In cooperation with uAvionix, the FAA has just released a SAIB (Special Airworthiness Information Bulletin) regarding altitude reporting from the uAvionix echoUAT. The SAIB seeks to raise awareness of the potential for echoUAT “failing to provide the barometric pressure altitude information in (the) ADS-B Out message.” How and when can that happen, how did this SAIB come to be, and what does this mean to our customers who rely on their echoUAT devices? Read on to find out.
The Magic
Released in 2017, echoUAT was one of uAvionix’s first General Aviation products, and the precursor to the radically successful skyBeacon and tailBeacon. We have been gratified to witness the incredible response people have had for a product that “just worked”. The echoUAT has seen over 1 million flight hours of successful use, affordably bringing the safety benefits of both ADS-B Out and ADS-B In to a passionate group of aviators.
echoUAT was only ever made available to the non-certified (experimental) aircraft market. Simplicity of retrofit and integration was key to keeping the total cost of ownership at an acceptable level for pilots of these aircraft. To ensure that simplicity and rule compliance, the echoUAT directly obtains its barometric pressure altitude from the transponder’s Mode C replies. It does not integrate its own barometric pressure sensor (altitude encoder) because doing so would have caused possible conflict with other issued FAA guidance regarding barometric pressure sources.
When skyBeacon and tailBeacon were TSO and STC certified, a great deal of work went into creating both a technical solution and regulatory approval for a unique hybrid sensor package and algorithm that fuses data to determine an accurate pressure altitude, corresponding with the transponder reported altitude. Even when transponder reported altitude is not available for significant periods of time, the ADS-B system will accurately report pressure altitude as if it had been directly plumbed to the static pressure system, calibrated to exactly correspond with the transponder’s altitude encoder, and at a higher update rate than could otherwise be provided. While the exact technical solution is beyond the scope of this article, Arthur C. Clarke would have likely considered it to be “indistinguishable from magic.”
The Gap
In the years since the ADS-B rule was published, the FAA and uAvionix have collected countless data points on the performance of ADS-B and its benefits to safe aviation. These years of use have, however, highlighted a gap in the echoUAT’s implementation that the FAA is no longer comfortable with.
We have established that the echoUAT does not incorporate the “magic” mentioned above, partly because it was the precursor to skyBeacon and tailBeacon, and partly because it does not have its own barometric pressure sensor. Because it relies solely on Mode C replies from the transponder, it only has valid altitude data when the aircraft transponder is being interrogated by ground radar or a TCAS system. The gap mentioned in the SAIB is in those environments when Mode C replies are not being generated. This, by its nature, happens primarily outside of ADS-B rule airspace, or at low altitude. In cooperation with the FAA, uAvionix analyzed a multitude of flights where radar coverage had become unavailable, such as landing at a remote airfield. In those cases, barometric pressure altitude data could become unavailable and cease to be transmitted in the ADS-B message.
The Solution
A number of solutions were considered to address this concern, including applying the learnings from our later skyBeacon and tailBeacon developments. Ideally this would have required no updated hardware, and have been a software-only fix. Unfortunately, after flight testing and discussions, it could not be agreed upon that the data would be fully rule compliant in these non-radar service areas.
uAvionix decided the most robust solution, and that which undoubtedly would satisfy all parties, was to incorporate the “magic” from skyBeacon and tailBeacon. To do so, echoUAT needed an independent barometric altitude sensor. Enter echoALT.
echoALT is a small form factor altitude encoder, specifically designed to be wired inline to an echoUAT. It comes with a harness that makes it essentially plug-and-play. The echoUAT software was also updated to process information from the encoder, and implement the altitude fusing “magic”.
In late 2023 we began notifying customers of the existence of echoALT, offering it for free, or alternately providing discounted upgrades to our latest tailBeaconX or other TSO-certified ADS-B transmitters. In April 2024, concurrent with our first echoALT shipments, a mandatory Service Bulletin was issued. Feedback has been overwhelmingly positive, and throughout 2024 we have been shipping echoALT to customers, while providing support for the simple upgrade process. To get an idea of how simple the update is, you can watch a support video detailing the process here.
The FAA, with uAvionix’s full support, wanted to ensure customers were aware of the existence of the Service Bulletin, and has now done so by issuing the aforementioned SAIB. If you’re an echoUAT user, and this is the first you’ve heard of echoALT or the issue discussed in the SAIB, we encourage you to fill out the form here to request an echoALT or alternative upgrade path. All good things must come to an end, and the period for obtaining a free echoALT under this program will end soon.
Exceptions to the Rule
echoUAT is an extremely capable device, and can be connected to a wide variety of equipment. There are installations where the altitude is being fed directly from an EFIS or existing transponder. In those cases, is it required to install the echoALT? That answer is nuanced. There may be installations where the update rate and latency from other altitude encoders will clearly meet the ADS-B rule. But because the installations are not controlled in the same way that they are in certified aircraft, uAvionix cannot guarantee the validity and timeliness of that data. As such, installing an echoALT is a simple way to ensure that your installation is fully compliant, and we highly recommend doing so.
As always, don’t hesitate to reach out to our incredible uAvionix support team with any questions.
Blue Skies,
Ryan Braun, COO
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