There are some moments that defy explanation.
I was recently on a routine installation overseas, working on an Airbus AS350 B2 audio system upgrade. Given the wiring was relatively new and in serviceable condition, the decision was made not to rewire the system, but rather re-pin/terminate the existing wiring to accommodate the new audio panel. I call this type of work “transplants.” They save time and cost, but are a very intensely focused “operation” due, in part, to having to operate methodically, moving one wire over at a time (this is particularly necessary if the existing wiring is not marked or identifiable).
This type of work also binds you to the existing wire lengths, which must reach wherever the new equipment — in this case the audio panel — is to be placed. In this instance, it did not leave me a lot of length to work with. Add in a hot climate and time pressure, and the experience was one where very little error could be afforded.
I pulled the wires back one by one, noting their source on the existing audio panels drawing and re-terminating them to their destination on the connector for the new audio panel. The process was moving along well and I was confident I would complete the job in a single day. Then it came time to test the system. I powered up the panel and configured it — and the pilot headset emitted an audible dull tone.
Such a sound is often attributed to audio hi and lo signal wires being swapped. But the mechanic I was working with was adamant that the sound was the warning horn, and I agreed with this — especially given that when we turned the horn switch off, the sound went away.
Ground runs proved even more confusing, as the pilot reported that the horn was working, but not at the correct trigger points, which were functions of Nr (main rotor rpm). As I sat through another run up cycle, I could hear the horn. Imposed on it was another sound, this one more dull and lower in frequency. I looked at the pilot. “THAT’S the horn!” I said, as it became clear that the tone we heard before was an imposter — and not the actual horn audio. We were chasing our own tails and causing our own problem(s). I took a step back to inventory what we knew — and what we didn’t. It was late in the day, everyone was tired and frustrated, and in those times its often hard to think clearly.
I knew the horn audio was coming into the audio panel on a fail-safe direct audio input. I removed the wire and connected a headset adapter to it so that I could feed the horn audio directly into the headset. The tone was now barely audible and very faint. It was clear that the tone was some level of noise coming from the horn audio generator. Given our limited resources, taking the troubleshooting any further into the audio generator would require spares we didn’t have (or the time to procure them).
I was left with one futile move, which was to relocate the audio wire to an alternate direct audio input on the audio panel. Upon doing so — and truly expecting no better a result — I was floored when I put on the pilot’s headphones and found the audio to be dead silent. No tone, no noise.
The aircraft was pulled outside, a subsequent ground run was conducted and the horn circuit functioned as it should with no anomalies. As of this writing, the audio panel OEM was not able to provide internal schematics for the panel or an explanation as to why one direct audio input differed in amplification from the other. As much as the issue was resolved in terms of operation, it felt (and still feels) very unfinished in my mind.
Every so often, against all odds, you just figure things out. You don’t quite know why it works, but you feel like further digging will just invite Murphy’s Law an opportunity to inject itself into the situation. It’s not a solution I particularly like, but when no other resolve presents itself, sometimes you have to accept that a fix is a fix.
