En Route Data Comm, Not Ready for Prime Time?

Pilots flying business jets and airliners in the U.S. are getting accustomedstarting to get used to digital clearances, now that the FAA has implemented Data Comm at 55 U.S. airports with Tower Data Link Service (TDLS)-equipped facilities. The next step is Data Comm for en route operations, yet there are some issues that need to be resolved before this takes place.

A 146-page draft advisory circular—AC 90-117, Data Link Communications—outlines the plans for further implementation of Data Comm services. The comment period for the draft AC ends on May 15. One of the key issues has to do with “push-to-load” capability, where a digital clearance is not only sent to an aircraft but also can automatically be loaded into the FMS.

Currently, most pilots using digital clearances at the TDLS airports transmit the clearance request and receive the clearance digitally, but still must input it into the FMS, instead of using the push-to-load feature (which isn’t available on all FMSs). 

Any aircraft equipped with Future Air Navigation System (FANS) avionics can take advantage of  the new controller-pilot datalink communications (CPDLC) digital clearance (DCL) service at the TDLS airports. The advantage of DCL is that the clearance and any revisions are sent digitally to the aircraft, and there is no need to call clearance delivery on the radio to request the clearance. This is especially efficient at busy airports where it is often difficult for pilots to break into the frequency to request a clearance on the radio. With DCL, even though they are then inputting the clearance into the FMS, the process keeps the pilots in the loop, ensuring that they are confirming all the elements in the clearance as they update or input the flight plan. 

Implementation Issues

At first glance, push to load seems like a natural step, removing the burden of all that button pushing to get the clearance into the FMS. Yet there are some issues with pushto-load, and while the draft FAA advisory circular implies that push to load will be mandatory for CPDLC en route operations, it might not yet be ready for implementation.

One issue involves when a pilot might receive an en route clearance and whether there is enough time to make sure a push-to-load clearance is correctly entered into the FMS. Clearly, the descent into a busy terminal area might not be the best time to let a push-to-load clearance go straight into the FMS, because the pilots need time for a quality control check before the autopilot blindly follows the flight plan. Obviously, making any complicated update to a flight plan in busy airspace is challenging, but some might question where it is a good idea to allow this to happen automatically.

Another issue is that FMSs on aircraft equipped for FANS operations are already DCL capable, but not necessarily push-to-load capable. But these aircraft still could benefit from en route CPDLC because they have the equipment needed to send and receive digital messaging, just not push-to-load clearances. 

Basically, as Universal Avionics pointed out in a recent blog on this subject, the FAA needs to create guidance to help avionics manufacturers “produce a system that is fully compliant with correctly loading a route in a manner that ensures the route on the aircraft continues to match the route the controller are using on the ground.” This would also help ensure that the avionics manufacturers “have additional time to develop solutions, once the system is perfected for both forward-fit and retrofit aircraft.” 

Pushto-load is already under way on some aircraft, and there have been problems with incorrect information being loaded into the FMS, and pilots and controllers disagreeing about the route to be flown. Among the recommendations submitted by Universal Avionics is this, which urges the FAA not to make push-to-load mandatory: “Eliminate requirement for 'push-to-load,' and base approval on system designs that satisfy safety, performance and interoperability requirements.”