Saturday, November 15, 2008
Ducked a bullet today
Whew. I haven't got a soft jug after all.
No, that's not what you think. To be precise, the No. 4 cylinder in my Cessna's aero engine isn't suffering from low compression.
Aircraft engine health is measured by many things, but the most important is the compression in the cylinders. To test the compressions, a mechanic removes one of the two spark plugs from each cylinder and pumps the cylinder full of compressed air at a pressure of 80 pounds per square inch, then turns the propeller so that the piston is driving against the pressure. Ideally -- but rarely, and usually with a brand new engine -- there is no leakage of air past the piston, resulting in a differential reading of 80/80 psi.
Usually, however, there is a teensy, tiny bit of leakage of air past the piston rings, so that the reading is, say, 78/80 psi. Anything in the 70s is considered fine, but if the reading drops into the 60s, then it's likely that you've got a problem that over time will only get worse. Below 60 means that your engine is losing too much power and it's time to break the piggy bank.
Maybe the piston rings are wearing out slowly and you're facing perhaps the installation of a whole new "jug," or matched cylinder and piston (about $2,500 for each of four jugs in the 100-horsepower engine of a Cessna 150, at right). If the readings are low in two or more cylinders, you might have to replace them all (a "top overhaul" in aviation parlance) -- or, at worst, have a complete major overhaul done on the engine, in which the whole thing is taken apart and anything that's worn beyond limits (almost everything) is replaced. That can cost $12,000 and usually does.
At my airplane's annual inspection in the beginning of September, my No. 4 cylinder compression read 62/80. Gulp. The engine had been run 971 hours since it last had undergone a major overhaul in 1994, or just past half the usual time before overhaul of 1,800 hours. Quite often one or more cylinders on a Continental O-200A engine gets unacceptably soft at the halfway point, and this is when aircraft ownership gets painful.
Fly the plane five or six more hours, Gordon, my mechanic, told me. Then we'll do another differential pressure test and see which way the problem is trending. If the pressure is still "soft," he said, we'll pull the jug and send the cylinder out for honing, or interior polishing, to get rid of any scoring that might be causing loss of pressure. Then we'll put new rings on the piston and reassemble the jug, then reattach it to the engine block. That'll be about $800 or $900, he said with an airy wave.
It might just be that the two piston rings have vibrated their way around the piston so that the gap in each ring lines up perfectly with the other one, yielding a tiny opening for air to leak through. If this is so, running the engine a few hours may cause the rings to rotate some more, closing the opening and returning pressure to where it should be.
I met Gordon at the airport today to run that pressure test.
First, Cylinder No. 1. Pressure: 78/80. Then Cylinder No. 2: 73/80. No. 3: 76/80. And, with a deep breath, Cylinder No. 4. "Look here," Gordon said. I let out my breath and peered at the gauges. 76/80! Yay!
The gaps in the rings had simply lined up at the time of the annual, then returned to their usual staggered positions.
The test cost $25 for Gordon's time. Not $800. Not $2,500. Not $12,000.
And so I shall keep Old N5859E a few more months at least, until the looming depression has either steamrollered my net worth or it hasn't.