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and replacement and the downtime for this King Air. Add to that the costs incurred by flying this King Air back and forth to my shop. I was borderline despondent.
My wife attempted to placate me with logical discussions such as: “Troubleshooting is like peeling an onion; you can only go layer by layer, and you can’t always get to the crux of the matter on the first try.” This was of no help at all.
We borescoped the wire bundles, starting at the GCU and worked our way out. We looked for something inside the pressure vessel but found nothing. Totally deflated, we labored on and removed the leading edge yet again. We examined every inch of the wire bundle with magnifying glasses. There was no evidence of arcing on any portion of the airframe. Parts of the wire bundle were encased in spiral wrap, showing no traces of arcing, but we removed it anyway. At long last we found a sign: a small black smudge on four little wires – and one of them went from the line contactor to the GCU. Eureka!
Success at last
We repaired that wire. We replaced the Q6 and Q7 transistors in the GCU for the last time and put everything back together. That R/H generator stayed faithfully online from then on. Hot diggity dog!
The saga was over, and we’d won the war. It’s a great feeling when everything finally works. My only lingering problem was understanding how replacing the transistors at Q6 and Q7 translated into $1,000 core billbacks each time I returned a GCU. When John did the job it was $50 in parts and an hour of labor – go figure that one!
This seemingly simple squawk turned into quite the wild goose chase. It’s been years since it happened, but
“Troubleshooting is like peeling an onion; you can only go layer by layer, and you can’t always get to the crux of the matter on the first try.”
John and I discussed this situation ad nauseam, then soldiered on. I rented a GCU breakout box and teed it into the system. We flew the aircraft and confirmed that the GCU was operating normally. Then we wondered if we had a chafed wire problem triggered by pressurization. Going through 12,000 feet measured around 3.0 psi in the cabin.
A close-up look at the cause of the perpetual problem – a small black mark on part of a wire bundle.
14 • KING AIR MAGAZINE
FEBRUARY 2025