Of all the things that could go wrong during the first couple flights of a newly assembled kit project, it would have to be a problematic ADS-B Out system as the one and only squawk for our Van’s RV. On the first flight, the local tower controller must have known things in our little cockpit were a bit … busy. He didn’t mention that the Garmin ADS-B transponder system in the airplane wasn’t outputting ADS-B while tooling around in the Class D airspace and on the fringe of the close-proximity Class C. It wasn’t until I pulled a PAPR for Public ADS-B Performance Report from the FAA’s website did I realize the system was flunking—badly—important bits of data in its output.
While you should get a report after a new installation or after maintenance or removal/reinstallation of an existing system, it doesn’t hurt to pull these reports from time to time so you know that the system is 100 percent. This is important when you buy a new-to-you airplane. If able, do it during the prepurchase process so you know you’re not getting stuck with a system that doesn’t work. Don’t rely on the controller to tell you it isn’t working—they don’t always speak up when workload is high, I was told. In fact, a friend flew his airplane for at least a year through the busiest airspaces in the country with a faulty system and not one controller said a word—until one finally did. I’m hearing more reports from pilots who are being called out for having faulty ADS-B in mandate airspace, so maybe the leniency has come to an end, while the discussion about using ADS-B transmissions for user fees continues.
The PAPR process (unlike fixing a bum system) is stone simple. Go to the website where the PAPR is typically available one hour after you fly the aircraft in ADS-B airspace, but in my troubleshooting I found that the PAPR can be delayed by more. Before requesting the report, gather the specifics of your particular system. Unless you’ve installed the system yourself as we did in our airplane, you’ll count on the installer to leave a trail with exact configuration elements of the interface. You will need to know whether the system in your aircraft operates on 1090ES or UAT (or both) and the manufacturer/model, plus the transmitting address type. This is usually programmed as the give-away tail number of the aircraft. You also need to know the exact nomenclature of the ADS-B-interfaced WAAS GPS—again, the make and model. The most popular system models are listed in drop-down menus, but if it isn’t listed, you’re required to state the brand and model in a separate box in the request. Our plane has the Garmin GPS20A receiver, but it wasn’t listed. Stating it in the comments section was sufficient to generate a report.
What’s It All Mean?
Unless you’re a good study on ADS-B Out, the stuff that’s spit out on a report will probably be a head scratcher. But the FAA makes it somewhat easy—anything highlighted in red is problematic. The main image at the top is the actual report from our Garmin system, which shows issues with the system’s integrity and accuracy. It was the 100 percent failure of NIC (Navigation Integrity Category), SIL (Source Integrity Level), and SDA (System Design Assurance), which all pointed at a problem with the ADS-B system’s WAAS GPS position source. In our case, this was a dedicated remote Garmin GPS receiver that was connected (so I thought) with the Garmin remote ADS-B transponder over a serial data line. I was able to straighten it out by talking with Garmin tech support, looking back through the wiring, and getting into the configuration menus to make the changes.
The good news is that the FAA provides a PAPR user’s guide to help analyze the report. For amateurs troubleshooting their own system, it’s helpful because it describes—in plain language—what some of the fault codes might be that show up on the inflight report. Without getting too far in the weeds, the FAA assesses equipment performance in four major categories, including any missing elements that are required in the ADS-B Out broadcast, integrity and accuracy (NIC/NACp/NACv/SIL/SDA), kinematics (the aircraft’s changes in baro and geo altitude, velocity and horizontal position), plus other checks that include the proper formatted 24-bit ICAO address and other configuration settings.
Last, don’t underestimate the importance of a properly working transponder/Mode C system. It’s integral to ADS-B Out transmissions. I’ve seen instances where an altitude encoding (Mode C) problem could fail the baro altitude data, plus the loss of geometric altitude data from the GPS will fail the geo altitude data. If you’re building your own aircraft, it pays to work with a capable avionics shop to test the ADS-B Out system. You’ll need to work with a shop anyway—to test and sign off the two-year transponder certification required by FAR 91.413. If you plan to fly IFR, they’ll need to accomplish FAR 91.411 as well. If you botched a configuration setting (or the wiring) in the ADS-B Out interface, they’ll spot it and hopefully save you some grief later on. We’ll look at the current market for ADS-B Out solutions in an upcoming Smart Aviator report.
Will a faulty system be readily apparent on tracking sites like FlightAware?
Not necessarily, it depends on which fields are bad. For example the GPS position may be correct even though NIC or NAC (the quality/error flags) are out of range, in which case your FlightAware track could look perfect even though your system is failing to meet the required parameters. ADS-B failures could also be caused by other equipment in the airplane, for example the comm radio can block the GPS when transmitting due to harmonic distortion, in which case installing low pass filters on the transmit output could help.
Another useful resource is getting the complete set of ADS-B message events for your flight. This is especially valuable if your ADS-B Out system is nearly passing with only intermittent failures. It shows exactly when & where the failures occur. It comes as a track log that you can load into applications like Google Earth. You can request these by emailing the FAA: 9-AWA-AFS-300-ADSB-AvionicsCheck@faa.gov.