Along-time, out-of-state customer called to say his R/H generator went offline mid-flight. He’s got an F90, but this could happen to any King Air. For troubleshooting purposes, I suggested he swap the GCUs (Generator Control Unit) to see if the problem went to the other side, and it did. I had an exchange GCU drop-shipped to him and it was installed on the right side, putting the GCU that came off the left side back in its original position. Everything worked fine on the ground.
On the next flight, going through 12,000 feet, the right generator dropped offline again, so he brought the aircraft to my shop. We performed standard troubleshooting, such as ringing out all the wiring between the GCU and the generator to confirm continuity. We found no obvious “smoking gun,” but tweaked a few things, changed the GCU and it ran great again … on the ground.
Frustration
On the next flight, going through 12,000 feet, the right generator dropped offline yet again (by then I was getting frustrated!). I will spare you the excruciating details and the expletives uttered during this protracted fix, which came to be known as “The Great GCU Saga.”
John, one of my avionics guys, and I go way back; he’s 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 meshes with mine, and between the two of us, we have unraveled quite a few mysteries; but this one had us stumped.
In our fight to resolve this conundrum, we went through four or 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 okay, indicating there were no shorts in the wiring. All systems ran great on the ground and the R/H generator was fine on the flight home. Hooray! On the next flight … you guessed it, the R/H generator went offline.
The exasperation multiplied exponentially; the expletives proliferated. Meanwhile, I was getting teardown reports (and the inevitable core charge bill backs) on exchange GCU cores from earlier in the process. Two different shops squawked blown transistors at Q6 and Q7. This was like crack 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.
Despair
At this stage, I think we had even exchanged the generator. I was totally beside myself and beyond frantic about the costs mounting up for my customer. I wasn’t even thinking about the labor cost. I was in agony over the GCUs, the core bill backs, the generator repair and replacement, etc. My wife’s attempts to placate me with logical discussions such as: “Troubleshooting is like peeling an onion, you can only go layer by layer, you can’t always get to the crux of the matter on the first try…” wasn’t helping at all.
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 were thinking we had a chafed wire problem that was triggered by pressurization; going through 12,000 feet measured around 3.0 psi in the cabin.
We borescoped the wire bundles, starting at the GCU and worked our way out. We were looking 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 which also showed 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, as well as the Q6 and Q7 transistors in the GCU again, and put everything back together – the R/H generator has stayed faithfully online ever since. Hot diggity dog! It’s a great feeling when everything finally works. My only lingering problem is understanding how the replacement of Q6 and Q7 translated into $1,000 core bill backs each time I returned a GCU. When John did the job, it was $50 in parts and an hour of labor – clearly I’m in the wrong business!
This GCU saga was one for the books for a number of reasons. The location in the leading edge isn’t subject to much expansion/contraction, so why did it short out consistently when going through 12,000 feet? And why didn’t the megger testing indicate a short in the leading edge wire bundle? The wire bundle was properly tied up and even had spiral wrap added in places. Everything appeared totally normal each time the leading edge was removed.
This was quite the wild goose chase and completely unexpected in a King Air. At one point in my career, I maintained a couple of Citations for about 10 years. Chafed wires were an everyday problem. Those Citations became the bane of my existence.
Keeping the Craft in Beechcraft
King Airs do not typically get chafed wires. What happened with this F90 is rare for any King Air. In production, when the wire bundles go in, care is taken to allow for the expansion and contraction brought on by fluctuations of altitude, temperature and pressure. At least that’s how it used to be.
It’s been awhile since I’ve visited the factory, and now the Beechcraft line is under the same umbrella as Cessna. I do not wish to cast aspersions; I just hope that the standards of excellence achieved under the watchful eye of Olive Ann Beech will be continually upheld and implemented.
I wish everyone a fabulous holiday season, a productive and prosperous New Year, and a King Air free of chafed wires.
Leave a Reply