Supplemental type certificates (STCs) exist to essentially improve on the original type certificate or type design of a specific aircraft type/model. For aging aircraft, it’s fair to say there are a lot of options across all systems from which you can choose. The end goal, of course, is to improve safety, efficiency, or save costs—or some combination of the three.
A client reached out this past winter wanting to upgrade their helicopter’s cockpit to a single-panel glass display, complete with interfaces. This included a radio altimeter, traffic avoidance system, and a GPS/NAV/COM unit. We began looking at whether STCs already existed that would satisfy this request and meet the user’s requirements and budget. We were fortunate to partner with a local repair station that was able to sell us the rights to use their STCs, allowing us to perform the installations.
This process was mostly seamless, with just the odd hiccup that comes with fitment, configuration, and testing. As we got to the last couple of days onsite, we ran up against a hard fault with the traffic alert and collision avoidance system (TCAS). It was inoperative and not providing much in the way of details as to what might be amiss.
Usually, with this sort of issue, the configuration is likely the cause. Configuration settings allow the units to communicate with one another and speak the same language. They also allow discrete inputs and outputs to be set and are responsible for customizing the function of the system in a specific helicopter type, and to other systems it’s linked to.
STCs include a document called an ICA (Instructions for Continued Airworthiness). This spells out the system’s operation, maintenance instructions, removal and replacement procedures, limitations, and configuration settings. It is a road map for all things related to the newly installed system, and we referenced it during the initial setup and configuration.
On my last night on this job, I lay awake thinking about what I could be missing. The TCAS remained inoperative, despite the wiring being intact and connected as it should be. Configuration settings were set per the ICA, and I was truly stumped.
Once I stepped away from the issue, it came to me. I recalled discussing the coaxial cables for the TCAS system, as there are eight of them: four per antenna. Length was critical, and the only other parameter of note was the cable loss—the signal attenuation as a result of the cable type and/or the quality of the connections.
The manufacturer’s installation manual, from which the STC was born, stipulated that several cable types were qualified for use with the system. Each had its own cable loss value or factor.
As I swiped through the manual on my phone, I opened an email containing the ICA document. In that moment, I could feel the energy shift as I closed in on the cause of the problem.
The ICA document contained a specific section for configuration settings, which included the antenna cable loss values. But it only factored in one cable type. Because we used one of the other approved cables, the values differed. It was one of my “self-inflicted wounds,” whereby something outside of the product had created the issue.
The next morning, I got to the aircraft and updated the cable loss values. In the same process, I discovered a diagnostics page in the TCAS software that allowed me to see live data. The system told me what values it was measuring on each cable, and which cable or cable connector could be creating the fault. Due to the incorrect values being input initially, the system was faulting most of the eight coaxial cables.
At the end of the day, we place our confidence and trust in the products and related documents that are available to us. I’d like to believe they are bombproof, but so long as humans are in charge of the design and approval process, there will always be room for error or oversight.
This example is a minor one, but it serves as a reminder that when things are not as they seem, you must remember to look under every rock for a solution.
