Many evolutions in the data link technologies and implementations required an update of the IFATCA (Controller to Pilot Data Link Communications (CPDLC) policy. Airlines growing use of VHF Data Link Mode 2 (VDL2) has offered a reliable sub network for CPDLC, Future Air Navigations System (FANS-1/A+) has removed the risk “out-of-date” messages and Aeronautical Telecommunications Network (ATN) Protected Mode CPDLC has eliminated the risk of “misdelivered” messages. But among other deficiencies to comply with ICAO ATN Standards and Recommended Practices (SARPs), FANS protocol is still vulnerable to this risk, eventually posing a safety threat in continental high density airspace when voice read-back is removed in a near future. Considering the lack of technical solutions to this FANS misdelivery risk, IFATCA opposes the use of CPDLC with FANS aircraft in continental airspace and requests ATN CPDLC only in high density airspace, and ultimately FANS replacement by ATN data link in oceanic airspace when ICAO ATN SARPs are upgraded.

1.1  In the field of data link and Controller Pilot Data Link Communications (CPDLC) a lot of progress has been made in the last years. As the Provisional Policy of the Federation has not been changed since the Annual Conference of 2003 held in Buenos Aires, Argentina, the Executive Board (EB) considered it necessary to task the Technical and Operations Committee (TOC) to perform a review of the policies of IFATCA in regards to data link.

1.2  The Federal Aviation Administration (FAA) has decided to halt its CPDLC program that was launched in Miami. The stop of the program was due to increasing structural costs and also to the fact that there was a lack of sufficient data link equipped aircraft. On the other hand, Europe is moving forward with its LINK2000+ programme. A growing number of “pioneer” airlines are equipping their aircraft with VHF Datalink mode 2 (VDL2) and Aeronautical Telecommunications Network (ATN) CPDLC capability to communicate with Air Traffic Control (ATC) via datalink. The first LINK2000+ center is Eurocontrol Maastricht Upper Area Control Center (MUAC) in the Netherlands. The DFS (Germany), Skyguide (Switzerland) and ENAV (Italy) are expected to join the programme in 2008. Portugal, France, Spain and the UK are planned to join by 2011. MUAC is using today a Future Air Navigation System (FANS)/ATN dual-stack ground system to accommodate both ATN pioneer aircraft using VDL2 and non-standard FANS-1/A aircraft.

1.3  During the last few years, a lot of changes in the CPDLC field have occurred. Just a few developments:

2.1  CPDLC does enhance safety in situations of blocked frequencies by offering an alternate means of communications to deliver “check stuck microphone” messages to all equipped aircraft and a solution to pass critical clearances to maintain aircraft separation via CPDLC. Besides, CPDLC improves communications capacity by reducing R/T-load, one of the currently most limiting factors for high density continental airspace in Europe and North America. The idea of sharing the communication load between the executive controller and the planner (or coordinator) looks also very promising.

2.2  Two technologies are used nowadays to support CPDLC: FANS-1/A and ATN. In Oceanic airspace, FANS-1/A is the first and only technology available to provide datalink services, but also Automatic Dependent Surveillance – Contract (ADS-C). In continental airspace, ATN is the only technology recommended by the ICAO. At MUAC, however, both FANS and ATN flights are handled through a dual stack ground system.

2.3  After the publication of the Industry Rapid Reaction Force (IRRF) report, an industry group set up in 2002, 17 points or issues intrinsic to FANS-1/A were identified that could prevent the accommodation of FANS-1/A aircraft into continental high density airspace where ATN-compliant operations are foreseen. The proliferation of FANS- 1/A into continental high density airspace was not stopped, regardless of this list of issues. IFATCA raised objections on the use of FANS-1/A in high density continental airspace. The main reason for this was the fact that not all the previously identified safety concerns and operational problems with the use of FANS were resolved, therefore a safe and efficient traffic handling for those aircraft could not be assured.

2.4  The FAA has decided in 2005 to stop its CPDLC program at Miami Center, a program that was initiated in 2002. The reasons invoked were unexpected maintenance costs and poor aircraft equipage rates. Only 30 aircraft were equipped, while at least 100 airframes were expected on a daily basis. Besides this, there were over 200 equipped aircraft needed so that the benefits would exceed the costs incurred. The approach chosen by the FAA, to favour the voluntary equipage of aircraft, was not successful in an airline industry that was very concerned about the net losses suffered after the September 11th 2001 recession. On the other hand, the European Link2000+ is more successful. The program is now reaching a total of more than 320 CPDLC aircraft equipped today and this is done mainly through financial incentives. “Pioneer airlines” are equipping their fleet with ATN CPDLC capability passing through VDL2. This technology is now considered as the most capable and reliable sub-network that is enabling ATN-based CPDLC-operations. The high equipage rates are a result of the development of the VDL2 technology, a technology which is beneficial to both the airlines operational communications and the Link2000+ CPDLC program. Airlines are adopting this new technology for their own airline communications improvement and with a simple software option they can change over to ATN CPDLC operations. It must be made clear that VDL2 is not only offering a high bandwidth for ATN CPDLC communications when the aircraft is ATN equipped, but also is increasing speed for all ACARS communications including FANS communications when the aircraft is FANS equipped.

Many differences in the message sets between FANS and ATN aircraft would make it difficult to present the same interface to the controllers handling a mixed data link fleet without degrading the overall performance.

2.9  Based on the IRRF report, and the ensuing heated safety debate for the MUAC FANS-1/A accommodation for continental ATN-airspace, it was concluded that a “latency timer” was needed to detect FANS expired (“out of date”) messages. The risk was that a clearance arrives onboard but is not read immediately by the pilots; the message remains on the display long after the clearance validity has expired and then eventually the pilot reads the message and complies with it. The solution used to mitigate this risk was the voice read-back. In an upgrade to FANS-1/A, the technical solution provided by FANS designers is to include a “latency” timer, a timestamp in each message that helps detect when the pilot does not give an answer within a specified period (usually 120 seconds). That change was named FANS-1/A+ and is now delivered to all new aircraft since 2005 and proposed as a retrofit on existing FANS-1/A equipped aircraft. This improvement eliminates the risk of out-of- date messages, but this retrofit is not mandatory for the time being.

2.10  Even with the FANS-1/A+ upgrade mentioned above, there is still the second risk that justified the use of voice read-back as mitigation action: misdelivery of FANS messages. When a controller wants to send a CPDLC message to an aircraft, she/he selects on his display the Flight ID (the callsign). The ground system is responsible for initiating the transfer to the aircraft but it must determine first the network address of the aircraft. For ATN aircraft, the network address is the ATN address whereas for FANS aircraft, this is the Registration Number (the Tail Number, or ship number as North Americans would say).

Using the information received from the aircraft at the logon time (Flight ID & ATN address for ATN aircraft, or Flight ID & Registration Number for FANS aircraft) and maintained in an addressing data base, the ground system retrieves the network address from the flight ID (this is the Flight ID / Tail Number correlation). Many errors can occur during this correlation:

• Software error when creating the CPDLC message
• Loss of integrity in the Flight Plan Database
• Undetected use of out-of-date flight plan messages or AFN (ATS Facilities Notification, the FANS equivalent for Context Management)
• Errors in the distribution or the processing of AFN messages

FDPS distribution errors and out-of-date information cannot easily be mitigated by software assurance and have an accumulative effect: a single error in the chain of functions performing this Flight ID / Tail Number correlation can result in a wrong Tail Number returned to the Flight Data Processor from the data base.

The Flight Data Processor then forms the CPDLC message, including the false Tail Number in the message header and the message checksum. The message will be delivered to the aircraft with the wrongly selected Tail Number and the onboard verification of the checksum’s and the message header’s Tail Number will be satisfactory. If the received message is a profile changing message and appears applicable in context, the crew will comply with it, potentially leading to a hazard.

With ATN aircraft, PM-CPDLC adds in each message checksum the Flight ID. If there is a wrong Flight ID / ATN address correlation by the ground Flight Data Processor and the message is sent to a wrong ATN address, it is easy to detect the misdelivery onboard thanks to the difference between the message checksum incorrect Flight ID and the aircraft actual Flight ID. FANS messages only include the Tail Number which is at the same time used for addressing, so the lack of the Flight ID is a critical FANS deficiency to detect messages misdeliveries.

3.1  Regional CPDLC implementations in high density continental airspace (e.g. in Miami and Europe) have highlighted the importance of high equipage rates for any successful CPDLC implementation. Therefore, it is a good sign that more and more airlines equip their fleet with VDL2 for CPDLC. It is regrettable that this is only confined to the European Region and that no other global programme for use of CPDLC based on ATN is planned.

3.2  FANS technology has evolved to try and meet ATN requirements. The FANS-1/A+ upgrade has solved the out-of-date messages issue but the risk of clearances misdelivery still remains unresolved. Instead of limiting the number of FANS aircraft as the PU-40 Safety Analysis recommends in order to mitigate this risk, it is preferable to revert to voice R/T with FANS transoceanic aircraft inside high density continental airspace. It is regrettable that such a mixed mode operation between ATN aircraft handled via CPDLC and FANS aircraft handled via voice R/T (along with small and State aircraft) is at this stage the only solution, but no safe alternative for the accommodation of FANS aircraft in that airspace has been proposed so far.

3.3  In oceanic airspace, the operational benefits of FANS are fully acknowledged and recognized. FANS in that environment obviously cannot be replaced as long as ATN cannot meet and match the same operational standards and performances as FANS.

3.4  ATN standards are regularly evaluated and improved to increase efficiency and reliability. For instance, PM-CPDLC is eliminating the risk of message misdeliveries, so that voice read-back mitigation is not necessary any more. There are nevertheless still many problems to be solved before ATN can be used in oceanic and remote areas. The ICAO ATN ADG develops the new ATN specifications needed for oceanic operations, and are searching for a safe and operational solution for a datalink convergence from ATN & FANS to new ATN standards. Till the moment those solutions are found, IFATCA should monitor the developments.

It is recommended that;

4.1  IFATCA Policy on page 3 2 4 11 of the IFATCA Manual:

All implementations of CPDLC must demonstrate full compliance with ICAO ATN SARPs. However, in Oceanic and Remote Regions, where it can be demonstrated that CPDLC implementation improves controller-pilot communications, it is recognized that non ATN compliant technologies may be deployed during a transitional phase.

The ICAO ATN SARPs and their progressive development form the definitive basis for any future CPDLC implementation.

is deleted.

4.2  IFATCA Policy is:

All implementations of CPDLC must demonstrate full compliance with ICAO ATN SARPs. However, in Oceanic and Remote Regions, where it can be demonstrated that CPDLC implementation improves controller pilot communications, it is recognized that non ATN compliant technologies may be deployed during a transitional phase.

The ICAO ATN SARPs and their progressive development form the definitive basis for any future CPDLC implementation.

In high density ATN CPDLC airspace, FANS aircraft shall be handled via voice R/T for safety reasons.

and is included in the IFATCA Manual on page 3 2 4 11.