Controller Pilot Data Link Communications (CPDLC)

What is CPDLC?

CPDLC is a data link application that allows for the direct exchange of text-based messages between a controller and a pilot.

CPDLC greatly improves communication capabilities in oceanic areas, especially in situations where controllers and pilots have previously had to rely on a Third Party HF communications relay.

The following link provides a high-level overview of CPDLC. Note - when the presentation loads, click on the word "More" to the right of the start button to select full-screen mode.

http://prezi.com/7erqio0gm3x7/cpdlc-overview/?auth_key=1870e4f4a458fca2c71b97729a7aee28254bf9dc

Apart from the direct link, CPDLC adds a number of other benefits to the ATS system, such as:

  • Allowing the flight crew to print messages, if an onboard printer is available.
  • Allowing the flight crew to load specific uplink messages into the Flight Management System (FMS), reducing input errors.
  • Allowing the crew to downlink a complex route clearance request, which the controller can automatically include in the uplink clearance, if approved, without having to type a long string of coordinates.
  • Specific uplink messages arm the FMS to automatically downlink a report when an event, such as crossing a waypoint, occurs. This automation assists with workload management for the flight crew and the controller.
  • Specific downlink messages, and the response to some uplink messages will automatically update the  Flight Data Record in some ground systems.


CPDLC Versus ADS-C

CPDLC and ADS-C (Automatic Dependent Surveillance Contract) have a number of similarities.

  • CPDLC and ADS-C are both data link applications.
  • In the airborne implementation they are both controlled by the FMS.
  • While they are separate entities, and it is possible to have one without the other (the pilot can select ADS on and off independently), the reality is that an FMS unserviceability is likely to lead to the loss of both applications.
  • Both applications accept messages from the ground system and return responses.
  • Both applications are capable of initiating downlink reports based on messages from the ground system.
  • It takes only one logon from the aircraft to allow a ground system to connect with both applications.

However, for all of the similarities, there are just as many differences. The connections established between an aircraft and a ground system differ for each of the applications.

CPDLC is a communications application and uses the concept of Data Authority. There can only be two Data Authorities, and therefore a maximum of two ATS Units (ATSUs) connected to the aircraft for CPDLC at any one time. Only one of the Data Authorities, the Current Data Authority (CDA) can communicate with the aircraft.

ADS-C, on the other hand, is a surveillance application and does not inhabit the Data Authority world. The maximum number of ATSUs that can have ADS-C connections simultaneously with the same aircraft is five for all aircraft models other than the B744. The B744 can only have a maximum of four simultaneous ADS-C connections.

The CPDLC connection has in-built integrity with a number of security mechanisms.

The FANS 1/A ADS-C connection has no such security. If a suitably equipped ground station, perhaps a research facility, knows the ACARS address and tail number of a particular aircraft, an ADS-C connection can be established from anywhere on the globe.

In the B747-400 implementation, the flight crew is only aware that the ADS-C application is active. In the B777 and later implementations, the flight crew is aware of the number of ADS-C connections established and the ICAO identification code of each ADS-C connected ATSU.

Another difference between the two applications is human interaction. Although the connections are established at a system level, the CPDLC application is a hands-on crew tool. The crew is aware of the ATSU with the active connection; and they actively use the functionality to send response messages, request messages and reports. They are also aware of when a transfer from one ATSU to another has occurred.

The flight crew can turn the ADS-C application on and off, and only the crew can initiate and cancel an emergency reporting mode, but other than that, the ADS-C application generally operates without flight crew interaction. When a logon occurs, the ground system automatically establishes a series of contracts directly with the avionics. The contracts are generally defined off-line, but in some ground systems, the controllers are able to construct their own contract contents.

The contracts define the type of reporting required, the timing of these reports, the information contained in the reports and the frequency of reporting.

The avionics will automatically send a report to the ground system whenever a contract-defined event occurs.

Unlike CPDLC, ADS-C also operates with minimal input from the controller.


The Data Authority

As previously discussed, an aircraft can have a maximum of two CPDLC connections. Only one of these connections can be active at any one time. The ATSU able to exchange CPDLC messages with the aircraft is known as the Current Data Authority (CDA). The connection between the aircraft and the Current Data Authority is known as the active connection.

The next (adjacent) ATSU to communicate with the aircraft by CPDLC (usually the next FIR on the cleared route) is known as the Next Data Authority (NDA) and is nominated by the CDA by the sending of an uplink NDA message to the aircraft containing the four-character logon code for the next ATSU that will send a Connection Request message. The connection established between the aircraft and the Next Data Authority following the Address Forwarding process is known as an in-active connection.

If the NDA message is not received by the aircraft, a Next Data Authority does not exist and termination of the connection with the Current Data Authority will leave the aircraft without CPDLC connectivity.

Similarly, if the next ATSU is not datalink equipped, an NDA message is not sent to the aircraft. Prior to the FIR boundary, the crew is instructed to establish voice contact with the next unit.

In this case, the End Service message sent by the Current Data Authority will terminate the active connection, and the aircraft will not be CPDLC connected with any ATSU until an initial logon is performed prior to the boundary of the next datalink equipped unit. The initial logon to the next unit is a pilot responsibility.

It is possible to use the Address Forwarding process to instruct the avionics to logon to a data link ATSU beyond a non-data link area, but this would lead to a unit having CPDLC communications with an aircraft operating in another FIR. The South Pacific States have agreed not to perform this process under normal conditions, but it may be a useful option in an emergency situation.

When the pilot performs an initial logon to an ATSU, that unit automatically becomes the Current Data Authority because it is the only unit connected. The CPDLC connection is active, however, in the FANS 1/A environment, the ground system can not determine that it is the Current Data Authority until a downlink message is received from the aircraft.


Connection Timings

For an aircraft departing from an airport within an ATSU's FIR the CPDLC connection sequence will generally occur on receipt of a logon from the aircraft. This will be an initial logon and the aircraft will either logon to the unit while parked at the gate, or at some point after departure (for example, when climbing through 10,000 ft).

For an aircraft inbound to an FIR the connection sequence to that FIR is managed by the design of the individual system. Some systems will connect ADS and CPDLC on receipt of the logon, others connect the applications at different times based on automated phases of the flight plan.

An initial logon (when transiting a non-datalink FIR prior to the boundary) will generally occur somewhere between 15 and 45 minutes prior to the aircraft's boundary estimate.


CPDLC and Jurisdiction

Some CPDLC ATSUs consist of only one sector. A logon and a subsequent active CPDLC connection with that ATSU allows the sector to exchange CPDLC messages with the relevant aircraft.

In a multi-sector environment, where multiple sectors use datalink, the centre itself generally takes the role of Current Data Authority and an individual sector is allocated the ability to communicate with aircraft under the sector's jurisdiction. In the multi-sector environment the connection (and any open messages) is transferred with the transfer of jurisdiction. An End Service message is not sent for inter-sector transfers.


The FMS

Below is a diagram of part of the B747-400 FMS HMI. The page currently being displayed is called the ATC Logon/Status page and is accessed by selecting the ATC button below the screen if no connections currently exist.

Below the function buttons is an alphanumeric keypad. To perform an initial logon manually the pilot types the four-letter ICAO designator for the ATSU. The flight number entered must be exactly the same as that submitted in the flight plan. This text will appear on a line below INDEX. The pilot then selects the LOGON TO button to move the designator to where the four boxes are displayed.

The pilot then enters the flight number and selects the FLT NO button. This action moves the flight number into the appropriate position and then activates a SEND button at the top right of the display. On selection, the pilot will see LOGON SENDING then LOGON SENT and finally LOGON ACCEPTED.

B747-400 FANS Display

The CPDLC Message Sets

The first CPDLC message set was created in 1993 by a non-profit industry body known as RTCA Incorporated and published in a manual designated as the DO-219. This manual has since been updated and combined with the ADS-C specification in the RTCA/EUROCAE DO-258A/ED100A.

The intention was to capture the most common communications phraseologies used in the oceanic voice environment and to create a list of "pre-formatted" CPDLC text elements sorted into a number of categories. To cover unusual situations a free text capability was added to allow controllers and pilots to create their own specific messages.

Boeing added one extra downlink message element to the basic message set for the FANS-1 CPDLC package. As discussed elsewhere, the ICAO Operational Data Link Panel (OPLINK) added somewhere in the order of 100 extra messages to the RTCA/EUROCAE message set for use over the ATN. Since then, the LINK 2000+ implementation in Europe has used a subset of the overall message set, and the SC214/WG78 Working Group has taken the ATN-based message set to around 360 uplink messages, including D-Taxi messages, in comparison with the 183 uplinks available to the FANS-1/A message set. To avoid confusion any description on this site will be refer to the FANS 1/A message set.


Messages and Elements

Individual entries in the message set are known as message elements. A CPDLC message can be created from a single message element, or a concatenation of up to five message elements.

Messages may be constructed solely from pre-formatted message elements, solely from free text message elements, or from a combination of pre-formatted and free text message elements.

Flight crews and controllers both have the capability of sending multi-element messages, however, multi-element clearances or clearance requests should be avoided. A flight crew or controller receiving a message containing multiple clearances or clearance requests for which not all can be accepted is required to send an Unable response to the entire message and then as period of negotiation on which elements are acceptable will ensue. Single-element clearances and requests do not have this issue.


CPDLC Dialogues

CPDLC dialogues are exchanges of CPDLC messages between a controller and a flight crew.

There are some simple rules governing CPDLC dialogues:

  • All CPDLC dialogues must be closed wherever possible.
  • A dialogue opened by CPDLC must be closed by CPDLC, unless connectivity has been lost.
  • A dialogue opened by voice must be closed by voice, not CPDLC.

The correct operation and synchronisation of the CPDLC system is dependent on the receipt of a valid response for any message that technically requires closure.

If a message requiring a closure response is sent and then subsequently negotiated by voice, that message must still be closed.

The automatic transfer of a CPDLC connection across an FIR boundary depends on certain messages being closed prior to the avionics receiving an END SERVICE message from the Current Data Authority. If a message exchange is not completed prior to the End Service message being received by the aircraft, then any open messages with the transferring FIR are deleted by the avionics.

Text and Variables

Pre-formatted message elements can be purely text-based, such as "DUE TO AIRSPACE RESTRICTION", or can be a combination of text and variables, such as "CLIMB TO AND MAINTAIN [level]".

The text of the message element is shown in upper case characters and the variable field is shown with the variable name displayed in lower case characters enclosed in square brackets. In most modern ground systems a variable field is generally displayed to a controller as a button that will display a pop-up menu from which the intended value can be selected.

A free text element contains only text. 


Element Numbers

Each pre-formatted message element is assigned a unique identification number.

For example, the element "CLIMB TO AND MAINTAIN [level]" is always uplink element number 20.

Depending on the urgency attribute, an uplink free text element is either number 169 or 170, without regard to the content.

An uplink message consisting of 5 different free text elements will be constructed of 5 message elements assigned number 169, whereas a message consisting of 5 pre-formatted elements will be made up of five elements each assigned their own unique identification number.

Neither the controller nor the flight crew will see these numbers as part of the message.


Message Attributes

Each message element is also assigned a number of attributes within the code. The attributes include urgency, alert and response requirements. The Response Attribute determines the type of response required to close each message dialogue.

For Uplink messages:

  • The attribute W/U means that a WILCO or UNABLE response is required to close the dialogue.

  • The A/N attribute requires either the AFFIRM or NEGATIVE response.

  • The R attribute requires a ROGER response.

  • The NE attribute means that the W/U, A/N, or R responses are Not Enabled for the flight crew. These messages are considered by the ground system as a self-closing as they do not require a downlink to close the dialogue.

 Note: Despite the NE attribute not requiring a technical closure response, most of these messages, such as CONFIRM ASSIGNED ALTITUDE, do require an operational response (e.g. ASSIGNED ALTITUDE [level]). The responses are correctly displayed to the crew providing that the correct uplink is sent - for example, if the same message is sent as free text, the correct response is not available to the crew.

For downlink messages the choices are a little more basic:

  • The Y attribute requires a response from the controller.

  • The N attribute does not require a response.

As mentioned earlier, system processing of the various message attributes means that the message is presented to the controller in a recognisable form. The attributes are not sent within the message, but are translated by the receiving system based on the message element identification numbers.

The controller and flight crews do not see the attributes.

For a multi-element message, the overall response attribute is the highest-level attribute of the indivdual elements.


Message Intent

The message elements of the original RTCA DO-219 message set were published without the intent being defined.

The intent of each element was defined later during the work of the OPLINK Panel and adopted by the Informal South Pacific ATS Coordinating Group (ISPACG) for CPDLC operations within the South Pacific.

Obviously, unless a free text message is carefully created, only the author may be fully aware of the intent of the message, especially when the author and the recipient may have languages other than English as their primary languages.


MINs and MRNs

To re-cap, individual element numbers are assigned to each uplink and downlink message element. In the FANS-1/A system each uplink message element is assigned an individual number between 0 and 182 inclusive, and downlink elements are assigned numbers between 0 and 80 inclusive. Message elements always retain the same element number.

It is possible for one message to be constructed from a total of five different message elements and, therefore, five individual element numbers.

Each full message, whether consisting of one element or five, is assigned a Message Identification Number (MIN) by the originating system.

The MIN is a number between 0 and 63 and its purpose is to allow a CPDLC dialogue to be tracked so that uplink and downlink messages belonging to the same dialogue are correctly paired and closed off.

The networks, over which the messages travel, can sometimes lose sychronisation, which means that a response to an earlier message can arrive later than the response to a subsequent message. It is important that controllers and flight crew are aware of which response relates to which message.

For any system receiving a message with an attribute requiring a response, the MIN of that message becomes the Message Reference Number (MRN) for the purposes of the response.

As an example, an aircraft sends the following downlink message to a ground system:

REQUEST CLIMB TO FL310.

This message is assigned a MIN by the avionics (say 17).

A downlink clearance request contains an attribute requiring a response. The controller returns an uplink clearance to the aircraft: CLIMB TO AND MAINTAIN FL310.

The uplink is assigned its own MIN by the ground system (say 23). The MINs assigned by different systems are unrelated.

The MIN of the downlink (17) becomes a MRN for the response. As a result, the avionics will know which message to pair the response with, i.e., which message is being referenced by the response.

The downlink request and the uplink response are closed in the avionics and the ground system, but because the uplink clearance also contains a response attribute, the dialogue is still open.

The response attribute of a clearance message requires either a WILCO or an UNABLE response from the flight crew. The MIN of the uplink clearance (23) now becomes an MRN and is returned with the WILCO. Due to the MRN, the ground system will know which message the WILCO is referencing.

The WILCO downlink does not require a response, so this dialogue is now closed.

Controllers may be conducting dialogues with a number of aircraft simultaneously.


Arming the FMS

There are three uplink messages that arm the avionics to perform functions automatically. These messages are:

REPORT REACHING [level],

REPORT PASSING [position],

and REPORT LEVEL [level].

When one of these messages is received by the avionics, the flight crew of Boeing aircraft is presented with an ARM prompt on the UPLINK and VERIFY REPORT pages of the FMS. Selecting the ARM prompt on either page will arm the report for transmission.

When the specified level is reached, the specified position is passed, or the specified level is being maintained, the avionics will automatically send the appropriate downlink report message (e.g. REACHING FL310).

The REPORT LEVEL [level] element should be used when asking for a report that the aircraft is maintaining the assigned level.


Loadable Messages

The flight crew can load specific uplink message elements into the FMS.

The crew is presented with a LOAD prompt when a message containing one or more of these elements is received, provided that there is not a pending modification in the FMS flight plan.

Loading one of these elements will either modify or replace details in the active FMS flight plan.

This functionality allows flight crew to load long or complex route clearances received from an ATSU directly into the FMS without having to manually enter waypoints, reducing the possibility of "blunder" errors.


Updating the FDR

A number of uplink messages will also automatically update the Flight Data Records held in some systems on receipt of a WILCO response from the crew.

The variables updated by these specific message elements include the Cleared Flight Level (CFL), speed, and some components of the route clearance.


Emergency Mode

The flight crew of Boeing aircraft can set the datalink emergency mode by two methods. The sending of a CPDLC MAYDAY message automatically places the CPDLC and the ADS-C functionality into Emergency Mode. The crew can also select ADS-C Emergency Mode independently of CPDLC.

Emergency Mode can only be initiated by the crew and can only be cancelled by the crew.

The Boeing FANS-1 package automatically adds some extra message elements to the MAYDAY message, such as an abbreviated position report containing current position, time at current position, altitude and current speed. If the pilot selected level is lower than the current altitude when the MAYDAY message is sent, then the DESCENDING TO [level] element is also added.


Pre-formatted Versus Free Text

There are some important differences between pre-formatted and free text message elements.

As we have seen in the previous section, the intent of a free text message may not be properly understood.

Other differences are:

  • Some pre-formatted uplink elements arm the avionics to automatically send a downlink report when a specific event occurs (eg. passing a waypoint).

  • Some pre-formatted uplink elements can be auto-loaded into the FMS (eg. route clearances).

  • Some pre-formatted uplink elements automatically update the Flight Data Record (FDR) of some systems on receipt of a WILCO response from the crew (eg. vertical clearances).

Free text messages do not perform any of these functions.

Another major difference between pre-formatted message elements and free text elements is in the delivery of the message.

For a message constructed of pre-formatted elements, only the message number and any variable contained in the element are transmitted. The message number is then decoded by a file of message elements stored in the receiving system.

In contrast, the entire free text message is sent to the receiving system.

As only the number and the variable(s) are transmitted, there is some scope for the tailoring of the pre-formatted message elements to suit the user’s environment. An extreme example of tailoring would be the presentation of the elements in the user’s native language. The danger of this approach is that the intent of the message may be compromised in the translation.

The important point to recognise here is that message text seen by the controller does not alter in any way the text seen by the flight crew, as the message text is decoded from the airborne file.

The procedures for the sending of uplink messages state that free text shall only be used when an appropriate pre-formatted message element does not exist, or as an additional amplification of a pre-formatted element.

 

Decoding a CPDLC Position Report

Ground systems may display a CPDLC position report to controllers in a particular format, but the fields in the message remain the same. There is also a difference in the number of fields between Boeing and Airbus aircraft and optional fields for turbulence and icing, but these are not often used. The following graphic is the format used by at least one ground system manufacturer for a Boeing position report without icing or turbulence included. An Airbus report would also contain fields for Groundspeed, Vertical Direction, Vertical Rate, True Track and True Heading.

 

CPDLC Integrity

A clearance delivered by CPDLC requires no specific readback by the crew.

This is due to the last four characters tacked onto the end of the coded message. This code is a check sum of the message header, the message contents and the sending time-stamp expressed as bits.

In simple terms the code is a sample of the entire message. This code is known as a Cyclic Redundancy Check (CRC) and each CRC is analysed by the receiving system. If the CRC is intact, then the message and its contents have not been corrupted during transmission.

The CRC check ensures that CPDLC has one of the highest integrity ratings of any aviation system, especially when compared with voice communications.

The only "readback" required in response to a clearance delivered via CPDLC is a WILCO message from the crew.

The current status indicates that the logon message has been accepted by YMMM and YMMM is the Active Centre. The Active Centre is displayed after the CPDLC connection has been established.

When Address Forwarding occurs, the Next Data Authority will be displayed as the NEXT CTR.

ADS-C is automatically armed when the avionics system is turned on. For ADS-C not to be armed at logon, the pilot would have to manually select ADS-C off.

The ADS-C EMERG Mode activation is a toggle switch. The prompt changes from SELECT ON to SELECT OFF. There are no other indications that the ADS-C Emergency Mode has been selected ON if the button has been selected inadvertently by the crew.

Selecting ATC COMM to OFF will disconnect the aircraft from CPDLC.


What the Pilot Sees - The B744 CPDLC FMS Display

ATC Logon / Status Page

 

The ACT CTR (Active Centre) in the graphic is KZAK, meaning that the aircraft has an active CPDLC connection with KZAK Centre. The Active Centre is displayed after the CPDLC connection has been established.

When Address Forwarding occurs, the Next Data Authority will be displayed as the NEXT CTR. In the diagram above a logon has been accepted by the next centre (NTTT) prior to a transfer across an FIR boundary.

ADS-C is automatically armed when the avionics system is turned on. For ADS-C not to be armed at logon, the pilot would have to manually select ADS-C off.

The ADS-C EMERG Mode activation is a toggle switch. The prompt changes from SELECT ON to SELECT OFF. There are no other indications that the ADS-C Emergency Mode has been selected ON if the button has been selected inadvertently by the crew.

Selecting ATC COMM to OFF will disconnect the aircraft from CPDLC.


Uplink Messages

An uplink level clearance has been received (at 1926z), but not yet actioned - hence the OPEN status.

The pilot has three response options - STANDBY, REJECT or ACCEPT.

If the REJECT prompt is selected the FMS will display the correct response message based on the coded response attribute carried by the uplink message (UNABLE). The ACCEPT prompt will display the correct response based on the response attribute (WILCO).

 

 


 

This view is after the pilot has accepted the uplink message and sent the response.

By selecting the ACCEPT prompt the FMS displays the correct response based on the coded response attribute carried by the uplink message. The correct response to this uplink message is WILCO.

 

Downlink Altitude Request

 

The ATC Request page provides the pilot with access to a number of downlink request message categories.

For a level-change request, an option to reach the requested level via a cruise climb is available.

The pilot can also choose to append one of a number of elements to amplify the request, such as DUE TO PERFORMANCE, DUE TO WEATHER, or AT PILOTS DISCRETION.

Once the message has been constructed, it must be viewed on the VERIFY page prior to sending.

 

The constructed message is viewed on the VERIFY page prior to sending.

The verify page also provides the pilot with the option of adding a free text element to the message.

Emergency Messages

The emergency page provides access to a number of message selections. The DIVERT TO message defaults to the destination, but can be changed to an alternative landing point.

If selected, the downlink element DIVERTING TO [position] VIA [route clearance] is added to the emergency message. If entered by the pilot, ie. if the destination is other than flight planned, then the [route clearance] variable is likely to be DCT (direct) until otherwise advised.

If the pilot's altitude bug is set to a level that is below a tolerance from the current level (150 feet), the downlink element DESCENDING TO [level] is automatically added to the emergency message.

SOB relates to the fuel remaining and the Souls On Board the aircraft. Both values are "known" by the FMS at any one time. The downlink element [remaining fuel] OF FUEL REMAINING AND [remaining souls] ON BOARD is automatically added to the emergency message.

Downlink messages require that the pilot verifies the report on the VERIFY page prior to sending.

The verify page also provides the pilot with access to free text elements. In the case of an emergency message, the free text element automatically has an "urgency" delivery attribute. This is the only time that the pilot has access to the urgency free text attribute.

 

Display Issues

There are a number of display issues across the various FANS 1/A equipped aircraft. As the system has evolved and new models have utilised larger displays in the cockpit, there are a number of differences between how information is displayed to the flight crew between "large-display" and "small-display" cockpits. Models such as the B777, B787 and B748 have the large displays, while models such as the B737, B747-400, and B757/767 have the small displays. Of particular interest is the display of pre-departure clearances and route clearances.

In the small-display models a single-element route clearance message (UL#80) displays only the words CLEARED ROUTE CLEARANCE. To see the actual components the flight crew either has to print the message, or to load the message into the FMC. There have been occasions where flight crew have believed that this message meant "cleared as per flight planned" and have taken no action as intended by ATC. Printing the clearance can be problematic for two reasons: a) the carriage of a printer is not mandatory, and b) the printer has a lower level of certification and there is no guarantee that a printed message will be exactly the same as the clearance displayed. The large-display models display the full contents of the clearance message, generally over multiple pages.

While route clearances are an issue if the crew has not been properly trained, there is usually no issue as the components of the clearance generally load correctly. However, there has been an issue identified during some testing by the RTCA/EUROCAE SC214/WG78 working group related to the display of the predeparture clearance.

The predeparture clearance (UL#73) (PDC) contains additional information fields that are not part of the actual route clearance, such as the SSR code, the airborne frequency, and an altitude restriction. In the small-display models a single-element predeparture uplink displays simply displays as PREDEPARTURE CLEARANCE. However, there is now a difference at this point with the other route clearance elements for the Boeing small-display models. To see the fields contained in the PDC message the flight crew must print the message, but once again, the carriage of a printer is not mandatory. The other option is to load the clearance into the FMC, but as the additional fields (SSR, Frequency, and Altitude Restriction) are not clearance fields, on loading, these fields are erased and discarded and not displayed to the flight crew. The Boeing large-display models display the full contents of the clearance (including the extra fields) across multiple pages (see the graphic below). However, Airbus models do not load the PDC message.

One mitigation proposed is to extract the additional fields in the ground system and add them as additional message elements. In this case, the flight crew in the small-display models will see the PDC message and the additional information displayed on receipt of the message, but flight crew in large-display models will see the information twice, once as fields within the PDC message and again as additional elements. The only way to really resolve this issue is for the ground system to have some additional intelligence and to know when to add the additional information (small-display) models and when to leave it out. But with FANS equippage spreading to othe airframe manufacturers, such as Gulfstream, it becomes a complex task to track all capable models and their individual idiosyncracies, when they are all effectively complying with the same standards. It is an onerous task and one that the ground system manuafacturers should not have to carry.

The graphics below show a single PDC element message on a small display, a PDC element plus two additional elements on a small display, and a single PDC element across two pages of a large display.

                                                                                           Small display - single PDC element

 

                                                                        Small display - PDC element and two additional elements

 

                                                                              Large display - single PDC element - page 1 of 2 

 

                                                                               Large display - single PDC element - page 2 of 2

 
ATC Data Link News Copyright © 1999, Craig. J. Roberts - Page last modified: November 10, 2016