Friday, March 14, 2008

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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