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My partner in this battle was John, an avionics tech
who worked part-time for me. Although sidelined by a bad
back, his brain fired on all cylinders. He was a fantastic
“sparky” – my term for avionics technicians. I swear
those guys are wired differently from the rest of us, pun
intended. His approach to troubleshooting meshed with
mine, and between the two of us we had unraveled quite
a few mysteries. But this GCU issue had us stumped, and
we were not happy. 26
In our fight to resolve this conundrum, we went through 18 at least five GCUs, though I think one was an out-of-box
failure. We sent the starter generator out for repair and
found the field was out of specs; we were hoping that
would do the trick. But in the subsequent flight, after about 20-25 minutes while going through 12,000 feet, the same problem reared its ugly head.
Exasperation
We megger tested all the wires going in and out of the GCU. A megger tester will tell you if there is a short, then it’s up to you to find it. Everything checked out OK, indicating no shorts in the wiring. All systems ran great on the ground and the R/H generator was fine on the subsequent flight home. Hooray! On the next flight ... you guessed it, the R/H generator went offline.
The exasperation multiplied exponentially, and the expletives followed suit accordingly. The core charge billbacks from the GCU cores were piling up. John and I scrutinized the teardown reports on those cores and found a possible clue to the problem. Two different shops squawked blown transistors at Q6 and Q7. This was like manna from heaven to John because the King Air maintenance manual includes a breakdown of the GCU. We saw that Q6 and Q7 run the line contactor, so we used a power supply on the line contactor to see if it was drawing the correct amperage – it was. We even left it going for four hours to see if it would break down from the buildup of heat – it did not. Just when we thought we were on to something, we hit another dead end. We were crushed.
Despair
At this stage, we had even exchanged the starter generator. I was beyond frantic with the extravagant parts costs mounting up for my customer. I gave no thought to what this was costing me in labor. I had no intention of charging my customer for that. Fortunately, this customer trusted me implicitly and had the patience of Job the entire time. I, however, was in agony over the exchange GCUs, the core billbacks, the generator repair
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FEBRUARY 2025
KING AIR MAGAZINE • 13