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What Is Going On With ATN B1 (CPDLC) Functionality Issues in Europe?

What Is Going On With ATN B1 (CPDLC) Functionality Issues in Europe?

Making Sense of the Looming ATN B1 Mandate

With the European Aeronautical Telecommunication Network Baseline 1 (ATN B1) mandate set to take effect February 5, 2020, many people are trying to understand what’s going on in the world of ATN. Now we have a mandate looming and a continuously changing subset of the White list and Black list (just renamed the “Logon List” and the “Logon Ineligible Avionics” list for better clarity). Interpreting the rules is as complicated as ever. How did it get this way? The answer is: it’s complicated. Let’s try to break it down into some simple-to-understand explanations and explain what Honeywell is doing to adapt to the ever-changing ATN development.

In Honeywell EPIC platforms, what controls CPDLC is referred to as the Communication Management Function (CMF). A CMF is an integrated datalink communications component of the EPIC system and is the heart of the datalink system. In cockpits where avionics are less integrated, the functionality resides in a Communication Management Unit (CMU) but can be thought of as the same thing for the purpose of this article. Although CPDLC has been used in many parts of the world, Europe is the only area where the ATN B1 network is deployed. All other regions use the ACARS (FANS) network. When ATN B1 was conceived in the late 1990s, it was designed to be a faster, more reliable way of relieving frequency congestion across most of Europe, using a higher speed radio referred to as Very High Frequency Datalink Mode 2 (VDL Mode 2). By comparison, FANS uses an older network consisting of VDL Mode 0, although the United States is implementing VDL Mode 2 for its Enroute Data Comm. VDL Mode 0 is not compatible with the ATN network. The goal of the ATN B1 network focused on essentially two things:
1. Higher speed data transmissions/minimized message latency
2. Lower instances of provider aborts (think dropped calls on a cellphone)

ATN B1 is a much more complex set of protocols than traditional ACARS. The ATN B1 environment continues to evolve and various European Air Navigation Service Providers (ANSP) (ATC centers) are at different levels of maturity. ATN is a system that was mandated and rolled out relatively quickly and, some would argue, before it was validated and proven ready by the ground networks and airborne avionics.

The February 2015 mandate was subsequently pushed out to February 5, 2020, due to poor network performance and lack of deployment by many centers. The entire industry, including ATC ground systems, air-ground network communication systems, and avionics (both air transport and business aviation), struggles to meet the challenges of this environment.

A good analogy would be cell phones and when they first became common. Most people remember the early days of flip phones, with their poor call quality and regularly dropped calls due to the emerging infrastructure. This is similar to where the ATN B1 network is today with the exception that the ATN B1 network is being closely monitored for performance and is continuously tightening the network performance standards in an effort to make the system reliable.

Whereas with cell phones, users wanted better call quality and fewer dropped calls, the ATN B1 development project seeks to minimize round-trip delay time and provider aborts (drops in controller/pilot message connectivity). The busy European airspace is a challenging environment. External factors that can induce conditions causing provider aborts include:
· High amount of VHF traffic – causes congestion on the VDL Mode 2 frequencies
· Terrain – mountains can interfere with line of sight between a ground station and the aircraft; bodies of water can cause unintended VHF signal reflectivity that confuses air/ground radios
· Number of ground stations and proximity to other aircraft in the area – can cause RF signal collisions

What About the White List?
The initial implementation of the ATN B1 network in 2014 on Honeywell EPIC-equipped aircraft satisfied all industry requirements and passed all system validation, verification, and certification checkpoints. The ATN network has since evolved and become strained with more and more aircraft using VDL Mode 2 and the continued proliferation of non-safety operational messages—both of which have impacted network robustness. In 2015 several agencies within Eurocontrol devised something called the Logon List (previously the White list). The Logon List is an operational approval currently used by the following air navigation service providers:
· Maastricht Upper Area Control
· Skyguide
· Nav Portugal

These ANSPs restrict ATN B1 logons within their FIRs only to aircraft with avionics known to have acceptable provider abort rates. These air navigation service providers now also maintain the Logon Ineligible Avionics list (previously called the Black list) of avionics versions that exhibit excessive provider aborts. Aircraft with avionics noted on the Logon Ineligible Avionics list will not be permitted to establish an ATN PM-CPDLC connection using ATN B1 within their FIRs (see circled areas in Figure 1).

Figure 1. FIRs Implementing the Eurocontrol Logon List

Table 1 lists the same information in tabular format.

Table 1. FIRs Implementing the Eurocontrol Logon List

An air navigation service provider uses the Logon List (the White list) to assess avionics performance within the congested VDL Mode 2 network. As avionics updates are deployed, Eurocontrol gathers data on the aircraft’s PM-CPDLC performance – usually over a three-month period. The air navigation service providers that use the Logon List review the observed network performance data and decide whether the avionics configuration is acceptable to stay on the Logon List. If the new avionics version exhibits excessive provider aborts, the air navigation service providers move the avionics configuration to the Logon Ineligible Avionics list (the Black list). The avionics manufacturers are typically informed of this Logon Ineligible Avionics list decision with a limited one-month advanced notice. The Logon List criteria have gotten much tighter over the past five years resulting in Logon List operators being moved to the Logon Ineligible Avionics list from one month to the next.

The Mandate (Datalink Implementation Rule 29/2009)
The ATN B1 mandate, effective February 5, 2020, requires that operators in European airspace above FL 285 must be equipped with ATN B1 or meet the exemption criteria. The operational impact of being on the Logon Ineligible Avionics list (the Black list) is that the aircraft will not be permitted to establish a PM-CPDLC connection with air navigation service providers in their FIRs and will have to revert to voice communications with ATC. It is important to note that some air navigation service providers do not use Eligible/Ineligible lists (see Figure 1 and Table 1). It is also important to understand that, while the Eligible/Ineligible lists are an operational requirement for establishing ATN sessions with ATC, they are not a mandate requirement for complying with Datalink Implementation Rule 29/2009. The rate of 1 Provider Abort per 100 hours of CPDLC session time is a mandate requirement that the end-to-end ATN network is striving to achieve over time.

From the Eurocontrol Frequently Asked Questions:

“If my aircraft are not on the white list, are they still considered compliant with the data link regulation and will they suffer any restrictions?”

“The White list is an operational response by some ANSPs to improve the performance of data link by not allowing aircraft that are observed to perform poorly from logging on. Whether or not an aircraft is on the White list does not affect whether an aircraft is considered compliant with the regulation. Aircraft not on the White list will still be considered compliant with the regulation if they are equipped in accordance with the regulation. However aircraft not on the White list are not permitted to log on to use data link by the ANSPs that implement the White list. There are currently no plans from the White list ANSPs to apply any restrictions to aircraft not on the White list other than not allowing those aircraft to log on to data link.” (See Eurocontrol Frequently Asked Questions.)

On April 10, 2019, Eurocontrol notified Honeywell, Dassault, Embraer, Gulfstream, and Pilatus that all EPIC aircraft with CMF 3.0 or earlier would be placed on the Logon Ineligible Avionics list, due to the excessive number of Provider Aborts observed on certain European aircraft flying certain routes.

Honeywell performed an exhaustive root cause corrective action analysis consisting of ground-based robustness testing and six weeks of flight testing throughout the United States and Europe. The recommendation is for all EPIC aircraft to adopt EPIC CMF Block Point 3.4 and EPIC VDR Mod U. Flight tests with this configuration show ATN PM-CPDLC performance that meets Logon List criteria. Honeywell is working with Dassault, Embraer, Gulfstream, and Pilatus to make EPIC CMF Block Point 3.4 and EPIC VDR Mod U certified and available on all in-service and future EPIC platforms.

Honeywell emphasizes the importance of operators continuing to update CMF and VDR software versions as they become available and adapt to an evolving ATN network. The Logon List criteria will continue to get tighter in future years until the ATN network meets the Safety Performance Requirement of one provider abort per 100 hours of ATN PM-CPDLC sessions. This is an industry-wide challenge and Honeywell has invested heavily in making sure its ATN system exceeds performance requirements. Table 2 lists Honeywell’s current Data Communication products and Logon List status as of this publication.

Table 2. Current Data Communication Products and Logon List Status

Eurocontrol maintains the Logon List on behalf of the air navigation service providers and makes it publicly available.

Note: Download the Fielded Loads document for more specifics about the CMF versions by platform.

It is important to understand that most business and general aviation operators will fall under an approved exemption list, where no ATN PM-CPDLC or VDL Mode 2 Multi-Frequency requirements are levied upon the aircraft. Being exempted means that there are no operational restrictions imposed on an aircraft without an ATN B1 installation. The final exemption list from the European Commission can be found online here. In simple terms, the exemption list provides relief to the following aircraft:
· Aircraft with a Certificate of Airworthiness prior to 1995
· Aircraft with a Certificate of Airworthiness before December 31, 2003, and ceasing operation before December 31, 2022
· Aircraft with a Certificate of Airworthiness prior to January 1, 2018, with a datalink system (ATN)
· Aircraft having a maximum seating capacity of 19 passengers or less AND a maximum certified takeoff weight of 100,000 pounds or less
· State aircraft
· Aircraft flying for testing, delivery, or maintenance purposes or with data link constituents temporarily inoperative under conditions specified in the applicable minimum equipment list.

One other important distinction: If an aircraft is exempted from the requirement but has an approved ATN eligible system, the aircraft can still log on and get the benefits, provided they have the appropriate authorizations.

Going Forward
Honeywell has invested significant effort and resources in improving the VDL Mode 2 performance of our Datalink products. Every year for the past five years Honeywell has performed flight tests of each CMU, CMF, and VDR software update within congested VHF airspace. Flight test results have shown that the real-world airborne environment cannot be simulated or fully tested in a laboratory or ground-based environment.

VDL Mode 2 connectivity problems are a major contributor to poor performance. Honeywell data shows that VHF congestion at altitude is so prevalent that an aircraft can hear up to 60 VDL Mode 2 air/ground transmitters at a given time. This congestion results in a non-ideal environment for aviation communication. This is a systemic issue with the VDL Mode 2 network and is not resolvable with avionics updates alone. Honeywell is engaged with Eurocontrol and the communication service providers to openly share the findings and results from Honeywell flight tests that should drive air/ground updates that benefit the overall aviation community—airlines, operators, airframers, and air traffic controllers.

In addition to annual flight tests, Honeywell continues to participate in industry associations and standardization meetings to identify avionics improvement opportunities. Honeywell’s Datalink product roadmap has multiple software updates planned over the next several years that will provide:
· Value-added operational improvements
· Operational cost saving opportunities
· Pilot human-machine-interface improvements
· Cyber security improvements

Any in-service issues identified with VDL Mode 2, ATN PM-CPDLC, or any Eurocontrol recommendations necessary to support the Safety Performance Requirement will be incorporated into future Datalink product updates. Each product update is additive, such that any future VDL Mode 2 and ATN robustness improvements will build upon the proven performance of previous versions and will continue to accommodate changes as necessary to comply with the Eurocontrol Logon List.

Program Pilot David Rogers supports EPIC and NG FMS-equipped Cessna and Gulfstream aircraft for Honeywell Flight Technical Services. He can be reached via email at