Controller Pilot Data Link Communications (CPDLC)

An In-Depth Look at CPDLC, and What in Means for Air Travel


Airways and jet routes are becoming increasingly more crowded, as global air traffic is projected to grow 4.7% through 2037. As air traffic continues to expand, air traffic control services must be able to adapt to growth while not necessarily being able to expand themselves. Adding more controllers into the frey is not always a viable solution because the only way to do so is to break the airspace down into smaller sectors which then requires yet another frequency and series of arbitrary boundaries, i.e., increased probability of human error.

Analog Origins

The problem with segment saturation is particularly problematic over oceanic routes where hundreds to thousands of flights take place daily, the busiest being Shanwick Oceanic Control which controls about 80% of all North Atlantic air traffic which translates to approximately 1,400 flights per day.  Shanwick operates 20 high frequency (HF) frequencies and two very high frequency (VHF) frequencies to handle all of this traffic, but that is somewhat occlusive for the purpose of this discussion.


VHF frequencies are limited to line of sight which restricts their use generally to only about 200 miles maximum with good weather. In an oceanic center, VHF frequencies will be limited to inbound and outbound aircraft which are near the coast; all traffic thereafter will be moved on one of 20 HF channels. There will be nearly two thousand flights per day competing for space on 20 channels in a 24 hour period.


To succinctly describe the problem, there comes a time when too many attempt voice communications at a given time and step on each other. Also, controllers reach a point of task saturation where they can no longer handle any more traffic. The routes themselves are not saturated, particularly with the advent of RVSM where air traffic can travel in much closer proximity to each other than in previous years.

Digital Text Solution

A 4.7% annual growth on global air traffic will add somewhere around ~66 flights per day to Shanwick Oceanic which will obviously become incrementally more each year as the average is based on the previous year’s numbers. In order to more effectively, thoroughly, and positively communicate with the traffic, an alternative to voice communications was sought out and has been realized in the form of sending secure text messages via data link. The results during research and simulation by the FAA were excellent, with CPDLC reducing voice transmission by up to 75%.


In a nutshell, message traffic which would have at one point been handled via voice communication can now be done in a text format. Messages are securely transmitted between the designated air traffic controller and the aircraft through the use of the flight management system (FMS).


On top of the benefits of substantially reducing ATC workload by processing information over text, there is an inherent safety feature in the text method. English is the international language of aviation, but that is not an accurate indicator of how well it is actually spoken at any given location. A text message is a verifiable and accountable method of transferring information without confusion.


Speaking of confusion, take a look at some of the waypoints and intersections on a high chart the next time you get the chance; some are easy to pronounce phonetically like ‘TMACK’, but others are impossible to say phonetically such as ‘JADDE’, ‘ROTMN’, or ‘ASHUW’. Many times, controllers will spell them out in an exchange which could be conveyed much more accurately from the start in text format. Or consider instructions which include raw lat-longs with no fix, waypoint, or NAVAID. These are valuable minutes of airtime which many pilots are vying for where the data can be disseminated much faster and more accurately through CPDLC

Data Link Communication Overview: What is it?

Data link is a term to describe generically data link systems and subnetworks. It is important to understand primarily that data links still must abide by VHF and HF rules, and that both the airborne equipment (the aircraft) and the ground-based ATS units.


There are two broad categories of data link used technology presently in use, which are Aircraft Communication Addressing and Reporting System (ACARS), and the Aeronautical Telecommunications Network (ATN). The information regarding these two systems is far too complex to get into here, so just remember this: they are not interoperable. ACARS and ATN are two different systems which are not compatible.

Operational Use of CPDLC

One very important limitation of CPDLC to understand is that it is a strictly non-urgent communication method. All urgent communication will be conducted through voice from now until forever; you cannot replicate the brevity of vocal communication with written text, or convey the urgency of the situation the way voice inflection does.

NAS Data Communications

Through a partnership with Harris® Technology, the FAA has sponsored the definitive guide to data communications in the NAS. This pamphlet is not regulatory guidance, although it looks and is formatted about the same.


The booklet is broken down into different sections based on the different functions of CPDLC from the perspective of pilot and controller. Since this is a guidebook, it breaks down the message traffic between Boeing and Airbus aircraft; this is not an educational resource as much as it is an operational guide. Because it is so much to process, I’m going to summarize the chapters because it is an important document for anyone planning to use CPDLC.

Ch. 1- Intro

Nothing to summarize; self-explanatory.

Ch. 2 – CPDLC in the Flight Deck

To make a long story short, chapter two covers the specific messages you will receive on the FMS of a given make and model of aircraft. Why is this important? Pilots who hold multiple type ratings (not uncommon) need to be aware of the nuance between not only different makes, but even different models of the same make.

Ch. 3 – Departure Clearance Service (CPDLC-DCL)

A short but important chapter, this is the service which delivers a text version of the clearance. A personal aside here from years of experience in flight services, this is worth its weight in gold. I cannot count how many times I was given a clearance and couldn’t keep up with the routing, muffed a squawk code, spelled a transition wrong, or put down the wrong abbreviation of a NAVAID.

Ch. 4 – CPDLC – DCL Flight Crew Procedures and Guidance

This chapter has to do with the manual process of inputting message formatting, flight plans, etc., into the FMS.

Ch. 5 – Types of Departure Clearances (CPDLC-DCL)

Chapter 5 breaks down different departure clearances as the pilot approves the, or requests changes.

Ch. 6 – CPDLC-DCL Examples

A graphic, pictorial  chapter with real snippets of avionics systems screens.

Ch. 7 – En Route Airspace CPDLC Communications and Session Management

Once an aircraft has departed the airport and TRACON, they are then handed off to the En Route service, which are the 22 Air Route Traffic Control Centers (ARTCCs), commonly referred to as simply “Center”. CPDLC enabled aircraft can continue to utilize the system to into the en route potion of flight.


Since not all airports are CPDLC enabled, a pilot of a CPDLC enabled aircraft does have the option to pick up CPDLC services after departure.

Ch. 8 – Vertical Navigation – Altitude and Crossing Restrictions

Vertical navigation translates to altitude change requests. Flight crews are limited in their request actions to a single block and the preprogrammed reasons are “DUE TO WEATHER” or “DUE TO PERFORMANCE”.


Controllers have much more latitude in their vertical navigation instructions, which are all explained in the data communication guide.

Ch. 9 – Re-route and Lateral Navigation

En Route re-routing can be accomplished with CPDLC from either flight crew initiated requests or controllers instructions. Again, the usefulness of a text format is pronounced here where the nomenclature of a fix will not be lost in translation, and a lat-long request will not be confused by a misheard digit.

Ch. 10 – Emergency CPDLC Messages

In an addendum to my previous statement about CPDLC not being an emergency service, that is not entirely true but more of a common sense statement: text messages are the lesser responsive method of communication; vocal communication will always be the preferred method for emergency communication.


Should there be a need to use CPDLC text to declare an emergency, it can be done. Emergency messages will be forwarded to a supervising ATC desk so they are ensured to be received.


Some of the appendices are examples of message traffic for specific aircraft so pilots will have an accurate, constantly updated manual for their aircraft. The other appendices are examples of guidance for message traffic in various phases of flight.

What to Expect in Operation

Ground Control & Tower

In the upper left corner of the Airport Diagram, the communication and frequencies block, CPDLC will be in the list if the airport is equipped for it. If it isn’t, there will be nothing there to reference.


A full flight plan can be filed via CPDLC in the ICAO format, FAA Form 7233-1. Remember though, the flight plan must reflect the equipment actually on the particular airplane, not a default setting by the air carrier.

En Route

Route deviations due to weather or anticipated performance are standard requests by flight crews and they may request either adjusted heading or altitude.


A controller may issue an entirely new route of flight to the flight crew by CPDLC, which can be acknowledged by text, or the flight crew can request an amendment.

Departure from Domestic Airspace

Not all international airspace provides data link capabilities, and it is the pilot’s responsibility to make this determination in mission planning. Also, automatic transfer from KUSA to the next data authority (NDA) may not exist so the pilot must plan accordingly and terminate their session in order to reestablish later.

Implementation and Regulatory Guidance

United States

  • FAA AC 90-117, Data Link Communications. ACs are the FAAs immediate regulation on all aircraft eligibility and operational use of data link equipment. The AC is broken down into the following sections: data link communications overview, aircraft eligibility, communication service providers, operational use of data link systems, and performance monitoring, training, and reports.
  • Title 14 CFR Parts 21, 23, 25, 27, 29, 43, 91, 91K, 121, 125, 129, and 135. While they often cover the same territory, the Code of Regulations are not the same as an advisory circular. CFR are statutory, i.e., they are the law.


  • COMMISSION REGULATION (EC) No 29/2009 of 16 January 2009 laying down requirements on data link services for the single European sky.

International Civil Aviation Organization

  • Annex 6, Operation of Aircraft, Part I, International Commercial Air Transport.
  • Annex 6, Operation of Aircraft, Part II, International General Aviation.
  • Annex 10, Aeronautical Telecommunications, Volume II, Communication Procedures including those with PANS status.
  • Annex 10, Aeronautical Telecommunications, Volume III, Communication Systems, Part 1, Digital Data Communications Systems.
  • Annex 11, Air Traffic Services.
  • Annex 15, Aeronautical Information Services.
  • Annex 19, Safety Management.
  • Document 4444, Procedures for Air Navigation Services – Air Traffic Management.
  • Document 9694, Manual of Air Traffic Services Data Link Applications.
  • Document 10037, Global Operational Data Link (GOLD) Manual, ICAO Global Guidelines for Data Link Operations.


Due to the highly technical nature of CPDLC, ACARS, ATN, and data link systems in general, it is impossible to provide anything other than a broad primer. What is most interesting about the systems is that their lineage is traced back to about 1978, so the need for communication outside of voice was established four decades ago. Text messages are a significant part of the NEXGEN technology shift, so it could be said that it has been a long time coming.


If the simulation is accurate at all, a 75% reduction is voice communication would be a tremendous asset for air traffic services. It cleans up the voice communications so that urgent situations are much easier to identify and address. Text messaging reduces the confusion so prevelent to congested airspace, particularly the very dense regions of the world (Western Europe, both seaboards in the continental U.S., China, Japan, the Middle East). Again, the stress taken off of language barriers internationally is a prize safety factor. So many times flight crews receive an entire message but can only understand a portion of it due to conflicting dialects. This is a tech shift which should be gladly welcomed particularly by the international flight crew community.

Need assistance in obtaining your CPDLC (A056) LOA?  RVSM Solutions has been helping operators do just that from the beginning.  Give us a call or click here to find out more information!

References & Further Reading


Leave a Reply

Your email address will not be published. Required fields are marked *