Sometimes, positively-motivated actions can lead to negative or unintended consequences, especially if we fail to consider complex systems, potential side effects, or power dynamics. In the world of aircraft maintenance, an attempt to complete a repair or solve a problem without the whole picture can not only fail to address the original issue — it can make things even worse.
Case in point: While contracted to do some calendar-related maintenance items on an MD500, I was given a list of issues that had been affecting the aircraft for some time. Among these, the “Engine Out” circuit continued to operate with the aircraft on the ground, and the radio altimeter also continued to transmit on the ground. Both systems’ audio warnings were not audible in the pilot’s headset.
At first glance, one would assume these are two very different and independent systems — which they are — but often “on ground” related logic is achieved through a microswitch that is activated by the collective stick. When the collective in the full-down position it makes a ground logic circuit. This sends a signal to a variety of systems when the aircraft is on the ground, such as the traffic alert and collision avoidance system (TCAS), radio altimeter, ADS-B and weather radar systems.
This particular MD500 had a full set of completion drawings from the OEM — rare for its age, but vital in providing a roadmap of how the aircraft was originally wired. While reviewing the drawings and then inspecting the aircraft, I honed in on the aircraft’s hourmeter and ground logic circuit. There was a relay, a collective switch, and a multitude of interfaces to other systems — two of which were the radio altimeter and the engine out circuit. This made me pause, as schematically the aircraft drawing indicated everything should be working, or had been working at some point. Further discussion with maintenance staff started to reveal more of the story. At some point, the hourmeter relay was deemed to be inoperative. Unable to source a spare, the maintenance team modified the wiring, removed the relay, and restored the function of the hourmeter. But the other systems that relied on this ground logic became isolated from it as a result.
I was able to source a relay and restore the wiring to its original design, and the ground logic circuit was once again operational. But that still did not explain the lack of audio in the pilot’s headset from these respective systems.
Most audio panels use a direct audio input for critical audio alerts. I did a continuity test from the pinouts of the audio panel to the audio outputs of the engine out and radio altimeter circuits and found them to be open circuit. This seemed odd, but clearly they were not a direct connection — and the chances of both wires being compromised was unlikely. A quick check of the original audio drawings showed an audio summing amplifier in line with the alert audio inputs. This device allows multiple audio sources to share one audio input. I placed jumper wires across its connector to create a bypass circuit and tested for the audio again. Still nothing. Further investigation revealed that the direct audio configuration jumpers on the audio panel were installed for connection to the right-hand-side crew headset (this aircraft was configured for the pilot to be on the left-hand side). A quick check of the aircraft logbook showed an audio panel replacement had taken place a couple of months prior. It became clear that reconfiguring the jumpers for this particular installation was missed.
All told, the troubleshooting and repairs took a few hours. As I reflected on the the totality of the issues, I could clearly see the chain of events that led to these issues manifesting as they did. The intention to restore function to a system (the hourmeter) which was deemed inoperative, ultimately caused other systems to lose their full-scale function as a result of the repair. An hourmeter not counting hours is immediately obvious, but a radio altimeter (post initial repair) that continues to quietly display altitude while on the ground was not so noticeable. The engine out circuit being operational on the ground was also easily muted by simply pulling the respective circuit breaker; something the pilots defaulted to as an easy “fix.” As more time passed, the connected relationship of these systems became lost, and the issues became regarded more as individual problems — when in fact they were all one and the same.
