CPDLC
CPDLC stands for Controller–pilot data link communications and was initially conceived to overcome the limitations of using HF Radio for long distance communication over Oceanic areas - principally the North Atlantic. This first version of CPDLC was called FANS 1/A, or just FANS. More recently, a second version of CPDLC called ATN has been slowly introduced into the European Air Traffic Network, in this case to alleviate frequency congestion. Full implemetation within Europe is targeted for 2018. ATN is also expected to appear in the United States at some point, though no dates have yet been released.
CPDLC FANS 1/A
FANS 1 was developed by Boeing and later adopted by Airbus as FANS A, and hence it is now referred to as FANS 1/A. It allows two-way, text based messaging between a controller and a pilot when an aircraft is out of range of VHF voice-radio communications, or where HF radio is unsuitable or un-useable. Even today, a voice communication over HF can take quite some time, and is often not possible at all due to atmospheric conditions. FANS also allows a periodic broadcast of aircraft position, altitude and speed which is particularly useful where there is no radar coverage. This component is known as Automatic Dependent Surveillance-Contract (ADS-C). Across the Atlantic, it is fair to say that the majority of aircraft are now equipped with FANS, alleviating the tedium of broadcasting a position report on HF every 40 minutes or so.
On the aircraft, FANS uses ACARS (Aircraft Communications Addressing and Reporting System) as the medium for data transmission and reception of messages. ACARS itself uses the most appropriate (generally the most economical) of VHF, HF or SATCOM, depending upon what's available. CPDLC has a predefined set of text messages for clearances, requests and routine message traffic, as well as a free text form for those more unusual requests.
Air Traffic Networks across all five continents have an implementation of FANS, covering the Atlantic, Pacific, Indian Oceans and parts of Africa. Some of the Ocean facing European Air Traffic Networks such as London and Shannon also have FANS.
As an aircraft transits from one FANS air traffic sector to the next, the CPDLC FANS system will automatically transfer the aircraft to that next sector. This requires no input from the pilot, but we think that it does require that the Air Traffic Controller from the exiting sector, sends a message to the aircraft containing the next sector's address. Unlike all other ATC initiated messages, there is no notification that this "next address" message has been received. If the next sector field of the ATC Logon Status page is blank, then the transfer fails. As with many of the more subtle parts of CPDLC functionality, this is undocumented, but based on observed behaviour.
CPDLC ATN
ATN is a data link service that allows text-based communication between an ATC and the flight crew but doesn’t include a surveillance component. This type of CPDLC is intended for use in areas where ground surveillance already exists. i.e. No position reports are broadcast. To date, ATN has only been implemented in some Northern, Western and Central parts of Europe, but is expected to be Europe-wide by 2018. Four stations, namely London, Scottish, Shannon and Maastricht have implemented both ATN and FANS. Airbus refer to ATN as FANS B+, but the term is not widely used, and will hopefully be dropped in due course.
On the aircraft, ATN uses a dedicated VHF transceiver, known as VDL Mode 2, as its means of transmission of messages. As with all VHF transmitters, the range is limited to around 200nm so is only suitable over or near to land. Given that it will only be used in areas of dense traffic which are already under radar control, this limitation will not restrict its use.
As an aircraft transits from one ATN air traffic sector to the next, the CPDLC ATN system will, like FANS, automatically transfer the aircraft to that next sector. Again, this requires no input from the pilot, but does require an (un-notified) input from the Air Traffic Controller from the sector that is about to be exited (see FANS-FANS transfer). If the aircraft transits from an ATN sector to a FANS sector, or vice-versa, no automatic transfer of logon occurs. This is because the two systems, FANS and ATN use completely different equipment. There are rumours that this type of transfer will be implemented in the future, but it would require either extensive software updates to CPDLC equipment on the aircraft, or ATC initiated logons which currently does not take place.
Usage on the aircraft
This information relates to the 787 dreamliner, which was designed from the outset to have CPDLC FANS capability, with the addition of CPDLC ATN appearing in 2014. As far as I know, all 787s built from 2014 onwards will have both FANS and ATN CPDLC. For an airline such as British Airways, it means that registrations G-ZBJA to G-ZBJD only have FANS capability, with all subsequent registrations having both FANS and ATN, namely G-ZBJE to G-ZBJL and G-ZBKA to G-ZBKZ and beyond.
For aircraft with only CPDLC FANS (in the BA example G-ZBJA to G-ZBJD) there is little point logging on to European stations, as with the exception of London, Scottish, Shannon and Maastricht it won't work. Areas outside of Europe which have CPDLC FANS work well, and my own experiences of transiting the Atlantic Ocean and Canada, shows a system that seamlessly transfers between FANS stations. Note that these FANS only aircraft will quite happily attempt a FANS logon with an ATN sector, and will show the message "Network Ready" which gives the illusion of potential success. Alas it is doomed to failure, and will generate the only error message possible - the rather unhelpful "Re-logon to ATC Comm" after 10 minutes of nothingness.
The message "Network Ready" is displayed on the Logon page to indicate that the aircraft CPDLC system is ready, and implies no serviceability on the part of the Air Traffic station. Additionally, for ATN CPDLC to show "Network Ready", the aircraft also needs to be located within Europe. For example, when transiting from the Istanbul FIR to the Sofia FIR, with the Logon page displayed and an ATN CPDLC station selected, the status will change from "Network not Ready" to "Network Ready" as the aircraft enters the Sofia FIR.
For aircraft with both FANS and ATN, the system is more capable, but has more potential for errors.
Each aircraft contains a single lookup of CPDLC stations which the aircraft treats as being both ATN and FANS capable. Any station not on the list (i.e. any combination of four letters in the range A-Z) is assumed to be FANS only. Again this undocumented, but is best guess based on aircraft behaviour.
The initial CPDLC logon screen on the aircraft will attempt a  FANS logon with stations not on the aforementioned list. For those stations that are on the list, the pilot is presented with an option to select FANS if desired, otherwise an ATN logon will be attempted. The choice depends upon the subsequent sector types so that automatic transfers are utilised. For example, if one is starting the flight in either London, Shannon or Maastricht zones, and flying West or North the FANS is the better option. If travelling South or East then choose ATN.
The implementation of CPDLC on the 787 is not reliable within Europe. This is due to several factors, which include incorrectly coded station lists, unhelpful error messaging and poor documentation. Many of the failures to logon are as a consequence of the aircraft attempting the wrong type of logon due to incorrectly coded lists. This, coupled with no error reporting and poor documentation, means the pilot remains in the dark as to why the logon fails. As CPDLC becomes more mainstream over the next few years, this is sure to improve.
The current issues and failures with the 787s logons are as follows:
1) All the ATN capable 787s are coded to assume that all the European ATN stations can do both ATN and FANS but this is only true of EGTT, EISN, EGPX and EDYY. i.e. the aircraft will allow an attempt of a FANS logon to a non-FANS station. A FANS logon to an ATN station will time out (see #4 below).
2) An address mismatch between aircraft and flight plan - we were seeing this frequently on most of the 787-9s and this was caused by an error in the flight plan, which I believe has now been fixed. If you search for "CODE" in the flight plan you will find a six-letter alphanumeric code. This should match the aircraft code which you can find on the SYS => MAINT => Central Maint => Aircraft configuration page. If they don't match then the login fails. This error produces the message "Re-logon to ATC Comm" pretty much straight away.
3) If the ATC centre is not expecting you to logon, they will reject the logon, which results in an almost immediate "Re-logon to ATC Comm" message. This may happen as a result of a direct routing which takes the aircraft outside, or almost outside, of an ATC area that you were originally flight planned through. It can also happen when an ATC unit does not have complete coverage of their FIR. An example of this is the western part of Austria, where LOVV will reject the logon attempt.
4) Message times out. "Re-logon to ATC Comm" will appear 10 minutes after a logon attempt, if no response, successful or otherwise, has been received.
5) Flights transiting from a FANS zone to an ATN zone will have to re-logon manually.
6) If the logon status page shows "Network not ready" when everything else looks correct, it is possible to do both a soft and hard reset of the system. Both resets are achieved by initially selecting Comm => Manager => Master. To do a soft reset select "Data Link System Reset". For a hard reset, which will delete all existing comm messages, select "Restart Data Link". It can sometimes take a few hard resets to bring a reluctant ATN CPDLC back to life.
7) Another flight plan issue which is resulting in an immediate rejection of a logon attempt, is occuring in the newly implemented (March 2017) Prague FIR. Prague requires that equipment item "J1" is entered in the flight plan, which signifies that CPDLC VDL Mode 2 equipment is carried. The 787 responds to the rejection in the only way it can, ie "Re-logon to ATC Comm".
8) Another potential reason for a logon rejection is the so called CPDLC White List. This is a list of aircraft registrations that presumably indicates compliant CPDLC VDL2 equipment. I suspect this is being superceded by the "J1" flight plan equipment code. However, two countries, namely France and Switzerland, theoretically require it.
9) We are now seeing some CPDLC logon pages in Europe that do not show a FANS box. This should still cause the aircraft to attempt an ATN logon. However, given that the implementation on the 787 has hitherto allowed both types of logons (FANS and ATN) to all the European stations, it is possible that this single logon type is actually FANS. If this is the case, then a logon attempt would time out. The message from the aircraft, as always is "Re-logon to ATC Comm"!
