Let there be light

Just a fun weekend with one of the bigger bugmashers in the fleet.  The road to Eagle washed out and the company had a few heavy days of flying last week and another hard week scheduled this week.  They ran the Caravan out of hours on me and I got to work this weekend to knock out an inspection so they can perforate the atmosphere in a safe machine.  Wasn’t a big deal; the weather kinda sucked Saturday anyway.  I even believe we had an incipient tornado.  At least that’s what it looked like to me watching it come down the runway.  We had almost all of it that day:  Forrest Gump rain (comes from the top, the sides, and even up from the bottom), hail, winds, sunshine…  The only thing missing was snow and that’s not too far off.

So there I was, doing the pre-inspection engine run on the Caravan.  It’s fun, the thing’s got a big turboprop on the front and is the closest thing to a real live jet I’ve run since I was in the Air Force.  I’m checking all the systems and, naturally, the lights on the gauges in the pilot’s side instrument panel won’t come on.  Not a big issue this time of year ’cause it doesn’t get dark, but they still gotta work.  Shouldn’t be a hard fix, methinks, prob’ly just a bad ground or a broken power wire at the panel connector.  So back I go to get my trusty rusty tools and get things started.

I work with some great guys and a couple of them came in to lend a hand. One of them started tackling the problems, the “fix” part of things, while I kept running down the checklist, getting the “look” part of the inspection done.  He got around to the light issue after a bit and between the two of us, we started breaking the instrument panel down into individual molecules.  No broken wires, all the grounds were solid, everything checked fine with our meter, we even had the varying resistance from the dimmer like we should… so we started digging deeper. 

The Caravan’s a funny thing. (It’s a Cessna; the name went on before the quality went in.)  There are four knobs which control the cockpit lighting.  Each knob is actually two dimmer switches in one, with concentric adjusters on a single shaft.  The outside knob controls one set of lights, the inside one another set.  Without getting too deep into it, there are eights sets of lights, eight controllers, but only four knobs sticking out of the panel.  The rub with our problem was the inside controller worked fine, as did the other six. They all have the same power source and work off the same circuit so I knew we had power to the switch.  What we didn’t have was power to the lights.  It was the end of the day Saturday and we called it a day.  I was coming in solo Sunday, so I’d knock the rest of the job out on my own.

Deep in the side of the airframe are three little boxes.  They’re behind a big ass circuit breaker panel and under the dashboard so they’re a right bastard to get to.  On these boxes are eight transistors; one for each set of lights on the instrument panel.  I discovered this after a trip through the parts manual looking for a new dimmer switch.   We even have a couple of the little gems in stock!!  Eureka! , I exclaimed as the proverbial light bulb went on.  (Unfortunately it wasn’t the one I really wanted to light up.) I’ll swap out the left panel light transistor and do the aviation mechanic dance of joy.

So I get the books with the pictures, break the plane down into even smaller molecules and find the transistor boxes.  The three boxes are mounted on the side of the plane with the transistors actually where I could get to them. That makes for a nice change.  I compare the picture in the book with the verbage in the text and with the real thing in the airplane.  Looks like the picture; there are three transistors on the top two boxes and two on the bottom one.  The text says the top box has the transistors for the left panel, the right panel and one floodlight, and that the top transistor is the one I need to change.  Two screws, a little cussing and searching after I drop the frappin’ thing under the floorboards, and the new one’s in.  No lights.  Out comes the meter and lo’ and behold, the transistor’s good.  So are the other two on the box.   Let’s pull one of these three and see what lights that did work aren’t working now.  OK, that transistor runs the radio lights and is supposed to be on the middle box.  Any bets the yahoos at Cessna got the book wrong? 

Off comes the top transistor on the middle box, in goes a good one, and let there be light!   Get out the meter and imagine that,  the transistor’s reading open.  Commence to dancin’! All that’s left is to put all the molecules back into the shape of the inside of an airplane, do the mountain of paperwork, and go grab a frosty cold adult carbonated malt beverage.

I’m a firm believer that the guys designing things for Cessna were promised jobs at NASA or Boeing or Lockheed or one of the big sexy I’m-a-rocket-scientist kinda places when they were in airplane design school.  When those jobs fell through and they ended up at Cessna designing what amounts to the Chevy of the aviation world, they got pissed.  They took all that high-tech, fancy-schmancy schooling and put it where old farts like me have to fight with it.  They could have made this way easier.  Either that or they came home and found a mechanic fooling around with their girlfriend.  Oh, wait,  they’re engineers…..they’ve never had girlfriends……

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