FANS Datalink Update

Figure 1. MCDU ATC LOGON/STATUS Page

The second type of DCL is ACARS-DCL, which uses the ARINC 623 functionality of the FMS. ARINC 623 is an ACARS messaging standard that allows direct datalink communication with ATC for specific air traffic service (ATS) functions. This type of clearance is like an ACARS PDC in the U.S. An ACARS-DCL is obtained by using the ATS pages on the MCDU (Figure 2) and should not be confused with CPDLC-DCL. This type of DCL availability is depicted in the ACARS box on the airport diagram and is only used at airports outside the U.S.

Figure 2. MCDU ATS Pages

Prior To Logging On to KUSA…

CPDLC-DCL requires the operator to have a flight plan on file with the relevant equipage codes in field 10a of their ICAO flight plan indicating FANS capability. To specify a preference to receive a DCL, the appropriate delivery preference code is filed in the DAT/ sub-field of block 18. If the relevant DAT/ codes are not present, a logon to KUSA will not be established. This issue is still one of the most common reported for unsuccessful logons. 

Also, it’s very important to have the filed flight plan loaded into the FMS active flight plan prior to logging on to KUSA, as shown in Figure 3.

Figure 3. Filed Flight Plan Loaded Into the FMS Active Flight Plan

Logon Status

There has been some confusion among operators regarding whether or not they are actually logged on to KUSA. The timing of the logon status message differs slightly in the U.S. compared to Oceanic Regions providing enroute FANS CPDLC.

In Oceanic areas, the ATC LOGON/STATUS page will display LOGON ACCEPTED momentarily, and then within seconds display ATC COMM ESTABLISHED in the scratchpad. Next, the active Center address displays under the ACT CTR field.

When performing a FANS logon with a Tower in the U.S., LOGON ACCEPTED will display and remain, as shown in the first screen on figure 4. The CPDLC connection occurs only after the tower controller approves the CPDLC clearance, which happens at some point within 30 minutes prior to filed ETD. Once the controller transmits (pushes) the clearance to the aircraft, the LOGON ACCEPTED display will disappear and KUSA displays in the ACT CTR field, as shown in the second screen on figure 4.

Operators may think the FANS logon failed or the ATC LOGON/STATUS screen has “locked up” with only LOGON ACCEPTED displayed before a connection is established. As a result, multiple logon attempts are being made by crews while their aircraft is actually logged on. This causes an automatic disconnect by ATC and no other logon attempts will be accepted. The crew then has to revert to voice communication to obtain a clearance.

Figure 4. Logon/Status

Recommendation

If within 30 minutes of filed ETD and getting close to actual departure time with no DCL received, contact the tower via voice to see if they show you logged on. If attempting another logon, first turn off ATC COMM to ensure there is no conflict with ATC automation. Go to page 2 of the ATC LOGON/STATUS screen and select ATC COMM to OFF, as shown in the first screen of figure 5. If the logon was active, ATC COMM TERMINATED will display as shown in the second screen of figure 5.

Next, re-select ATC COMM back to ARM. Verify logon information is correct, then only one additional logon attempt should be made. If the second logon attempt fails, or the DCL is not received, the crew should revert to voice for clearance delivery.

Figure 5. Attempting a Second Logon

There Are Different Types of Clearance Messages

If the filed flight plan route has been accepted by ATC, a “cleared as filed” DCL message will be received by the crew. All elements of the clearance will be contained in free text, including assigned SID, flight plan route, climb instructions, initial cruise altitude, and squawk code. No loadable portion of the route is needed (or included), since there is no change. This is one reason why it is important to have the filed plan loaded prior to logging on to KUSA. Also note that the REJECT prompt is unavailable with a “cleared as filed” DCL (Figure 6).

                                 Cleared as Filed DCL                                                                                   No REJECT Prompt

Figure 6. Cleared As Filed DCL Message

Route amendments from ATC will either be uplinked as a partial re-route or full route clearance. A partial route clearance will only contain the amendment as a loadable portion of the route (push-to-load). This is the second reason why it is imperative to have the filed flight plan loaded into the FMS prior to logging on to KUSA. The loadable clearance will amend the previously loaded filed plan; if no filed flight plan is entered, the new amended flight plan can’t be loaded. The free text element reminds the crew to load the new route (Figure 7).

                                 Loadable Clearance                                                                                   Free Text Reminder

Figure 7. Reminder to Load the New Route

A full “cleared route clearance” will contain the entire new route as loadable. All fixes prior to the destination airport will be deleted and replaced by the new route when applied (push-to-load). A reminder: SIDs and STARs that are runway-dependent are not included in the loadable portion of the route and must be manually loaded.

You Can’t Modify a Flight Plan That Is Already Being Modified

If the active flight plan is either in a PEND or MOD state (depending on aircraft platform) (Figure 8), the APPLY / INSERT prompt will not be available for a push-to-load clearance modification (Figure 9).

Figure 8. PEND and MOD Flight Plan Status

Figure 9. APPLY Prompt Not Available

CPDLC-DCL Has More Operational Benefits Versus PDC

CPDLC-DCL has many benefits over the old ACARS PDC. DCL allows for “push-to-load” capability of amended departure clearances and multiple revisions to a clearance without having to copy a new clearance via radio and manually update the flight plan. PDC is restricted to only being delivered once per day per airport for an aircraft; once delivered, the clearance must be updated via voice. As mentioned earlier in this article, if an operator does not have the proper ICAO flight plan codes showing CPDLC-DCL as their preference, then a PDC will be issued and the FANS logon to KUSA will not be established, which has caused confusion for many operators attempting to receive a DCL.

A “Quick Topic” video outlining CPDLC-DCL can be viewed here. For more detailed operational videos and guides on CPDLC-DCL and other datalink systems, operators are encouraged to visit the Virtual Classroom section on the Pilot Gateway.

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Derek Fiedler supports RNP, Datalink, and Forge Flight Services for Honeywell Flight Technical Services. He can be reached via email at Derek.Fiedler@Honeywell.com.