the learning never stops!



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: 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: 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 purpose of CPDLC and:



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


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, ¶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 but is no longer expressed explicitly. (An older version of ICAO Doc 10037 listed it as RCP 150, but that reference is gone.) 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.

Log On


Photo: G450 MCDU, ADS Armed, from Eddie's aircraft.

[ICAO Doc 10037, ¶]

  • The logon is the first step in the data link process. A logon, initiated either by the flight crew or automatically following a contact request from another ATS unit, is performed prior to the ATS unit establishing a CPDLC and/or ADS-C connection.
  • The purpose of the logon is to provide the ATS unit with:
    • the information on ATS data link applications supported by the aircraft system (e.g. CPDLC, ADS-C);
    • the associated version numbers of the ATS data link applications;
    • the unique identification of the aircraft; and
    • additional relevant aircraft information required to allow the ATS unit to correlate the logon information with the aircraft’s corresponding flight plan.
    • Note 1.— For FANS 1/A, the unique identification of the aircraft is the aircraft registration and/or aircraft address; for ATN B1 the unique identification of the aircraft is the aircraft address.

      Note 2.— Under certain circumstances, it may be operationally desirable for an ATS unit to set up an ADS-C connection (perhaps for a single demand contract) without a preceding logon. When this is done, correlation with the flight plan can be achieved by requesting the optional flight identification group and checking this against the aircraft registration in the flight plan. See also paragraph 3.5.3 for guidelines on ADS-C connection management.

  • On receipt of a logon request, the ATS unit correlates the logon information with the relevant information in the flight plan held by the ATS unit. This ensures that messages are sent to the correct aircraft and that automation associated with ADS-C reports or CPDLC messages updates the correct flight plan.

[ICAO Doc 10037, ¶4.2.1]

  • A CPDLC connection requires a successfully completed logon procedure before the ATS unit can establish a CPDLC connection with the aircraft.
  • Prior to initiating the logon, the flight crew should verify the following:
    • the aircraft identification provided when initiating the logon exactly matches the aircraft identification (Item 7) of the filed flight plan;
    • the flight plan contains the correct aircraft registration in Item 18 prefixed by REG/;
    • the flight plan contains the correct aircraft address in Item 18 prefixed by CODE/, when required;
    • the flight plan contains the correct departure and destination aerodromes in Items 13 and 16, when required; and
    • The aircraft registration provided when initiating the logon exactly matches the aircraft placard, when the flight crew manually enters the aircraft registration. Refer to Appendix C, paragraph C.1 for aircraft types that require manual entry.
  • If any of the information described in paragraph do not match, the flight crew will need to contact AOC or ATC, as appropriate, to resolve the discrepancy.
  • Note 1.— In accordance with ICAO Doc 4444, the aircraft identification is either the: a) ICAO designator for the aircraft operating agency followed by the flight identification; or b) aircraft registration.

  • The flight crew should then manually initiate a logon using the logon address, as indicated on aeronautical charts.
  • Note 1.— Often the logon address is the same as the 4-letter facility designator but in some airspace a different logon address is used. Refer to Appendix B.

    Note 2.— Some aircraft (see Appendix C, paragraph C.1) implement FANS 1/A and ATN B1 capabilities as separate systems and do not comply with ED154A/DO305A. For these aircraft, the flight crew will have to select the appropriate system (FANS 1/A or ATN B1) to initiate the logon.

    Appendix C of ICAO Doc 10037 is primarily designed for Airbus and Boeing aircraft. In the case of a G450 or G550, for example, it tells us that "Independent FANS 1/A-ATN B1 aircraft have FANS 1/A+ and ATN B1 capability but do not comply with ED154A/DO305A. Only one FANS 1/A+ or ATN B1 is active at a time. The flight crew must manually select either FANS 1/A+ or ATN B1 prior to logon. There is no automatic transfer between FANS 1/A and ATN B1 ATSUs." But ATN B1 isn't really available for us yet (as of early 2017). So we must resort to whatever information Gulfstream has for us.

  • If there are no indications that the logon procedure was unsuccessful, the flight crew can assume that the system is functioning normally and that they will receive a CPDLC connection prior to entry into the next ATS unit’s airspace.
  • If an indication that the logon procedure was unsuccessful is received, the flight crew should reconfirm that the logon information is correct per paragraphs and and reinitiate a logon.
  • Each time a CPDLC connection is established, the flight crew should ensure the identifier displayed on the aircraft system matches the logon address for the controlling authority.
  • In the event of an unexpected CPDLC disconnect, the flight crew may attempt to reinitiate a logon to resume data link operations.
  • The flight crew may receive a CPDLC free text message from the ATS unit or a flight deck indication regarding the use of the message latency monitor on FANS 1/A+ aircraft. When this message is received, the flight crew should respond as described in Table 4-1 and in accordance with procedures for the specific aircraft type.
  • Note 1.— Procedures associated with the message latency monitor are applicable only in the European Region and are described in Appendix B, paragraph B.2.3.2.

    Note 2.— FANS 1/A aircraft do not support the message latency monitor. Refer to Appendix C, paragraph C.1, for availability of a FANS 1/A+ upgrade on different types of aircraft. Refer to Appendix C, paragraph C.11, for the specifications of the message latency monitor on different types of aircraft.

    This isn't really a problem. See: CPDLC Checklist / Latency Timer.


Photo: G450 MCDU, ADS Established, from Eddie's aircraft.

The log on procedure will vary with aircraft. In the case of a Gulfstream G450/G550, for example, the ADS-C log on takes places as a consequence of the CPDLC log on. The CPDLC FIR region code is entered into the ATC LOGON STATUS page of the MCDU after ensuring all other items are correct. The information is sent and once accepted the MCDU scratch pad will show "ADS ESTABLISHED" and the ADS ARMED entry will change to ADS ACTIVE.

For more about how to do this, see: Communications / CPDLC: Logon.

When to log on initially for data link services

[ICAO Doc 10037, ¶4.2.2]

  • When operating outside data link airspace, the flight crew should initiate a logon 10 to 25 minutes prior to entry into airspace where data link services are provided.
  • Note.— When departing an aerodrome close to or within such airspace, this may require the logon to be initiated prior to departure.

  • Where a data link service is only provided in upper airspace and where local procedures do not dictate otherwise, the flight crew should log on to that ATS unit in whose airspace a data link service will first be used.
  • When failure of a data link connection is detected, the flight crew should terminate the connection and then initiate a new logon with the current ATS unit.

Automatic Transfer of CPDLC and ADS-C Services between ATS Units

[ICAO Doc 10037, ¶4.2.3]

  • Under normal circumstances, the current and next ATS units automatically transfer CPDLC and ADS-C services. The transfer is seamless to the flight crew.
  • Note.— The flight crew should not need to reinitiate a logon.

  • The flight crew should promptly respond to CPDLC uplink messages to minimize the risk of an open CPDLC uplink message when transferring to the next ATS unit.
  • Note.— If a flight is transferred to a new ATS unit with an open CPDLC message, the message status will change to ABORTED. If the flight crew has not yet received a response from the controller, the downlink request will also display the ABORTED status.

  • Prior to the point at which the current ATS unit will transfer CPDLC and/or ADS-C services, the flight crew may receive an instruction to close any open CPDLC messages.
  • When entering the next ATS unit’s airspace, the flight crew should confirm the successful transfer from the current ATS unit to the next ATS unit by observing the change in the active ATS unit indication provided by the aircraft system.
  • When required by local procedures, the flight crew should send RTED-5 POSITION REPORT (position report). Alternatively, the flight crew may be required to respond to a CPDLC message exchange initiated by the ATS unit.
  • Note.— Since FANS 1/A aircraft do not report that the downstream ATS unit has become the CDA, the only way to confirm that it has taken place is for the ATS unit to receive a CPDLC message from the aircraft (refer to Appendix B).

Transfer voice communications with the CPDLC connection transfer

[ICAO Doc 10037, ¶]

  • Prior to crossing the boundary, the active ATS unit may initiate transfer of voice communications with the CPDLC connection transfer using any of the message elements containing CONTACT or MONITOR.
  • A CONTACT or MONITOR message instructs the flight crew to change to the specified frequency and may include a position or time for when to change to the new frequency.
    • when a MONITOR message is received, the flight crew should change to the specified frequency upon receipt of the instruction or at the specified time or position. The flight crew should not establish voice contact on the frequency.
    • when a CONTACT message is received, the flight crew should change to the specified frequency upon receipt of the instruction or at the specified time or position, and establish voice contact on the frequency.
    • Note 1.— Some States do not require HF SELCAL checks. If, following a MONITOR instruction, a SELCAL check is specifically required by operator procedures, this will usually be accommodated on the allocated frequency.

      Note 2.— If the next ATS unit provides CPDLC services, the flight crew should not expect that CPDLC will be terminated or suspended once voice contact is established per receipt of a CONTACT message, unless otherwise advised per paragraph

      Note 3.— CONTACT/MONITOR messages may specify a SATVOICE number, per paragraph, rather than a radio frequency.

  • If the ATS unit assigns a single HF frequency, the flight crew should select a secondary frequency from the same family. Further details of the composition of frequency families may be found in regional documentation.
  • Note.— In areas of poor radio coverage, the controller may append COMU-4 SECONDARY FREQUENCY (frequency) to specify a secondary frequency.

Exiting CPDLC and ADS-C Areas

[ICAO Doc 10037, ¶4.2.5]

  • Approximately 15 minutes after exiting CPDLC and/or ADS-C areas, the flight crew should ensure there are no active CPDLC or ADS-C connections. Ensuring that connections are not active eliminates the possibility of inadvertent or inappropriate use of the connections.
  • The flight crew should consult the current ATS unit prior to the manual termination of any ADS contract, even if it is suspected to be unnecessary or that its termination has failed.
  • In the event that the connection termination has failed, the flight crew should contact the ATS unit via voice or any other appropriate means.
  • Note.— ADS contracts are normally managed (e.g. established and terminated) by ATS units.

Operational Differences Between Voice Communications and CPDLC

[ICAO Doc 10037, ¶4.1.2.]

  • Development, testing, and operational experience have highlighted fundamental differences between voice communications and CPDLC. These differences need to be considered when developing or approving flight crew procedures involving the use of CPDLC.
  • For example, when using voice communications, each flight crew member hears an incoming or outgoing ATS transmission. With voice, the natural ability for each flight crew member to understand incoming and outgoing transmissions for their own aircraft has provided a certain level of situational awareness among the flight crew. With CPDLC, flight crew procedures need to ensure that the flight crew has an equivalent level of situational awareness associated with understanding the content and intent of a message in the same way.
  • Each flight crew member (e.g. pilot flying and pilot monitoring) should individually review each CPDLC uplink message prior to responding to and/or executing any clearance, and individually review each CPDLC downlink message prior to transmission. Reading a message individually is a key element to ensuring that each flight crew member does not infer any preconceived intent different from what is intended or appropriate. Reading the message aloud would bias the other flight crew member and could lead to the error of ‘reading’ what was read aloud as opposed to what was actually displayed.
  • Some uplink messages, such as complex or conditional clearances, require special attention to prevent the flight crew from responding to a clearance with RSPD-1 WILCO, but not complying with that clearance. To minimize errors, when responding to a clearance with RSPD-1 WILCO, each flight crew member should read the uplink message individually (silently) before initiating a discussion about whether and how to act on the message.
  • In a similar manner, each flight crew member should individually review CPDLC downlink messages before the message is sent. Having one flight crew member (e.g. the pilot monitoring) input the message and having a different flight crew member (pilot flying) review the message before it is sent provides an adequate level of situational awareness comparable to or better than voice communication.
  • If an operator uses augmented crews, the flight crew carrying out the ‘handover’ briefing should thoroughly brief the ‘changeover’ flight crew or flight crew member on the status of ADS-C and CPDLC connections and messages, including a review of any pertinent uplink and downlink CPDLC messages (e.g. conditional clearances).
  • The flight crew should coordinate uplink and downlink messages using the appropriate flight deck displays. Unless otherwise authorized, the flight crew should not use printer-based information to verify CPDLC messages as printers are not usually intended for this specific purpose.
  • Note.— For aircraft that have CPDLC message printing capabilities, there are constraints associated with the use of the flight deck printer. Printers may not produce an exact copy of the displayed clearance with the required reliability, and should not be used as the primary display for CPDLC. However, in some cases, printed copies may assist the flight crew with clearances and other information that are displayed on more than one page, conditional clearances and crew handover briefings.

  • When operating within airspace beyond the range of DCPC VHF voice communication, CPDLC is available and local ATC procedures do not state otherwise, the flight crew should normally choose CPDLC as the means of communication. The flight crew would use voice as an alternative means of communication (e.g. VHF, HF or SATVOICE direct or via a radio operator). However, in any case, the flight crew will determine the appropriate communication medium to use at any given time.
  • In airspace where both DCPC VHF voice and CPDLC communication services are provided, and local ATC procedures do not state otherwise, the flight crew will determine the communication medium to use at any given time.
  • Note.— ICAO Doc 4444, paragraph 8.3.2, requires that DCPC be established prior to the provision of ATS surveillance services, unless special circumstances, such as emergencies, dictate otherwise. This does not prevent the use of CPDLC for ATC communications, voice being immediately available for intervention and to address non-routine and time critical situations.

  • To minimize pilot head down time and potential distractions during critical phases of flight, the flight crew should use voice for ATC communications when operating below 10 000 ft AGL.
  • While the CPDLC message set, as defined in Appendix A, generally provides message elements for common ATC communications, the flight crew may determine voice to be a more appropriate means depending on the circumstances (e.g. some types of non-routine communications).
  • Note.— Refer to paragraph 4.6 for guidelines on use of voice and data communications in emergency and non-routine situations.

  • During an emergency, the flight crew would normally revert to voice communications. However, the flight crew may use CPDLC for emergency communications if it is either more expedient or if voice contact cannot be established.
  • Note.— For ATN B1 aircraft, emergency message elements are not supported. See Appendix A, paragraph A 4.9 for a list of emergency message elements.

  • Except as provided in paragraph, the flight crew should respond to a CPDLC message via CPDLC, and should respond to a voice message via voice (ICAO Doc 4444 refers).
  • Note.— This will lessen the opportunity for messages to get lost, discarded or unanswered between the ATS unit and the flight crew and cause unintended consequences.

  • If the intent of an uplink message is uncertain, the flight crew should respond to the uplink message with RSPD-2 UNABLE and obtain clarification using voice.
  • Note.— For FANS 1/A aircraft, some uplink messages do not have a DM 1 UNABLE response. On these aircraft, the flight crew should respond with DM 3 ROGER and then obtain clarification via voice.

  • Regardless of whether CPDLC is being used, the flight crew should continuously monitor VHF/HF/UHF emergency frequency. In addition, the flight crew should continuously maintain a listening or SELCAL watch on the specified backup or secondary frequency (frequencies).

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]

  • The following procedures only apply to normal (non-emergency) operations. While the intent of these procedures is to develop a standardised and predictable environment using a combination of voice and CPDLC, the decision on whether voice or CPDLC is the more appropriate medium for use in a given operational situation will be made by the pilots and controllers involved.
  • As a supplement to VHF voice, CPDLC is intended to be restricted to the passing of strategic information. Strategic information involves routine, non-time-critical communications, and includes examples such as the passing of amended flight levels, amended routes, speed control messages, frequency changes, and SSR codes, when the speed of delivery is not critical to safety.
  • Voice instructions and acknowledgments shall have precedence over CPDLC messages at all times. In the event that any ambiguity exists in a message or message exchange, then the controller/pilot shall revert to voice communications for clarification.
  • Flight crew shall either comply with uplink CPDLC instructions or respond with UNABLE, and shall respond to uplink messages using CPDLC whenever possible.

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:

  • Log on to CPDLC where you can.
  • Use CPDLC for all routine issues.
  • Confirm via voice any CPDLC instruction that changes the aircraft altitude, heading, or airspeed, adding the term "datalink" to let the controller know where the instruction came from. Example:
    • (Via CPDLC) CLIMB TO FL 290
    • (Via VHF) "London Control, November seven seven zero zero, data link climb flight level two niner zero."
  • Do not use ADS-C domestically.

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


Figure: Different ATSU/Aircraft Interoperable Connectivity, from ICAO Doc 10037, figure 1-2.


[ICAO Doc 10037, Table 1-1.]

  • ACARS ATS — ATS applications, departure clearance (DCL), oceanic clearance (OCL) and data link – automatic terminal information service (D-ATIS), supported by aircraft communications addressing and reporting system (ACARS).
  • FMC WPR — Flight management computer waypoint position reporting (FMC WPR)
  • FANS 1/A — Initial future air navigation system (FANS 1/A) ATS applications, CPDLC and ADS-C, supported by FANS 1/A over ACARS.
  • FANS 1/A+ — Same as FANS 1/A, except with additional features, such as the message latency monitor function
  • ATN B1 — ATS applications, CM and CPDLC, supported by aeronautical telecommunication network – baseline 1 (ATN B1):
  • VDL M2 — Very high frequency data link – mode 2

[ICAO Doc 10037, ¶]

  • The ATN B1 data link system relies on the aeronautical telecommunication network (ATN), which is provided and maintained by various communication service providers (CSPs) and/or ANSPs.
  • The ATN relies only on VHF (VDL M2) to meet the performance required for the intended operations.

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]

  • As a key component of the Single European Sky, the European Community has mandated operational use of air-ground data link, in the form of “Protected Mode” Controller Pilot Data Link Communications (PM-CPDLC), for aircraft flying above 28,500 feet (FL285).
  • Although complex in name, PM-CPDLC is straightforward in practice. The system is similar to text messaging on cell phones, allowing pilots and Air Traffic Control (ATC) to send pre-set or “canned” data messages between the ground and the aircraft. CPDLC messages enable automation of routine tasks that can take up to 50 percent of a controller’s time. Using data link systems can also mitigate common communication problems such as unclear radio communication or misinterpretation due to language differences or poor clarity.
  • Indeed, studies within the European Community have confirmed the capability of datalink services to provide additional air traffic control capacity. Datalink solutions provide airline and business aircraft operators with significant benefits at reduced cost, including enhanced flight operations efficiency and lower airline maintenance, administration and air traffic control costs.
  • Once all the modernization initiatives are complete, this should lead to a threefold increase in airspace capacity and a 50 percent reduction in air navigation costs.



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 Doc 10037, ¶4.3.2.]

  • System performance requirements have been established to support reduced separation standards. Specific latency times have been allocated to the technical performance, and flight crew and controller response times. Regional/State monitoring agencies analyze actual performance to ensure the technical and operational components of the system meet required standards. For example, to support RCP 240 operations, the flight crew is expected to be able to respond to a CPDLC uplink message within one minute.
  • For an ATN-B1 aircraft, the flight crew should respond to a CPDLC uplink message within 100 seconds to prevent the CPDLC uplink message from automatically timing out.
  • Note.— ATN-B1 aircraft use a CPDLC message response timer, which is set at 100 seconds upon receipt of the CPDLC uplink message. If the flight crew has not sent a response within this time:

    • the flight crew is no longer provided with any response prompts for the message;
    • the aircraft sends an ERROR message for display to the controller; and
    • the aircraft and ground systems close the dialogue.
  • When a CPDLC uplink message automatically times out, the flight crew should contact ATC by voice.
  • The flight crew should respond to CPDLC messages as soon as practical after they are received. For most messages, the flight crew will have adequate time to read and respond within one minute. However, the flight crew should not be pressured to respond without taking adequate time to fully understand the CPDLC message and to satisfy other higher priority operational demands. If additional time is needed, the flight crew should send a RSPD-3 STANDBY response.
  • Note.— For ATN B1 aircraft systems, if the flight crew does not send an operational response within 100 seconds after the RSPD-3 STANDBY was sent, the CPDLC uplink message will time out (refer to paragraph

  • If a RSPD-3 STANDBY response has been sent, the flight crew should provide a subsequent closure response to the CPDLC message.
  • Note 1.— In the case of a RSPD-3 STANDBY response, the uplink message remains open until the flight crew responds with a RSPD-1 WILCO or RSPD-2 UNABLE. If the closure response is not received within a reasonable period of time, the controller is expected to query the flight crew per paragraph

    Note 2.— Transmission times for messages may vary for a number of reasons including the type of transmission media, network loading, or the criteria for transitioning from one media to another (e.g. VHF/Satcom). Operational response times may vary depending on workload and complexity of the instruction or clearance.

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 Connections vs Contracts


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."

CPDLC Position Reporting

When do you need to do a CPDLC position report? It depends! ADS-C might have you covered but some ATSU's want them anyway. The only way to know for sure is to check Appendix B of ICAO Doc 10037. For example, position reports with Gander Oceanic are not required because they don't ask for them in Appendix B:


Figure: Gander Oceanic Remarks, from ICAO Doc 10037, Appendix B.

Position reports are required with Mauritus because they do ask for them in Appendix B. (CDA is the "Current Data Authority")


Figure: Gander Oceanic Remarks, from ICAO Doc 10037, Appendix B.

[ICAO Doc 10037, ¶] When using CPDLC to provide position information, the flight crew should report unnamed waypoints (latitudes/longitudes) using the ICAO format of nn[N/S]nnn[E/W] or, if both degrees and minutes are required, nnnn[N/S]nnnnn[E/W].

Note.— The flight crew and flight operations officers/dispatchers should not use the ARINC 424 format. ARINC 424 describes a 5-character latitude/longitude format for aircraft navigation databases (e.g. 10N40 describes a lat/long of 10N140W). The ATS unit may reject or be unable to process any downlink message containing waypoint names in the ARINC 424 format.

Position Reporting in a non-ADS-C environment

[ICAO Doc 10037, ¶]

  • When ADS-C is not available, the flight crew should conduct position reporting by voice or CPDLC. When using CPDLC, the flight crew should send RTED-5 POSITION REPORT (position report) whenever an ATC waypoint is sequenced, (or passed abeam when offset flight is in progress).
  • When using CPDLC for position reporting, the flight crew should send position reports only at compulsory reporting points and ensure that the position and next position information applies to compulsory reporting points, unless requested otherwise by ATC. The ensuing significant point after the next position may be either a compulsory or non-compulsory reporting point (refer AIREP form ICAO Doc 4444, Appendix 1).

Position Reporting in an ADS-C environment

[ICAO Doc 10037, ¶]

    Note.— In an ADS-C environment, the flight crew should not provide position reports or revised waypoint estimates by CPDLC or voice, unless otherwise instructed or under conditions in certain airspace as stipulated in AIP (or other appropriate publication)

  • If required by regional supplementary procedures or AIP (or other appropriate publication), the flight crew should provide a CPDLC position report when either of the following events occurs:
    • an initial CPDLC connection is established; or
    • the CPDLC connection transfer has been completed (i.e. at the associated boundary entry position).
    • Note.— Some ANSPs require a single CPDLC position report, even when in an ADS-C environment, to provide the controlling ATS unit confirmation that it is the CDA and the only ATS unit able to communicate with the aircraft via CPDLC (refer to Appendix B).

    In general you should not provide position reports or revised waypoint estimates by CPDLC or voice unless you see something that tells you to do so. Append B can be misleading. The subject is only brought up in the North Atlantic region where you are explicitly told "For ADS-C flights, the flight crew should not submit position reports via voice to reduce frequency congestion, unless requested by aeronautical radio operator." Makes sense, but don't let that talk you into thinking now you need to do so in other regions because the same phrase is missing.

  • The flight crew should include only ATC waypoints in cleared segments of the aircraft active flight plan. However, when an ATC clearance eliminates a waypoint, it is permissible to retain and report the point abeam of that waypoint since this ensures retention of meteorological data associated with the eliminated waypoint.
  • Note.— If the flight crew inserts non-ATC waypoints (e.g. mid-points) into the aircraft active flight plan and activates the change, the aircraft system may trigger an ADS-C waypoint change event report at the non-ATC waypoint, or include information about the non-ATC waypoint in the predicted route group, as well as the intermediate and fixed projected intent groups. As a result, the ADS-C report will include information about the non-ATC waypoint, which is not expected by the ATC ground system.

  • The flight crew should maintain the active route in the aircraft system to be the same as the ATC cleared route of flight.
  • Note.— If the flight crew activates a non-ATC cleared route into the aircraft system, the ADS-C reports will include information that will indicate the aircraft is flying a route that is deviating from the cleared route.

  • When reporting by ADS-C only, the flight crew should include ATC waypoints in the aircraft active flight plan even if they are not compulsory reporting points.

The Importance of FMS Waypoint Sequencing

You don't have to worry about flying a Strategic Lateral Offset because your FMS will sequence the waypoints even if your are two miles away from the intended course. But if you are further than the tolerance allowed by your FMS, it may not sequence. If that happens, the position report will not be made.


Figure: Waypoint sequencing anomaly, from ICAO Doc 10037, figure 4-6.

[ICAO Doc 10037, ¶]

  • The flight crew should ensure that waypoints are sequenced correctly. If an aircraft passes abeam a waypoint by more than the aircraft FMS waypoint sequencing parameter, the flight crew should sequence the waypoints in the FMS, as appropriate.
  • As shown in [the figure], when an aircraft passes abeam a waypoint in excess of the defined sequencing parameter (refer to , paragraph F.7) for specific aircraft types), the FMS will not sequence the active waypoint. If the flight crew does not sequence the waypoint, incorrect information will be contained in ADS-C reports, CPDLC position reports and FMC waypoint position reports – the next waypoint in these reports will actually be the waypoint that the aircraft has already passed.

Confirm Assigned Route Message


Photo: "Confirm Assigned Route" message, from Eddie's aircraft.


  • Coming soon to an FMS near you will be a new standard message from Oceanic ATC: “Confirm Assigned Route”.
  • If you’ve operated on the ‘half-tracks’ in the NAT recently, you’ll have seen this. So, what’s it all about?
  • Short and simple: with the half-tracks, the potential for Nav Errors are now (quite a lot, perhaps) higher than before. Waypoints are that bit more complicated, and 5030N 30W is a little too similar to 50N 30W.
  • When I first heard this a year and a half ago I dismissed it as something I didn't have to worry about since I operate above the tracks. (We can climb right up to 41,000 feet at any weight.) But lo and behold I have been getting this message routinely.

  • So, to prevent you reading back the clearance correctly and then screwing up the route in the FMS, Shanwick (and Gander from 01DEC16) will ask you via datalink “What are you planning to fly?” once you enter the Ocean.
  • All you have to do is ack the message, scroll through your route and check it looks OK, and send it back down to them. If it’s the same as your clearance, then that’s that. If not, or you don’t reply, you’ll get an additional telegram from Shanwick.
  • In Gulfstream PlaneView cockpits you are expected to select "REPORT" and then "SEND" without entering any free text. See: Gulfstream CPDLC Confirm Route.

CPDLC Weather Deviation

If you need to deviate for weather while using CPDLC, it pays to know Chapter 4 of ICAO Doc 10037. Be careful you ask for a weather deviation versus an offset.



Figure: Offset and weather deviation, from ICAO Doc 10037, Figure 4-2.

[ICAO Doc 10037 ¶4.5.1]

  • The flight crew may use CPDLC to request a weather deviation clearance or an offset clearance. The difference between a weather deviation and an offset are portrayed in [the figure].
    • A weather deviation clearance authorizes the flight crew to deviate up to the specified distance at their discretion in the specified direction from the route in the flight plan.
    • An offset clearance authorizes the flight crew to operate at the specified distance in the specified direction from the route in the flight plan. A clearance is required to deviate from this offset route.
    • Note.— CPDLC offers timely coordination of weather deviation clearances. However, the flight crew may deviate due to weather under the provisions of ICAO Doc 4444, paragraph 15.2.3. The extent to which weather deviations are conducted may be a consideration when applying reduced separations.

  • Flight crews should use the correct message element when requesting an off-route clearance.
  • Note.— The difference between a weather deviation and an offset affects how ATC separate aircraft.

    You have to stay on your offset course precisely; you can "meander" as necessary up to your cleared distance when cleared for a weather deviation.

Weather Deviation Requests and Offsets

[ICAO Doc 10037 ¶]

  • When requesting a weather deviation or offset clearance, the flight crew should specify the distance off route with respect to the cleared route of the aircraft. If the flight crew has received a off-route clearance and then requests and receives a subsequent off-route clearance, the new clearance supersedes the previous clearance (i.e. only the most recent clearance is valid).
  • images

    Figure: Weather deviation clearance up to 20 nm, from ICAO Doc 10037l, Figure 4-3.

  • Example 1: As shown in Figure 4-3, the flight crew requests a weather deviation clearance to operate up to 20NM left of route. The controller issues the appropriate clearance.
  • images

    Figure: Subsequent weather deviation up to 20nm left, from ICAO Doc 10037, Figure 4-4.

  • Example 2: As shown in Figure 4-4, the flight crew is operating on a weather deviation clearance up to 20 NM left of route and then requests another weather deviation clearance to operate up to a further 30NM left of route. They specify the deviation distance in the clearance request based on the cleared route rather in relation to the current weather deviation clearance. The controller issues the appropriate clearance.
  • images

    Figure: Subsequent weather deviation up to 30nm right, from ICAO Doc 10037, Figure 4-5.

  • Example 3: As shown in Figure 5-6, the aircraft then requests a weather deviation clearance to operate 30NM right of route. The controller issues the appropriate clearance. The flight crew expeditiously navigates from one side of route to the other in accordance with the above clearance.

    Note.— The ATSU applies the appropriate separation standards during the maneuvers.

Deviations Either Side of Route

[ICAO Doc 10037 ¶] When requesting a deviation on either side of route, the flight crew should request a weather deviation left and right of route using LATD-2 REQUEST WEATHER DEVIATION UP TO (lateral deviation) OF ROUTE.

Reporting Back on Route

[ICAO Doc 10037, ¶] When the flight crew no longer needs the deviation clearance and is back on the cleared route, the flight crew should send a LATD-4 BACK ON ROUTE report.

  • if during the weather deviation, the flight crew receives a clearance to proceed direct to a waypoint – and the flight crew responds to the clearance with RSPD-1 WILCO – the aircraft is considered to be on the cleared route. Therefore, the flight crew should send a LATD-4 BACK ON ROUTE report after they execute the “direct to” clearance; and
  • if the aircraft is off route during a weather deviation clearance and proceeding direct to a waypoint on the cleared route, the flight crew should send a LATD-4 BACK ON ROUTE report after the aircraft has sequenced the waypoint on the cleared route.
  • Note.— If a LATD-4 BACK ON ROUTE report is received while the aircraft is still off route, the incorrect information provided to ATC may affect the separation standards in use. Alternatively, the flight crew may consider requesting a clearance direct to the waypoint – on receipt of the uplink clearance, the procedure specified in item a) above applies.

Gulfstream Notes

G450 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:

  • Communications via CPDLC are through the NAV > ATC page
  • Communications to a Downlink Service Provider (DSP) also known as a Communications Service Provider (CSP), such as for a PDC or ATIS, is through the AOC (Aeronautical Operational Control) DLK page -- this system is also known as ACARS (Aircraft Communication Addressing and Reporting System)


This is just an example of how data link can be covered in an MEL. We have an MEL above and beyond the manufacturer's standard MMEL and I know you may not need either. But I think if you have CPDLC the FAA is going to require some mention in your MEL. (At least our LOA says so.)

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

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, 16th Edition, Procedures for Air Navigation Services, International Civil Aviation Organization, October 2016

ICAO Document 10037 AN/509 - Global Operational Data Link (GOLD) Manual, Advance edition (unedited), First Edition, 2016

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

Revision: 20170427