Serial Number : 2019 _ 003 Rev 2 Subject : Data Link Performance Improvement Options Originator : NAT SPG Issued : 08 July 2020 Effective : 08 July 2020

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1.
Purpose of Bulletin. The purpose of this bulletin is to provide guidance to North Atlantic (NAT) operators regarding options that are available to improve data link performance.

2.
Background. Application of the reduced lateral and longitudinal separation minima in the NAT Region is dependent on a smooth functioning FANS 1/A data link system. Various known data link related deficiencies in aircraft systems and poor data link performance have a detrimental effect on the air traffic control system and impede aircraft operator's efforts to obtain performance-based communication and surveillance (PBCS) authorizations. Many of these known deficiencies have already been fixed by aircraft manufacturers and software upgrades are available. To ensure the best possible functioning of the NAT air traffic control system, it is of utmost importance that aircraft operators always operate the latest available FANS 1/A related software version in aircraft that fly in the NAT high level airspace (HLA) and that the aircraft systems are configured in an optimal manner. Meanwhile, implementation of improvements and corrections is also a priority undertaking for the ground and network segments of the overall FANS 1/A system.

3.
The list of recommended data link performance improvement options provided in the Attachment to this OPS Bulletin describes the problems and solutions identified to improve data link performance. However, it should be noted that not all aircraft operators experience all these problems and therefore not all solutions apply to all aircraft operators. Additionally, while acknowledging there is confidence that the recommended improvement options would improve the data link performance, it should be noted that these updates might not be necessarily seen as sufficient to ensure a PBCS authorization. Aircraft operators are advised to consult with aircraft manufacturers for guidance regarding implementation of the improvement options.

4.
The certification status versus EUROCAE ED-122 / RTCA DO-306 standards and PBCS authorization requirements should be clarified by aircraft operators in coordination with the manufacturers concerned, recognizing the aircraft operators need to consider the economic and operational aspects and priorities.

5.1
The . This option removes HFDL as an available media so the "next-on-busy" function will not occur.
At the recent FAA PARC CWG40, an Iridium SATCOM equipped operator demonstrated the PBCS performance impact of HFDL "next-on-busy". Some other operators have also taken this action to place HF into "voice-only" mode. Iridium SATCOM operators, equipped with CMU-900, could take this action on interim basis prior to an available CMU software. See item 4 below on HFDL for similar recommendation.

Solution b): Work with Airbus and Rockwell
Collins to install software versions that disable the next-on-busy feature. (For the Rockwell Collins CMU-900 with recent software, this can be done with a database update).

VHF to SATCOM Transitions
2.1 Transitions from using VHF to using SATCOM, especially when they occur repeatedly in a short period of time, reduce datalink performance because the ACARS protocols are generally not designed to maximize performance but rather to minimize cost by persistently attempting to use less costly VHF.

Solution a): Disable VHF datalink just prior to entering oceanic airspace
Implement flight crew procedures to disable VHF datalink (usually by placing the VHF radio used for VHF datalink into voice mode) just prior to entering oceanic airspace or prior to leaving contiguous VHF coverage in order to proactively force SATCOM use. Conversely, enable VHF datalink when exiting oceanic airspace or entering contiguous VHF coverage.

"Ack-and-toss"
3.1 ACARS router (CMU or equivalent) avionics may for various reasons acknowledge receipt of a FANS uplink message but then fail to deliver the message to the avionics that host the FANS applications. This is commonly known as "ack-and toss" behaviour.

Solution a) Rockwell Collins CMU-900 software problem
For the 747-8, Boeing has certified core software -202 that fixes this problem. For the 737, 747-400, 757, 767 and MD11, Rockwell Collins is certifying core software -014 that fixes this problem. For the 747-400, 757, and MD-11, Boeing and Rockwell Collins are investigating certification opportunities.

Solution b)
Boeing 777 AIMS-2 software problem Boeing developed AIMS-2 BPV17.1 software that fixed this problem.

Solution c) Airbus A320/A330/A340 software problem
This problem occurs only in the ATSU CSB/CLR7.1 to 7.4 software versions. Airbus is developing the CSB/CLR7.5 software that fixes this problem. The issue is also fixed in the CSB/CLR9 software under development.

Solution Manually prevent HF datalink use
Prevent HF datalink use manually by implementing flight crew procedures to disable HF datalink (usually by placing the HF radio used for HF datalink into voice mode).

5.1
The DLMA has observed that some performance problems are caused by the challenges of effective CSP internetworking when an aircraft operator chooses to use one of the two global CSPs (ARINC or SITA) for VHF and the other global CSP for SATCOM.
(The DLMA also realizes that some aircraft operators configure their avionics to first prefer regional DSPs, such as Avicom in Japan, which has not been shown to affect performance).

Solution: For aircraft operators that do not configure their avionics to first prefer a regional CSP, use the same global CSP for both VHF and SATCOM
It is likely that the data link performance will be improved if the same global CSP is used for both VHF and SATCOM.

Large Pilot Operational Response Time (PORT) values
6.1 PORT is one component of the Actual Communications Performance (ACP), the other being the Actual Communications Technical Performance (ACTP). For an uplinkdownlink CPDLC transaction, PORT captures the human portion of the transaction time and ACTP captures the technical (mainly network) portion of the transaction time. Accordingly, large PORT values reduce performance.

Solution:
Implement flight crew procedures to respond to CPDLC messages with STANDBY when appropriate. Promptly submit service requests to Boeing for SATCOM problems and provide any requested information (such as SDU logs).

Unknown causes
8.1 If a data link performance problem has an unknown cause, then the DLMA recommends submitting a problem report at http://www.fans-cra.com/ so that the DLMA and other involved stakeholders can attempt to determine the cause.

Maximising access to the Classic Aero Ground Earth Station (GES) services:
9.1 In the Inmarsat SATCOM system, there are a multitude of transmission paths available via the different ground stations and satellites. If one path fails, the aircraft may be able to switch to an alternate path provided the Operator Requirement Table (

PROBLEM / ISSUE SOLUTIONS / ACTIONS
Some of those fixes may improve data link performance and most of them fix issues that cause problems for pilots and air traffic controllers in the use of data link.
10.2 To ensure the best possible functioning of the NAT air traffic control system it is of utmost importance that aircraft operators take care to always operate the latest available FANS 1/A related software version in aircraft that fly in the NAT high level airspace and to ensure that the aircraft systems are configured in an optimal manner. A list of recommended aircraft avionics software versions is provided in the table below.
10.3 It should be noted that new software versions that fix several known data link problems will become available for many aircraft types within the next year. Operators are advised to seek information from aircraft manufacturers about the status of those new software releases.