Saturday, February 23, 2008

Snow, Ice, Lions, Tigers, Bears and other dreadful things...

It's been a pretty back week for weather. I think 50% of my work commutes were through some kind of ice, sleet, freezing rain, snow, or similar. We currently have that weird stuff that we get here in Central Ohio, which is snow with a hard crust of ice on top. The Arizona Chamber of Commerce simply has to be behind this in some way. If it's not them, it must be Florida.

I have an update on the engine tear-down and reassembly from A&P school, though. You may remember that the engine we tore down wasn't one of the many "stable horses" normally used for the purpose; this one was pulled off of one of the hangar planes because it exhibited very serious internal damage after one of the ground handling classes ran it. Specifically, it destroyed 6 out of 8 spark plus (there are two per cylinder on an airplane engine) and got a very bad case of combustion chamber acne on 3 of the 4 cylinders. It was obvious that something had been ingested into the cylinders to cause that damage, but it was not obvious what it was. We found nothing in the cylinders when we tore down the engine that would provide any kind of clue.

Everyone pretty much shrugged and decided to just reassemble the engine and hang it back on the airplane. Chalk it up as a mystery and press on, as it were. Well, there we were, hanging pretty much the very last piece of kit back on the engine when one of the guys noticed something. We were in the process of hanging the heat cuff (shroud) back around the exhaust pipes (which is just a piece of metal that surrounds the exhaust pipe and directs some of the heat into the cabin for the comfort of the folks when it's chilly outside) when he noticed that the piece of screen that covers the carb heat inlet was torn.

Carb heat, for you non-pilots, is simply warm air that is added to the incoming air for the carburetor to keep ice from forming. You can read more about it on Wiki, should you choose to. If not, this is the most important thing to note:

Usually, the air filter is bypassed when carb heat is used.

Because of that, they placed a screen over the hole in the shroud to catch any big pieces of junk before they could get into the engine. That's all well and good, right up until the screen itself decides to contribute pieces of itself to the fuel/air combustible mix:

While it is by no means a certainty that this is what caused the damage, it is a fact that we weren't about to put this back on the airplane! That piece of screen has attached to the shroud with a almost-circular "donut" of sheet metal, held in place with five rivets. The teacher was wondering what it was going to cost to buy a new shroud since there was no sheet metal class this quarter when I chimed in that I had already had said class, and would be thrilled to do the repair myself.

So, I stopped by the hangar on Wednesday to pack up my riveting stuff (it was good to see my Gucci rivet gun (so named fir its astonishing color) again - it's been too long) and other supplies.

I had a little trouble drilling out 2 of the rivets and ended up making the holes a little bigger, but that was ok since the original rivets were #3 (3/32" diameter) and could easily be replaced with #4 (4/32" diameter) size rivets. The teacher had some metal screen on hand (which I'm pretty sure came from the Aviation Parts aisle at the local Lowe's store) that I used to replace the old, torn screen. That should be good enough since, as I have pretty much the entire class saying now, I can guarantee 100% that it won't fail in flight. (You know, because there is absolutely no way in the world that any of the hangar queens will ever fly again).

That was a pretty fun job, and as we're hoping to actually start and run the engine on Monday, I should know pretty soon whether it worked or not.

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