes having the latest approved version of the Honeywell PRIMUS EPIC system, comply with the interoperability requirements of RTCA D0-258A for AFN and ADS-C operations. Additionally, airplanes equipped with ASC 059 (or later approved Revisions) demonstrate compliance with RTCA DO-258A for CPDLC operations.

  1. Interoperability requirements for ATS applications using ARINC 622 Data Communications (FANS 1/A Interoperability Standard) comply with RTCA DO-258A.

  2. AFN, ADS-C, and CPDLC are also approved for oceanic and remote operation within the NAT and in areas outside of the NAT. The proper datalink capability must be noted on the filed ICAO flight plan: block 10 should include "J" and "/D", and block 18 should include "DAT/SV".

NOTE: This constitutes engineering approval only. Operational approval must be obtained from the local authority (FSDO) prior to using ADS-C and / or CPDLC capability. Requirements and operational guidance are found in AC 120-70x.

Gulfstream Nomenclature

[G450 Aircraft Operating Manual ¶2B-33-10] CPDLC is an ATS (Air Traffic Services) application that permits pilots and air traffic controllers to exchange messages using a datalink connection. CPDLC includes a set of clearance, information, and/or request messages that correspond to existing phraseology used in current ATC procedures. The term "ATC Comm" is used by Honeywell in place of CPDLC.

[G450 Aircraft Operating Manual ¶2B-33-10] ADS-C permits an ATC center to request the aircraft automatically transmit aircraft data generated from onboard navigation systems.

[G450 Aircraft Operating Manual ¶2B-33-10] The AFN (ATS Facilities Notification) function permits the pilot to log on to an ATC center to begin ADS or CPDLC operations.

Other Gulfstream Notes:

Book Notes

Portions of this page can be found in the book International Flight Operations, Part IV, Chapter 5.


Advisory Circular 120-70C, Operational Authorization Process for Use of Data Link Communication System, 8/3/15, U.S. Department of Transportation

Asia/Pacific Information Package, FAA Flight Technologies and Procedures Division (AFS-400), June 15, 2012

FANS-1/A Operations Manual, FAA Aeronautical Communications Aviation Safety (AVS), Version 6.0, 25 September 2008

Guidance Material for ATS Data Link Services in NAT Airspace, The North Atlantic FANS Implementation Group (NAT FIG), The North Atlantic Systems Planning Group (NAT SPG), Version 19.1, 14 September 2009

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013

Gulfstream Operating Manual Supplement for G350, G450, G500, and G550 Airplanes, Supplement Number GAC-OMS-4, Future Air Navigation System (FANS 1/A), Automatic Dependent Surveillance (ADS-C), Controller Pilot Data Link Communications (CPDLC), Revision 1, July 1, 2012

"Honeywell White Paper, Review of Aviation Mandates," A60-1307-000-000, May 2014.

ICAO Doc 4444 - Air Traffic Management, Fourteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2001 *

ICAO Doc 4444 - Air Traffic Management, Fifteenth Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, 2007 *

* Not all of Doc 4444 seems to have been reproduced in the 15th edition, so you might need to look at the 15th edition and then then 14th edition for some sections.

ICAO Global Operational Data Link Document (GOLD), International Civil Aviation Organization, Second Edition, 26 April 2013