The two Lycoming engines in GXRP are about 70% done with their expected lifespan. They are showing annoying but not alarming signs of this.

There are several ways in which aircraft piston engines can show signs of decay. Metal in the oil can indicate impending failure with the crankcase widgetry – something that cannot be repaired short of an overhaul. Poor cylinder compression can indicate leaky intake or exhaust valves, which too suggest impending failure, but at least can be repaired with a cylinder exchange. Increased oil consumption accompanies wearing piston rings, which is thankfully a gradual process.

GXRP has the last of these problems. Oil usage per hour of flight is well below the manufacturer’s “alarm” limits, but well above that of a fresh clean engine. What’s worse: some of the oil that’s disappearing is entering the cylinders’ combustion chambers, and can foul spark plugs. Oil-fouled plugs tend to not fire, and show up on the engine monitor and as excessive RPM drops during the runup magneto checks.

During this phase of the piston ring wear-out, there is not enough oil spewage to foul spark plugs in flight, so the magneto check glitches are somewhat of a nuisance. In fact, oil-fouled plugs tend to clear up within seconds of a lift-off, when the engine produces full power. That’s partly because then the piston rings are squeezed back into their normal, high-combustion-pressure physical dimensions; everything is warmed up and expanded; things just seem to seal better. Note that oil fouling on spark plugs is not burned off the same way as lead fouling, by highly leaned medium power runs.

If one runs the engines on the ground for a long time, bad news. The piston will smear oil into the combustion chamber, but the oil is not blown or burned back out. Lucky me: as a part of an electrical system repair in GXRP, a number of low-power ground runs were performed. This appears to have collected just enough oil to interfere with both spark plugs of a particular cylinder. This grounded the plane again, right after it came of maintenance. ARGH. At least the problem wasn’t a more nasty metallic omen like stuck valves.

In the short term, I need to clean the plugs one way or the other, go flying, and get back to a cleaner state of cylinder existence. In the longer term (next few months), I expect to need some flavour of cylinder repair. Perhaps I will outright replace all 12; perhaps I’ll have a few individual ones re-honed and piston rings replaced; perhaps the situation will stabilize enough to defer a solution until next year; perhaps a very unlikely in-flight upset will prompt immediate maintenance.

The cost of what sounds like the most conservative approach (outright cylinder replacement with new parts) is high enough to counteract my normal problem-solving tendency to go for the higher-cost but longer-lasting fix. Plus, the work is so invasive to the engine guts that, should collateral damage occur, it would be significant in hazard and expense. So conservatism in this case might best be practiced with the “it’s working well enough; attempts to fix it can hurt more than help; let’s wait a while” option.