A&P School: Class completed

It was a long 10 weeks, but a very valuable class. The experience of removing an O-320 from a Piper and completely disassembling it, putting it all back together, hanging it back on the airplane and actually seeing it run was fascinating and rewarding. I’ve always treated the engine as some kind of mysterious, sacrosanct, magical lump of aluminum and steel, and having seen the inner workings has reduced some of the awe-factor and replaced it with a more useful and appopriate understanding of how it works, and even more importantly, how to maintain it.

For example, I’m more sure than ever that I need to get an Exhaust Gas Temperature (EGT) gauge in my panel. For those that are unaware, an aircraft engine does not set its fuel/air mixture automatically the way a car engine does. As the aircraft climbs into the thinner (less dense) air, the fuel/air mixture has to be manually leaned to keep it from running with too rich of a mixture. Running too lean, though, can cause serious and expensive damage to the (very expensive!!) engine.

The proper way to do this leaning is to ease out the mixture knob while monitoring the temperature of the exhaust gases coming from each cyliner. As the mixture becomes leaner, the temperature of the gases will increase. If the mixture is leaned too far, the temperature of the exhaust gases will begin decreasing. The point where the first cylinder ‘peaks’ is a critical point to know, since you will lean no further than that point and will, in fact, richen it back up just a smidgen.

The cylinders will not all peak at the same point due to the somewhat imprecise fuel/air metering provided by a carburated engine. Because of this, it really isn’t sufficient to have a single EGT probe, but it’s a heck of a lot cheaper to do so. In theory, you could do some form of ground test to try to determine which cylinder peaks first and put the single probe on the exhaust coming from that engine, but I think it is better to have a reading for all four cylinders.

Engine monitoring is an area where you can spend a lot of money. Even a basic electronic four cylinder EGT/CHT (CHT is Cylinder Head Temperature) costs almost $2,000. There are, of course, other parameters that can be measured, and it takes very little effort to run the price of engine monitoring to $3000+. I think I just need the EGT capability, though. CHT doesn’t really tell me that much, but EGT is something that can and should be used on every flight. The only question is whether to just monitor one cylinder, or go ahead and get a piece of kit that can do all four. It’s really just a question of spending $100-ish, or $400-ish.

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It runs! Well, more like lopes…

After a prolonged operation of moving other aircraft out of the way, were managed to pull the Warrior out of the hangar to test run the re-assembled engine. I received yet another harsh reminder of the true cost of taking night classes in the winter: it’s COLD out there.

We had pre-oiled the engine before pulling the plane out of the hangar to ensure that all of the engine bits had at least some access to lubrication when we first started the engine, so it should have been a simple matter to just crank it up and run it. Snapita, of course. What appeared initially to be a dead-ish battery (and in this case, given the hugely deflating, anti-climatic failure to turn over, I prefer the word “flaccid”) turned out to be a problem in the wiring. An hour of fiddling around with that, wishing all the while that, if not Rome, at least something was burning to provide some heat, the wiring was jury-rigged enough to allow another attempt.

It started right up. No apparent leaks, and all of the parts (at least those visible to the naked eye) remained attached. Success!

Well, partial success. The engine steadfastly refuses to rev up any higher than 1,300 RPM. At that point, it just quits. Fuel starvation, it appears, which indicates a problem in the carburetor. Now, here’s something we didn’t know: when the initial damage to the pistons and cylinder heads occured, they were running the engine to try to determine the cause of (wait for it…) fuel starvation at higher RPMs. Late to be learning that, in my opinion, but nothing to be done. We did all we could do, and now it will be up to the advanced engine class to find the fault with the carb. Which, of course, they will do inside, where it’s warm.

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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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Piston Acne

When I last wrote on the topic of A&P school, we had removed the O-320 from the Piper Cherokee in order to tear it down and see if we could determine why it was eating its sparkplugs. Since then, we’ve removed the cylinders and pistons, all of the accessories, and split the crankcase. We were careful to look for foreign objects in the cylinders and/or exhaust as we did so, but found nothing. What we did find was that in cylinders 1, 2, and 4, the pistons, the valves, and the inner surfaces of the cyliner heads were beat to hell and back. They look like an over zealous antique furniture counterfeiter went after them with a small length of chain. Every inch of the surfaces was nicked and scratched. If you imagine what it would look like if you tossed a hardened steel washer into the cylinder and ran it for an hour, you have a good idea of what condition these parts are in. And, we have no way to explain what happened!

We’re going through the instructional exercise of precisely measuring all of the innards, determining a multitude of arcane things like washout, rod bearing journal diameters, piston pin diamter, etc. before putting it all back together. With the limited budget the school has, there’s no way that they’re buying new pistons and cylinders, so we’re going to reassemble it with the ones we have and hope that it runs. The reassembly is not going to be easy; the same level of conscientious marking of parts and their locations that was exhibited when removing the engine from the airplane (which is to say “none”) went into its disassembly. Nuts, bolts, clamps, etc. are all neatly piled in abject disarray in a tin pan. It’s going to be interesting trying to sort all of that out!

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Back in school: Engine Maintenance class

The class schedule has finally rolled around to the two things that I required before starting again: a subject interesting to a piston engine airplane owner, and a class not taught by the former instructor that I had so many problems with.

Class actually started Monday night, and one of the first topics covered was the syllabus. There has been a significant change to this class, as it turns out. This used to be the engine overhaul class, but the focus has shifted to maintenance. While I was initially a little disappointed in the change (I have been looking forward to tearing an engine apart for a couple of years), I soon realized that this is a very positive move. In reality, the chances of my ever overhauling an engine are so close to nil that they could safely be described as “never.” Maintenance, on the other hand, well, that happens all the time.

Tonight we got our team project assignments, and you can rest assured that I wasted no time volunteering for the three person team that is assigned to remove the O-320 from the Piper Cherokee and tear it down in an attempt to determine why it is destroying the electrodes of both spark plugs in the #1 cylinder within just a few minutes of run time. The first step was obvious: remove the engine from the airplane, bring it into the shop, and tear it apart. I had asked last year whether we would ever have the opportunity to remove an engine, and was told no. With the change in the focus of the class, that disappointing answer has obviously changed.

It turns out that removing an engine is fairly easy for three guys to do in just a couple of hours. I can safely predict, though, that it’s going to be one hell of a lot harder to put back on! The airplane was somewhat unfortunately located in the hangar – it was within a couple of feet of the drainage grates. Of course, I managed to get two washers to roll straight into the grates after dropping them. Like a magnet, I’m telling you, those grates are just like a big magnet!

We got the engine into the shop and removed the alternator, magnetos, vacuum pump, valve covers, and a good bit of the baffling (I’m baffled as to how I’m going to get a wrench onto the last nut, though – it’s a tight fit) before we ran out of time. The “tool” classes go a lot faster than those interminable lectures last year, and I’m looking forward to going back next Monday.

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