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 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, 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:


  • RCP Specification: RCP 150
  • Interoperability: ATN B1, ATN B1-FANS 1/A

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.]

  • When operating within airspace where CPDLC is available and local ATC procedures do not state otherwise, CPDLC should be the primary means of communication. Voice should be used as the backup communication medium (e.g. direct VHF, direct HF, third party HF, Satellite voice).
  • While the CPDLC message set defined in Appendix A provides for ATC communications, voice may be a more appropriate means depending on the circumstances, e.g. some types of non-routine communications. Refer to paragraph 5.8 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 they are unable to establish voice contact.
  • The response to a CPDLC message should be via CPDLC, and the response to a voice message should be via voice.
  • If a conflicting CPDLC and voice clearance/instructions is received, the flight crew should obtain clarification using voice.
  • If the intent of an uplinked message is uncertain, the flight crew should reject (UNABLE) the message. The flight crew may use either CPDLC or voice to confirm the intent of the message.
  • Regardless of whether CPDLC is being used as the primary means for communication, the flight crew should continuously monitor VHF/HF/UHF guard frequency. In addition, the flight crew should continuously maintain a listening or SELCAL watch on the specified backup or secondary frequency (frequencies). On aircraft capable of two SATCOM channels, one channel may be selected to the phone number for the radio facility assigned to the current flight information region (FIR) to enable timely voice communications with ATS. The second channel may be selected to the company phone number to enable timely voice communications with company dispatch.

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 GOLD, figure 2-3.


[ICAO GOLD, Table 2-1.]

  • 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.
  • 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


  • 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 GOLD, ¶5.3.2.]

  • System performance requirements have been established to support reduced separation standards. Specific latency times have been allocated to the technical performance, based on flight crew and controller response times. Regional/State monitoring agencies monitor performance to ensure the technical and operational components of the system meet required standards. To support RCP 240 operations (e.g. 30 nautical mile longitudinal separation) the flight crew should respond to an uplink message within one minute.
  • Flight crew procedures should be developed to respond to uplinks as soon as practical after they are received. For most uplinks, 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 uplinked message and to satisfy other higher priority operational demands.
  • If the flight crew determines they will need a significant amount of time to respond to a message, they should send a STANDBY response.

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

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.)

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 E of the ICAO Gold Manual. It is a long and ponderous manual, to be sure, but make sure you read Chapter 5.

Position reporting in a non-ADS-C environment

[ICAO Gold Manual] ¶5.6.2]

  • 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 DM 48 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).

For the AIREP form, see Voice Position Reports.

Position reporting in an ADS-C environment

[ICAO Gold Manual] ¶5.6.3]

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 Regional Supplementary Procedures or AIP (or other appropriate publication) (See also Appendix E). 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:

a) An initial CPDLC connection is established; or

b) 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 ATSU confirmation that it is the CDA and the only ATSU able to communicate with the aircraft via CPDLC (refer to Appendix E). The flight crew should include only ATC waypoints in cleared segments of the aircraft active flight plan.

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.


Photo: ICAO Gold Appendix E example, from ICAO Gold Manual, Appendix E.

Much of what you do regionally depends on Appendix E. An example:

Waypoint Sequencing


[ICAO Gold Manual] ¶] 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 while flying in heading selected mode, the flight crew should sequence the waypoint in the FMS by executing a track offset that is within FMS waypoint sequencing parameters or flying direct to the next relevant waypoint.

Note.— As shown in Figure 5-2, when an aircraft passes abeam a waypoint in excess of the defined sequencing parameter (refer to Appendix F, 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.

Lat/Long Format

[ICAO Gold Manual] ¶] The flight crew should include in any CPDLC downlink message or FMC WPR, latitudes and longitudes encoded as waypoint names in the ICAO format. The flight crew should not use the ARINC 424 format.

Note 1 — ADS-C reports contain latitudes and longitudes rather than the identifier of the waypoint. Therefore, ARINC 424 waypoints do not affect ADS-C reporting.

Note 2 — ARINC 424 describes a 5-character latitude/longitude format for aircraft navigational database. (e.g. 10N40 describes a lat/long of 10N140W). This format is not an ICAO format and, therefore, the ATSU may reject any downlink message containing waypoint names in the ARINC 424 format.

It is clear CPDLC cannot communicate in the ARINC 424 format but it is less clear what the "ICAO Format" is. The only reference to standardized latitude/longitude is in the voice report appendix of ICAO Doc 4444.

More about: ARINC 424 Shorthand System.

[ICAO Doc 4444, Appendix A, §1, ¶3.] Report position in latitude (degrees as 2 numerics or degrees and minutes as 4 numerics, followed by "North" or "South") and longitude (degrees as 3 numerics or degrees and minutes as 5 numerics, followed by "East" or "West"), or as a significant point identified by a coded designator (2 to 5 characters), or as a significant point followed by magnetic bearing (3 numerics) and distance in nautical miles from the point (e.g. "4620 North 07805 West", "4620 North 07800 West", "4600 North 07800 West", LN ("LIMA NOVEMBER"), "MAY", "HADDY" or "DUB 180 DEGREES 40 MILES"). Precede significant point by "ABEAM", if applicable.

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 5 of the ICAO Gold Manual is your "Go To" source. Be careful you ask for a weather deviation versus an offset.



Figure: Offset and weather deviation, from ICAO Gold Manual, Figure 5-3.

[ICAO GOLD ¶5.7.1] 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 Figure 5-3.

a) 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.

b) 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. 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 GOLD ¶5.7.2] 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).


Figure: Weather Deviation, Example 1, from ICAO Gold Manual, Figure 5-4.

Example 1: As shown in Figure 5-4, the flight crew requests a weather deviation clearance to operate up to 20NM left of route. The controller issues the appropriate clearance.


Figure: Weather Deviation, Example 2, from ICAO Gold Manual, Figure 5-5.

Example 2: As shown in Figure 5-5, 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.


Figure: Weather Deviation, Example 3, from ICAO Gold Manual, Figure 5-6.

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 GOLD ¶5.7.3] There are a number of valid formats for the CPDLC [direction] variable. A number of aircraft types, however, can only request one direction (left or right) in weather deviation requests. When operating these aircraft types, the flight crew should request a deviation left and right of route using the following procedures:

a) Construct a weather deviation request for a deviation on one side of route using REQUEST WEATHER DEVIATION UP TO [specified distance] [direction] OF ROUTE; and

b) Append free text AND [specified distance] [direction] describing the distance to the other side of route.

Reporting Back on Route

[ICAO GOLD ¶5.7.4] When the flight crew no longer needs the deviation clearance and is back on the cleared route, the flight crew should send the report BACK ON ROUTE.

a) If during the weather deviation, the flight crew receives a clearance to proceed direct to a waypoint – and the flight crew accepts (WILCO) this clearance – the aircraft is considered to be on a cleared route. Therefore, the flight crew should send the BACK ON ROUTE report after they execute the “direct to” clearance.

b) If the aircraft is off route on a weather deviation clearance and proceeding direct to a waypoint on the cleared route, the flight crew should not send the BACK ON ROUTE report until they have sequenced the waypoint on the cleared route.

Note.— If a 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) applies.

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, 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 Doc 4444 - Amendment No. 1, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 1, 2007

ICAO Doc 4444 - Amendment No. 2, Procedures for Air Navigation Services, International Civil Aviation Organization, Amendment No. 2, 19/11/09

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

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: 20161001