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Controller-pilot datalink tackles miscommunications with text

In February 1989 a Boeing 747 hit a hillside on approach to Kuala Lumpur, Malaysia, killing all aboard. The air traffic controller’s instruction to “descend two-four-zero-zero (2,400 feet)” was interpreted by the crew as “descend to four-zero-zero (400 feet),” with tragic results.
Illustrations by Charles Floyd
Illustrations by Charles Floyd

Across the globe, in 2004, the pilot of a light twin cruising in night IMC mistakenly read back a descent clearance issued to another aircraft of the same model with the same last three characters in the N number. The controller did not catch the mistake and the aircraft struck terrain at 5,537 feet msl, again killing all on board. Aviation’s accident history is full of tragic demonstrations that miscommunication kills.

Controller-pilot datalink communications (CPDLC) holds the promise to drastically reduce misunderstood clearances, while also reducing frequency congestion, cutting down on departure delays, and saving time and fuel en route. Two pilots reading “DESCEND TO AND MAINTAIN 2400” on a display are not going to be confused as to the clearance. If a descent is sent by CPDLC directly to N434PA there is no chance N304PA thinks it was intended for them.

While at its essence CPDLC couldn’t be more simple—direct, text-based communication between aircraft and ATC—the implementation has been problematic, and the emergence of multiple CPDLC systems has only increased confusion. Looking at the four types of datalink systems currently in use through a consumer electronics model may help break down the complexities of the various CPDLC hardware, software, and connection requirements.

What are the pieces?
Consider sending a message to a friend from a tablet. We’ll first have to pick from several possible options of apps to use, some of which may be constrained by the friend’s own use of given apps or the inter-compatibility of the apps. For example, if we open the Gmail app on the tablet to send an email to a friend, they don’t have to be a Gmail user to receive the message. In contrast, a Facebook Messenger communication can only be sent and opened within its narrow ecosystem. Similarly, one of the two main protocols used in CPDLC is in broader use, while one is limited to a specific system running in defined geographical areas.

With the app open and message ready to send, we next need to get the message off the tablet. Depending on the tablet’s hardware it may only have the ability to send messages over Wi-Fi, with associated limits on where we can be physically while maintaining two-way communications. By spending a little more money on the tablet we can add a cellular radio in addition to Wi-Fi—now we’re much less limited geographically and can send messages from the road traveling between our house and a coffee shop, not just when in close proximity to each. However we send the message, at the next step a “toll-taker” comes into play. Ultimately someone is footing the bill to connect the device to the internet via an internet service provider (ISP). Maybe we pay directly to a cellular provider for monthly access, or maybe the coffee shop whose Wi-Fi we’re using is putting a penny of our latte’s cost toward its internet bill, but this step ultimately isn’t free.

Depending on the final destination of the message, that might be the only “toll” collected—if we’re sending an email to a friend, for example. In contrast, some apps or services add their own expense. Uploading not a message, but a photograph to an online storage host, for example, may have an annual expense associated with the service, in addition to the cost of connecting to the internet.


If not for COVID-19, the entire U.S. enroute structure would have CPDLC coverage by now. Current estimates for complete coverage are end of 2023.
If not for COVID-19, the entire U.S. enroute structure would have CPDLC coverage by now. Current estimates for complete coverage are end of 2023.
The southern “Blue Spruce” routes, popular with jets without the range to cross the Atlantic nonstop, fall in the datalink mandate airspace. This leaves aircraft without FANS two options: route farther north through the exempt airspace, or fly outside the vertical limits of the mandate.
The southern “Blue Spruce” routes, popular with jets without the range to cross the Atlantic nonstop, fall in the datalink mandate airspace. This leaves aircraft without FANS two options: route farther north through the exempt airspace, or fly outside the vertical limits of the mandate.

How did this all begin?
For more than 40 years aircraft have had the ability to send and receive textual messages through the Aircraft Communication Addressing and Reporting System (ACARS). Originally implemented as a system to automate the routine communications needed during every airline flight (e.g., takeoff time, ETA, landing fuel quantity), ACARS developed into a full-featured service presently utilized widely by both general aviation and airline traffic.

ACARS: What does it do?
Staying with the consumer electronics model, ACARS as a service can be loosely compared to using internet provider America Online (AOL) in late 1990s. While some interaction with the outside world occurs, the interaction is often filtered through a third-party service provider and largely occurs in a “walled garden” environment. For a single subscription fee that covers both network access to the system (paying the “ISP”) and the services provided, the pilot can perform flight planning on an app or computer before flight and then have the flight plan pushed directly into the avionics over the air; send and receive emails in flight; request weather information including METARs, TAFs, and textual ATIS (d-ATIS); and even order catering to be ready at the next airport.

The first ACARS feature that enters CPDLC territory (albeit in the most limited of ways) is pre-departure clearance (PDC). An official IFR clearance containing all the elements that would normally be received by voice, a PDC replaces the need to call clearance delivery. PDC does not come directly to the aircraft from ATC, however—it is routed through a third-party provider. Because of this, a pilot doesn’t even need ACARS capability in the flight deck to receive a PDC. Flight planning services such as ForeFlight and FltPlan offer PDC routing to aircraft without ACARS via email or text message to a phone or tablet.

There are limitations associated with PDC. It is only available for flights departing from roughly 70 of the busiest airports in the United States. Further, the system limits any aircraft’s receipt of a PDC from a given airport to one per 12- to 18-hour period. A pilot making two pick-ups at Teterboro Airport on the same day would need to pick up the second outbound clearance over voice. Finally, if a revision to the flight plan is made after delivery, for example for a weather reroute, the PDC system cannot accommodate the change; the pilot will need to receive the revision via voice communications.

Slightly different from a PDC is a departure clearance issued over ACARS, or ACARS-DCL. Currently in use in Europe, ACARS-DCL also is transmitted over an ACARS connection; however it is not routed through the third-party provider, but rather transmitted directly from ATC to the aircraft. This distinction is not of practical importance to the pilot, however, and the limitations of ACARS-DCL are similar to those encountered with PDC in the United States with respect to clearance changes.

The final ACARS feature that ties into the ATC system is delivery of an oceanic clearance. Prior to entry into oceanic airspace the airplane must receive a clearance distinct from the IFR clearance upon which it has been operating. Receiving this clearance textually over ACARS versus over voice provides the benefits of intelligibility that CPDLC provides, and eliminates the need for one pilot to leave the “domestic” controller frequency for a period of time while contacting the oceanic radio operator who will relay the clearance.

ACARS: What do I need?
The first piece we need to utilize ACARS is the appropriate “app,” or software option, in the airplane’s avionics. Depending on the aircraft, ACARS might come as standard equipment with the purchase of a new mid- or large-size jet, or it could be an à la carte option on lighter aircraft. The option can run more than $40,000 for software enablement alone.

We’re not done writing checks yet, though. Having the software alone is like having a tablet with no Wi-Fi or cell radio; we need one or more ways to connect our avionics to the outside world. Our Wi-Fi equivalent in aviation is a third VHF com radio, one that supports datalink over the VHF band (VDL). While also usable for normal voice communications, to support ACARS (or other CPDLC functions), this radio will typically be set to and left in data mode by the flight crew, functioning like a dial-up modem of old.

ACARS as a service can be loosely compared to using internet provider AOL in the late 1990s.Just as Wi-Fi only works within limited range of a hot spot, VDL is only useful within limited range of a VHF antenna. When operating in remote areas, notably over the open ocean, we need a longer-range communication option. Our second option, analogous to a tablet’s cell card untethering us from the limited range of Wi-Fi, is a satellite data radio. Typically using the Iridium network in light and mid-sized jets, and either Iridium or Inmarsat connection in larger jets, a satellite data radio allows for global or near-global connection, depending on the network in question.

With software and hardware in place, our last requirement is an active subscription through a service provider. ARINCDirect, Honeywell Forge, and Satcom Direct are currently the main options in the market, although flight planning and support providers such as Universal Weather and Aviation also bundle their services with the datalink support of one the main three providers to create further offerings. While the ACARS system on the aircraft must be set up specifically to the service provider, the option can be easily changed—the same way current eSIM technology lets a tablet be converted from the Verizon to AT&T network without a physical sim card swap.

And then there was FANS
Developed in parallel by Boeing and Airbus in the mid-1990s, the future air navigation system (FANS) 1/A (later 1/A+) introduced true two-way CPDLC. As global oceanic air traffic increased, more accurate surveillance and more reliable communication was needed to accommodate the greater number of aircraft needing to utilize the same blocks of oceanic airspace. Because oceanic traffic operates beyond line of sight VHF capability, communication has relied on long range high frequency (HF) radios. HF reception is not always reliable, however, and because of atmospheric effects aircraft can experience long periods of flight with no ATC contact.

Transmitting position information over satellite link (automatic dependent surveillance-contract, or ADS-C) and messages over the same link (CPDLC) allows ATC to operate FANS-equipped aircraft in much closer proximity when out of radar coverage. Before the FANS mandate was in effect aircraft were separated by roughly 60 nautical miles over the North Atlantic; with the most up-to-date FANS capability separation is cut to half, or in some cases a quarter, of that value.

FANS: What do I need?
As with ACARS, an aircraft must be equipped with appropriate software, hardware, and a subscription to utilize FANS. As FANS is built upon the ACARS communication protocol, an aircraft must start with all the required ACARS capability before adding FANS on top. The software alone can run near six figures, and for oceanic use of FANS a satellite link is essentially mandatory. While there are some certified installations of FANS that operate over VDL alone they are, for practical purposes, limited to use over land or near shore.

FANS: Where is it required?
FANS is the first CPDLC system to be currently mandatory in parts of the world. Of special interest to light and mid-sized jets is the North Atlantic datalink mandate. As of early 2021, nearly all of the North Atlantic between FL290 and FL410 requires full FANS equipage.

While there is an exception for flight in a defined “surveillance” corridor over Greenland and Iceland, it is critical to note that the “Blue Spruce” routes over southern Greenland, commonly used by light jets transiting the North Atlantic, fall outside the exempted area. Thus, use of these more southerly routes between FL290 and FL410 requires full FANS equipage; the only routes across the North Atlantic that fall entirely inside the exempted area pass through Sondrestrom Air Base (BGSF), several hundred miles north of the southern tip of Greenland. While pilots without FANS can request a climb through the mandated airspace, ATC does not guarantee an approval will occur at any given time.

Reading clearances removes ambiguity because of poor radio reception, frequency congestion, and accents of non-native speakers.Next up: ATN B1/PM CPDLC, née link 200+
Why use one name when three can do? In the late 1990s Europe conceived of a domestic CPDLC system to relieve communications congestion. The FANS system was perceived as too slow to serve the desired needs, so a new system was designed. While FANS was built using the existing ACARS protocol (not to be confused with the package of services also called ACARS), the aeronautical telecommunication network baseline 1 (ATN B1, often truncated to just ATN) system was built upon its own network and protocol, incompatible with FANS. Also called protected mode CPDLC (PM CPDLC), there is some good news with ATN—while the system is incompatible with FANS, the communication medium is one also used by ACARS and FANS—the VDL radio.

ATN: What do I need?
Only software and hardware. As equipage is partially mandatory over Europe (more on that later), the network costs are born by ATC, and no subscription is required. You will need the software, and again, unless it comes standard for your aircraft, it will be an additional cost above and beyond the ACARS and/or FANS “apps” you may have purchased. You will also need the third VDL radio option (no satellite data transmission allowed as it’s too slow), but that same radio can also be used for ACARS and/or FANS interfacing.

ATN: Where is it required?
In short, over Europe when flying above FL285. The program suffered extensive delays, however, which resulted in some generous exemptions being handed out. If an airplane has under 20 seats, and a maximum takeoff weight below 100,001 pounds, and was granted an airworthiness certificate before February 5, 2020, the exemption is (for now) permanent. For any new aircraft put into service, however, equipage is mandatory for flight over FL285.

ATN: What went wrong?
In short, a lot, but the full extent is still being identified. As the system became operational, but before the mandate was in place, a high level of “provider aborts,” or dropped connections, were experienced—leading to lost or timed-out messages, even for aircraft not experiencing connection problems. Several control areas reacted by banning all log-ons unless an aircraft could prove their avionics platform could demonstrate an acceptable abort rate. This “white list” became the current “logon list,” and requires an operator to complete an online form identifying their hardware and software configuration before being allowed to log on to ATN in several of the busiest blocks of European airspace. Once admitted to the logon list connection performance is monitored, and aircraft may find themselves kicked off should they experience a higher abort rate than other aircraft operating in the same area.

Eurocontrol points fingers at avionics software, while avionics manufactures are quick to point out that ground-side changes have been required to increase the performance of the system, and that the system is far more congested at altitude than ideal, with up to 60 ground stations being heard by a single aircraft. It’s important to note the distinction between the mandate and the logon list—an aircraft may be required to have ATN to fly above FL285, while simultaneously be prohibited from using ATN in designated control areas until its software is updated.

U.S. Data Comm
The final and newest system to discuss, the U.S. Data Comm system, will be of the most utility to a United States-based pilot. Data Comm is just “rebranded” FANS 1/A operating over a VHF (only) link. As such, an up-to-date FANS 1/A installation with a VDL radio can operate in the system without additional hardware or software. Even better, for aircraft that don’t need a full-blown FANS system capable of operating in oceanic airspace, the cost to enable Data Comm is a fraction of what it is for a complete FANS package. The combined hardware and software cost to enable Data Comm in a Garmin G3000 flight deck found in most in-production light jets (Cessna Citation M2/ CJ3+, Embraer Phenom 100EV/ 300E, and HondaJet Elite) is less than the price of just the satellite hardware needed for full FANS installation.

U.S. Data Comm: What does it do?
The first piece of Data Comm, CPDLC-DCL, mustn’t be confused with ACARS-DCL. Recall that ACARS-DCL is simply a European PDC transmitted directly from ATC to the aircraft; DCL in the United States, in contrast, is a system with significant improvements upon PDC. There are no limitations to the number of DCL clearances received at a single airport in any span of time, clearances can be revised and re-sent, and best of all, clearances and revisions can be directly loaded into the active FMS flight plan with a button push. Nearly all airports that support PDC also support DCL, with only roughly a dozen of the PDC airports still PDC-only.

The second piece of Data Comm, en route CPDLC, is still in limited roll-out, with only five en route centers—Oakland, Kansas City, Indianapolis, Washington, and Minneapolis—presently supporting CPDLC service. As with departure clearances, revisions to routes can be directly loaded into the active flight plan with a button push, cutting down on the potential for in-flight data entry errors. Expect to see further centers operating CPDLC this year; impacts from COVID-19 have been the main reason more are not online.

U.S. Data Comm: What went wrong
As did ATN in Europe, domestic CPDLC experienced problems during rollout that led to restrictions on participation. FDC NOTAM 1/3379, in effect until September 26, 2022, states in part: “GA AND BUSINESS AVIATION ACFT ARE PROHIBITED FM USING EN ROUTE CPDLC EXC APPROVED TRIAL PARTICIPANTS.” The good news is that aircraft with updated software are likely to be eligible to participate in the “trial,” and thus able to use en route CPDLC. L3Harris, a primary contractor on the Data Comm system, publishes a list of eligible hardware and software versions of various avionics platforms and instructions on how to receive permission to participate in the en route trials.

Two final requirements
Beyond required the hardware, software, and subscription (if needed), one or two more elements are required to utilize CPDLC. First, proper completion of the equipment codes in fields 10 and 18 of the ICAO flight plan form tells the CPDLC system the aircraft attempting to connect has the necessary equipment, and so to allow a connection. An airplane without the correct code for en route Data Comm (J4 in field 10, which means “I can communicate on FANS 1/A over VDL Mode 2”) won’t be able to establish a connection. The FAA finds a large number of flight plans are filed that do not have proper equipment codes entered, so research and care is required here.

Finally, for Part 91 operations outside of the United States, a letter of authorization must be granted before the CPDLC system can be used; the application process is lengthy and demanding. Documentation of the completion of acceptable pilot training is one requirement, and a “table top exercise” (oral exam by an FAA inspector) is likely to be required, ensuring the pilot understands the intricacies of the CPDLC system operation. While not mandatory for domestic CPDLC, the pilot training should properly be considered a must-do for all, given the complexity and pace of change of this evolving technology.




Neil Singer

Neil Singer is a corporate pilot, designated examiner, and instructor in Embraer Phenoms and Cessna Citations. He has more than 10,000 hours of flight time with more than 20 years of experience as an active instructor.

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