Often in avionics, we see OEMs battle it out for product and market share supremacy. You have the big dogs, the little ones and everyone in between. Creating a single platform in which these products are compatible can be a challenge. Sometimes manufacturers control their proprietary data in such a way that interfacing to their equipment can only be done WITH their equipment. For example, you may have an existing radio altimeter or autopilot and want to upgrade your GPS navigator — only to find that the two won’t integrate easily. This was very much the case in the early days of my career, before high-speed data busses and ARINC standards took effect. Cross-manufacturer compatibility has improved leaps and bounds since then.
Audio systems — the analog ones at least — have always had a level of “plug and play” to them. You could easily swap out a more dated audio panel in an aircraft for a more modern one, or one that offered more options. In most cases, the existing wiring is retained or added to, but our preference is to rewire the entire system to ensure we are not indirectly adopting pre-existing wiring issues.
Recently, we rewired the entire avionics suite on a BK117. Just shy of 80 pounds (35 kilograms) of wiring was removed and about 50 lb. (22 kg) was installed — a clear indication of what modern avionics can do for your weight and balance, never mind the weight saving in the equipment itself.
Part of this consideration is the avionics wiring integration (how things connect and talk to each other) and how to wire these systems to one another. We installed a tactical radio, which had multiple transceivers built into it. This meant multiple audio wires from it to the main audio panel. Per the installation manual, single shielded wire was called for, versus the usual twisted pair wire, which meant a smaller overall diameter of the wire harness (again resulting in less weight). The new audio panel was advertised as “pin for pin compatible” with the same input/output specifications as older models. What this translated to for us was knowing the internal audio “LO” connections were tied to ground. This meant we did not need to run a dedicated wire to the “LO” inputs (hence the single shielded wire would suffice).
We completed the installation and carried out a radio check in the hangar and all was deemed serviceable and satisfactory. A few weeks later, when the rest of the aircraft was put back together and during subsequent engine runs, the audio on the tactical radio was found to be weak. Both transmitting and receiving were “low volume.”
From a troubleshooting perspective, the pilot report revealed a lot. Being able to transmit means the push to talk circuit is operational. Being able to be heard and to hear audio means both audio circuits are operational. The next factor to consider was the transmit power and the condition of the coaxial antenna cable and the antenna itself. Having installed both new, the connections were inspected and found to be serviceable.
We adjusted the volume and gain levels of both the radio and audio panel and set to them to their maximum levels, a condition that is not normal or desired. Upon further review and having discussed the specifications of the new audio panel with the OEM, it was discovered that its audio inputs were floating. This meant the LOs are not internally grounded, but rather require a shared, hard wired external connection to ground (or the tactical radio’s LO outputs). Several short jumper wires were assembled and quickly inserted into the audio panel connectors and tied directly to ground. The audio levels were beyond acceptable limits and had to be adjusted, but ultimately proved that the missing ground connections now meant a complete audio circuit.
It was a reminder to always consider any and all fine print that may come with a new product that claims to be a “drop-in replacement” for its predecessor. Often, new products are designed to capture a larger compatibility scale and have some subtle differences. Depending on the level of integration, these may not get used or discovered. More reason to have discussions with the technical departments of the respective OEMs during the design stage of a new installation. That front-loading effort will often save time at the end of the project when time is often at a premium.
