Quickly finding Spark Plug Failure (Or Not)

With two spark plugs in each cylinder, and four to six (or more) cylinders in each engine, it may be inevitable that you'll see evidence of spark plug failure. The trick is to catch (and correct) the situation before it becomes catastrophic.

Two for the show

Piston engines have two spark plugs per cylinder for a couple of reasons:

• Redundancy. If one plug quits the other can continue to support combustion.
• Efficiency. Two ignition sources in a cylinder create a quicker, more complete fuel burn, transferring potential into power while the cylinder's valves are closed and explosive force acts directly on the piston.

Note: You know two plugs are more efficient than one, because you see a power loss (RPM drop) when switching to a single magneto (turning off one plug per cylinder) during your engine run-up.

Interesting indications

If you have the capability of monitoring all cylinders you'd see an interesting indication when switching to a single magneto-each cylinder's Exhaust Gas Temperature (EGT) will increase. But doesn't single-ignition operation reduce power output? Sure. But when a single spark plug is firing in a cylinder the fuel/air mixture does not burn as fast; the fuel/charge charge is still on fire when the exhaust valve opens, and the hot flame soars over the EGT probe in the exhaust manifold. Although there is less power being developed and the temperature inside the cylinder itself is indeed cooler, it looks like the EGT is hotter because of the indicating error created when active fire passes over the EGT probe. From where you sit in the cockpit, turning off (or losing) a spark plug causes that cylinder's indicated EGT to rise.

In-flight mag check

This means that, in the event of an in-flight spark plug failure, the EGT on the affected cylinder will increase. If you have a good engine monitor-and actively monitor indications-you'll detect the rise and can pretty easily distinguish between an ignition problem and others issues (like a stuck exhaust valve). How can you do this? Perform an in-flight magneto check. There's nothing that says you can't momentarily switch to a single magneto in flight, just like you do before takeoff. In fact, some spark plug failure modes will only show up at altitude, under cruise running conditions. In cruise (and, if equipped, on autopilot as your attention will be diverted), switch to a single magneto. All EGTs should rise. Switch back to BOTH and the EGTs should go back down. If an EGT does not rise on a single magneto, one of two things will happen when you switch to the other magneto:

• That cylinder will "go dark", or die completely, indicating a failed spark plug.
• That cylinder's EGT will remain high but constant on the other magneto, possibly indicative of a sticking exhaust valve or a disconnected magneto p-lead.

If you don't have an engine monitor it's harder to point directly to an individual cylinder or even a single spark plug, but you can still use RPM (with a fixed-pitch propeller or below the governing range of a controllable-pitch prop) and a single-point EGT to detect an anomaly.

Find a problem? Get the spark plug cleaned or replaced before conditions worsen and you have an in-flight engine failure.

Aero-tip of the day: Save money on troubleshooting and head off an engine failure by checking spark plug indications before and during flight.