CPDLC Gotcha: Clearance Busts

By Chris Shieff


Key Points
  • The FAA has published a new Safety Alert for CPDLC and partial route re-clearances.
  • Make sure you load your full SID manually into the FMS after you receive a partial reroute message (UM79).
  • Also, don’t mistake these partial reroute messages as being cleared to fly directly to the waypoint (a direct clearance would be a UM74 message).

Lessons from Teterboro

In 2022, the FAA recorded 20 aircraft deviations at KTEB/Teterboro Airport due to issues with CPDLC and partial reroute messages.

These incidents resulted from failure to reload SIDs after receiving a partial reroute UM79 message (where you are cleared to a particular waypoint via other waypoints en-route), requiring swift coordination with ATC to avoid traffic.

But the issue isn’t limited just to Teterboro – it could happen at any US airport, to any aircraft type receiving a clearance in this way.

Another thing to watch out for

Due to limitations in the formatting of CPDLC DCLs, they can be easy to misread or misunderstand. Take the following for example, courtesy of an OPSGROUP member.

A change to a clearance was received by a B777 at KJFK/New York during taxi and under considerable pressure to get underway or out of the way:

Unfortunately, in this instance the crew mis-interpreted their clearance as direct to the waypoint YNKEE. This was further compounded by the issue above – when the new route was loaded, their SID was dropped from the flight plan.

When they got airborne, ATC immediately began asking why they weren’t following the assigned SID – the result was a clearance bust. To their surprise, further down the clearance was indeed an assigned SID – the SKORR4. It was an understandable and easy miss.

The question remained though: what then is the intention of the top part of this clearance if not to clear the aircraft direct to YNKEE? We put this to the group, and received some useful feedback.

It maybe comes down to a machine readability issue. The section above the plus signs is required because of the way the clearance is written, and is related to the same issues as above. It will not contain a SID when you insert it.

In fact, some newer CPDLC systems don’t even show that section to the crew – only the information below the plus signs which contains the assigned SID. The full version is a confusing, and seemingly contradictory set up.

What about PDCs?

It’s probably worth a mention that these issues don’t affect PDC clearances. PDCs are different and are sent by a service provider via VHF datalink. No log on is needed, and only one can be issued for a flight number at specific airport over a 24 hour period. They also have to be read back via voice. PDC’s cannot be used to notify pilots of a change to the filed route. So it’s smooth sailing in that regard.

Further reading.

You can read the FAA’s new Safety Alert for CPDLC and partial route re-clearances here.

The FAA also has a handy guide on how to use CPDLC in US airspace. It covers the basics, along with departure clearances (DCLs), en route ops, speed/time restrictions, emergency use and free text.


More on the topic:

More reading:

Chris Shieff

Chris Shieff

OPSGROUP team member and Airbus pilot. Based in sunny Auckland, New Zealand. Question for us? Write to blog.team@ops.group.


  • NRL says:

    Only in America could something so simple be made to be that difficult.
    Look at the datalink clearances you get around the world and think maybe you’ve got it wrong.
    Dubai, London, and Singapore for example should be an easy copy.
    Late to the game and trying to make it too difficult

  • Matthew Newman says:

    Not only is the format of the message confusing, but the functionality of the Load + route is a safety issue. We spend nearly an hour of time carefully preflighting an aircraft and making sure all systems are set up and programmed correctly, and in one button push, all that is undone. In our Collins Vision system, when the new route is loaded, it not only drops the SID, it drops the cruise altitude in the flight plan setup, which then drops the performance, the V-speeds, the departure runway, the flight plan winds, and the speed profiles if they were modified from the defaults. Without a specific checklist to address each of these issues, your only recourse is to complete your entire FMS preflight from scratch. It would be much safer to just allow the pilot to manually adjust the FMS with the one waypoint that changes or needs to be inserted rather than forcing him or her to allow CPDLC to make the change to the FMS. I’m tempted to remove the 1FANS from my filing profile.

  • Ska says:

    Btw, the latest version of NAS Data Communications Guide is v13.0 (February 14, 2023) from https://www.l3harris.com/datacomm/

  • HBM says:

    The fragmented, out-of-order format of a change in the clearance when issued via CPDLC DLC is culpable. I have submitted numerous ASAPs, known by another acronym by my employer, about the confusion the disjointed formatting causes. Crews really need to stop and read the entire clearance, because the format is a real threat, especially if they’re time pressures and other distractions on top of the change. The load prompt on the FMC/CPDLC interface I use doesn’t load the new SID, so that’s another issue. Clearly a weak link on the National Airspace System in the US.

  • RT says:

    This happens at many other US airports, not just Teterboro. The safety mitigation is to scroll through the route point to point prior to takeoff.

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