CPDLC Gotcha: Clearance Busts

There’s been two pieces of news in the past week that has put CPDLC Departure Clearances (DCLs) in the United States on our radar.

The first was a warning issued by the friendly folk over at the Teterboro User’s Group, but which applies throughout the US NAS. The second was a report from an OPSGROUP member who generously shared a confusing ACARS clearance that ultimately led to a clearance bust.

The end result for both of these issues is the risk of misunderstanding a clearance, or not flying an assigned SID.

Here’s a rundown of everything we know.

Lessons from Teterboro

A recent spate of pilot deviations off the RUUDY 6 SID were found to be caused by a limitation in the practical use of CPDLC departure clearances.

And the issue isn’t specific to KTEB/Teterboro either. In fact, it could happen at any US airport, to any aircraft type receiving a clearance in this way.

The problem arises when you receive one other than ‘cleared as filed.’ When there has been a change or re-route, the clearance will include a new loadable route. The crew simply has to insert and activate it. Simple, right?

The issue is that by inserting it, your previously loaded SID will be dropped, along with its tracking and vertical constraints. But to make things even worse, the runway and performance data will remain. And so, it may be easy to miss unless you re-check your FMS carefully and re-insert the SID.

It’s not an error you want to discover once airborne.

This seems to be a limitation of the FAA’s Data Comm system, and applies to everyone – not just specific FMS’s.

But why is this happening?

The NBAA has reported that this is due to US SIDs being runway independent. In a nutshell, the problem doesn’t exist abroad where SIDs are runway specific. It’s not an easy fix. If the US were to try and follow suit, we could have a logistical nightmare on our hands.

The FAA is actively working towards better ways to mitigate this issue. But for now, it falls upon crews to double or even triple check their FMS when there has been any change. Be particularly alert when under time pressure or stress when this can become an easy trap to fall into.

A cautionary tale.

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 recently during taxi and under considerable pressure to get underway or out of the way:

Unfortunately, in this instance the crew mis-interpreted their datalink 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.

A special thank you to the member for sharing his story so that we can all learn from this experience.

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.

We’d love to hear more.

If you have other stories or info to share (completely anonymously of course) we’d love to hear from you. You can reach us on news@ops.group.

As always these issues take time to fix, and we’ll continue to keep you posted on any changes to the US CPDLC DCL system as they happen.

2 Comments

Jeff says:

8 October, 2022 at 11:34 pm

What I’m not getting in this story is what was the CPDLC message for in the first place? the original filed route, as indicated between the —- was SKORR4.YNKEE GREKI JUDDS. The NEW RTE when taken with the assigned departure is identical to the filed route. So this is not only confusing, and results in the SID being dropped from the FMC, but also seems totally useless. And it happens in NY airspace often. Maybe someone from ATC can weigh in on why we get these messages when the departure, squawk, freq, altitude or route hasn’t actually changed.

Michael says:

11 July, 2022 at 4:27 am

Thank you for this story.

We had exactly the same issue a couple of days ago and due to this story, we knew how to handle it!

CPDLC Gotcha: Clearance Busts