US Enroute CPDLC

The transition of pilot to controller communications away from dependence on voice over radio to the utilization of more effective and efficient communications via data link communications is foundational to airspace modernization efforts worldwide. For the United States NAS, the next phase of the FAA’s Data Communications (Data Comm) rollout, which adds U.S. Domestic Enroute CPDLC capabilities, is progressing well towards its goal of 24/7 operations in all 20 of the US Air Route Traffic Control Centers (ARTCCs) in the next several months.

As our readers are likely familiar, the initial phase of the Data Comm rollout in the US NAS referred to as the DCL (Departure Clearance) phase has been in operation for some time with capabilities fully operational at 65 airports. That includes many of the key business aviation airports (by eligible flights) including Teterboro, Van Nuys, Westchester, and Dallas Love Field.

As a foundational technology for NextGen and modernization of the NAS, the FAA outlined several top-level benefits of transitioning from radio voice communications only to datalink, both near term in dealing with inherently limited and increasingly overutilized VHF frequencies and longer term as necessary for the deployment of advanced services such as trajectory-based operations. As users of the airspace are painfully aware, communications on available VHF voice frequencies are increasingly challenged with increases in air traffic, especially in high density corridors. Pilot to controller communications that require long transmissions and readbacks such as clearance-related exchanges, can be particularly difficult in high traffic areas, particularly when weather is deteriorating, or other factors impact many aircraft simultaneously requiring communication with ATC.

By its very nature CPDLC allows for communications to be done more effectively via unicast (e.g., transmission to a single recipient) versus transmitting a message destined for one callsign on a frequency controlling several aircraft, removing the potential for a crew mistaking a message as intended for them when it was not as CPDLC requires no callsign recognition. CPDLC messages are sent to only the aircraft they are destined for. The controller can more easily select the intended destination for a given CPDLC communication, and that communication is transmitted only to the intended aircraft. In addition, datalink obviates the need for the non-flying pilot copying long, complex clearance information onto a kneeboard and then having to enter it manually into the flight management system accurately and completely, particularly in scenarios where workload on the flight deck is already high. Datalink alleviates that requirement via the FMS push-to-load function that is included in the US CPDLC aircraft equipage requirements for both DCL and Enroute CPDLC.

As intended, the DCL program has delivered significant benefits to users of the NAS. More than 5,900 aircraft have Data Comm avionics equipage that meet the requirements. Since its inception in 2016 nineteen US Air Carriers and seventy-nine non-US commercial and cargo operators, as well as greater than 40 business jet operators are using Data Comm for DCL in the NAS, based on figures published by the FAA in late 2022. The quantification of benefits from that usage over the period from inception to that date have been impressive:

• Over 13 million flights have received clearances over DCL
• Saved over 2.6 million minutes of airspace user time
• Saved over 3.4 million minutes of radio time
• Prevented over 159K readback errors
• Potentially reduced over 70.2 million pounds of CO2 emissions 

Adding the ability to utilize datalink communications as an augment to voice for CPDLC-equipped aircraft in the enroute phase will substantially increase the overall benefits in some of these measures as well as the overall role CPDLC is intended to play in the medium and longer terms effort toward the full realization of NextGen in the NAS.

In an update provided in late March of 2023, and illustrated by figure 1 below, 11 of the 20 Air Route Traffic Control Center (ARTCCs) in the US are now operating 24/7 for enroute clearance delivery via CPDLC for equipped and authorized operators in the NAS. As expected, airlines and business jet operators participating in the trial are already seeing significant benefits from the enroute functionality as well using some similar, quantifiable measures: reduced radio time and therefore decreased congestion on key frequencies, and the prevention of readback errors. The FAA and its primary Data Comm contractor, L3 Harris, are currently working on a comprehensive set of measurable, quantifiable benefits measures of enroute clearance delivery equipage and authorization that accrue to equipped/approved operators. 

Where is it Used and what are the Controller-to-Pilot Communications Included in US Domestic Enroute CPDLC?

Whereas DCL is relatively straightforward in its replacement of VHF communications with clearance delivery for pre-flight clearance transmission and acknowledgement, enroute requires a bit more explanation as to where and how it is used in augmenting a significantly larger subset of controller-to-pilot communications. At the outset it is important to outline that enroute is in fact an augment to voice Communications in the enroute phase, not a total replacement. Voice communications may always be used. Even though a particular flight may be logged into KUSA, you will be handed off to a Center voice frequency when initially entering Center control and expect VHF voice frequency changes throughout the flight – even when using CPDLC as the primary mode of communication for the communications outlined below.

Only Air Route Traffic Control Centers (ARTCC – e.g., “Albuquerque Center”) will provide enroute CPDLC services. CPDLC services are not provided while the flight is under control of the Terminal Radar Approach Control (TRACON) (e.g., “Phoenix Approach”). If a flight will enter TRACON airspace, and the flight will be under the control of the TRACON before reentering enroute airspace, CPDLC services will not be available until the flight is back under the control of an active enroute CPDLC facility – until all 20 ARTCCs are operational 24/7 and notwithstanding some important caveats, most notably that voice remains primary for all emergency communications! It is important to understand as well that using CPDLC is at the controller’s discretion. If the controller uses voice to communicate with the aircraft, pilots must respond using voice and not use CPDLC for that exchange. A crew of a particular flight that is equipped and authorized for enroute CPDLC indicates their intent to use enroute CPDLC for the flight via the flight plan, through use of a code in Block 18. However, flight crews also retain discretion to not to use enroute CPDLC services by not completing logon to “KUSA”, or if the aircraft already has a CPDLC connection, by terminating the connection providing they have accepted/rejected all pending clearances that require a response.

There are many types of communication with Center that can be accomplished via CPDLC as the primary communication mode for ATC clearances and pilot requests using CPDLC enroute. Center can use CPDLC to issue the following clearances via CPDLC message to a flight: 

• ATC route clearances
• Altitude and speed
• Controller uplink cancellations
• CPDLC Termination, Transfer, & Log Off 

In addition, Center can request a report from a flight such as CONFIRM ASSIGNED ALTITUDE or CONFIRM SPEED to which the crew can respond to utilizing the pre-formatted REPORT function. Frequency changes for handoff of VHF voice communications between sectors/Centers is done via the CONTACT message.

Flight crews can utilize enroute CPDLC to issue the following pre-formatted REQUEST messages to Center: 

• REQUEST DIRECT TO [position] – position must be on the current ATC-assigned route
• REQUEST [altitude]
• REQUEST CLIMB TO [altitude]
• REQUEST DESCENT TO [altitude]
• REQUEST BLOCK [altitude] TO [altitude]
• REQUEST VOICE CONTACT
• EMERGENCY MESSAGES 

ATC will respond to a pilot-initiated REQUEST message using CPDLC with either a new clearance approving the request or with an UNABLE response if they are not able to approve the request. 

Aircraft Equipage and Authorization Required

The aircraft avionics equipage that is specific to CPDLC/data comm for the purposes of meeting NAS requirements consists of the CMU (or CMF for integrated avionics systems) or ATSU on Airbus types, VHF Data Radio (VDR), and the Flight Management System (FMS) due to the push-to-load component. Further, the avionics requirements vary slightly based on the operation or operations desired: DCL only, or DCL and Enroute. For the latter, in order to utilize CPDLC for both functions, the aircraft must be equipped as follows:

• FANS 1/A (+) – which is provided by CMU/CMF or ATSU
• VDL Mode 2 – capability of the VDR
• VHF Data Link (VDL) Mode 2 Multi-Frequency (MF)
• “Push to load” capability enabling the pilot to incorporate received routing changes into the FMS 

For readers already familiar with DCL, note the more stringent equipage requirement for enroute CPDLC: The aircraft must be capable of VDL Mode 2 Multi-Frequency (MF) operation which requires support implemented within the CMU/CMF/ATSU. Note that for DCL only, FANS 1/A (+) VDL Mode 0/A or VDL Mode 2 suffices.

The current version of AC 90-117 furnishes guidance to operators and pilots desiring Enroute CPDLC services within the U.S. NAS. Part 91 operators do not require a Letter of Authorization for CPDLC operations within the U.S. domestic airspace. Part 121, part 135, and part 91K operators require OpSpec/MSpec A056 for CPDLC operations. The FAA’s Flight Technologies and Procedures Division, publishes an AO56 Compliance Guide that provides detailed guidance for operators wishing to pursue the OpSpec/Mspec when required. 

Additional Information

The primary purpose of this article was to provide readers with an overview of the US Domestic Enroute CPDLC functionality. Much more detailed information is readily available via several online resources. The FAA’s primary contractor for the Data Communications service is L3 Harris. L3 Harris maintains the Federal Aviation Administration (FAA) Data Communications (Data Comm) User Information website found here: https://www.l3harris.com/datacomm

That site provides numerous resources around all aspects of the US Domestic CPDLC program and is highly recommended as a first stop.
The FAA Flight Operations Group for Data Communications (AFS-410) also maintains a website with all applicable FAA documents related to the data link function: https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs400/afs410/datacomm

For additional information on datalink (and more), visit the free Virtual Classroom on the Honeywell Pilot Gateway.

J.P. Gorsky is a Naval Aviator and Honeywell Technical Sales Representative. He can be reached via email at John-Paul.Gorsky@Honeywell.com