Running up to high power and leaning aggressively on the ground didn’t fix the problem—the left mag still showed an excessive and rough fall-off in rpm. The usual suspects in this frequently re-enacted aviation whodunit included the left magneto (or its components), faulty mag timing, the ignition harness, and the bottom spark plugs (which are fired by the left mag).
The engine on this particular airplane had logged only a few hundred hours since overhaul, and the accessories mostly were less than two years old. Also, the airframe had received its annual inspection about three months prior, and the engine had exhibited none of these symptoms during the immediate post-maintenance flights—or since.
Advice on how to proceed ran the gamut from the crazy expensive (replace both mags, both ignition harnesses, and all dozen spark plugs for about $3,000) to the semi-expensive (replace the lower spark plugs for about $300). As it turned out, none of that was necessary.
Simply starting with the easiest and cheapest possible explanation—namely, cleaning and gapping the lower spark plugs (a task explicitly approved under the FAA’s owner-approved maintenance items in FAR Part 43, Appendix A)—solved the problem at next to no cost. The lower plugs in any horizontally opposed engine are particularly susceptible to lead fouling, and it became obvious as soon as the plugs were removed that caked-on lead was preventing them from firing efficiently. Once reinstalled, the mag checks on both sides were smooth and well within tolerances.
Here’s another example, this time from a mid-time Lycoming IO-540 with a recently installed digital engine monitor. The engine ran fine on the ground, and indications were normal during climb. But in cruise, the engine monitor showed troubling rises in both oil temperature and oil pressure. Spooky.
Mechanics suspected a faulty vernatherm—an engine valve that routes oil through the cooler or the crankcase, depending on its temperature and viscosity. It was quickly replaced but didn’t solve the problem, and the consensus was that the engine needed a complete overhaul, an unanticipated $30,000 expense.
Then, the aircraft owner stumbled upon the answer that should have been obvious from the very beginning. It’s a physical impossibility for oil to rise in both temperature and pressure simultaneously. It’s an inverse relationship—when one goes up, the other goes down. A mechanic installed standalone, direct-reading oil temperature and pressure gauges, which showed that the engine was operating normally. There never was an engine problem at all, only errant indications caused by faulty solder connections to the digital sensors.
All this isn’t to say that really complicated and expensive problems don’t exist. They do. But exploring the easy and cheap solutions first seldom hurts—and it may just help a lot.
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Don’t leap to complicated and expensive
The Frugal Flier’s first tenet of aircraft maintenance is this: When troubleshooting any aircraft mechanical problem, start with the simplest, cheapest explanation and work your way up the complication and expense scale from there.
Here’s a recent case in point. While running up an engine prior to flight, a magneto check showed an excessive rpm drop in the six-cylinder Continental O-470. The engine ran rough, sputtered, and backfired on the left mag. The right mag checked fine, and the engine ran smooth and strong on both mags.
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