EAA Sport Aviation - January 2012

intervals before moving on to the next phase. If the monitor samples every six seconds, run on each individual magneto for a minimum of one minute; if the monitor samples every two seconds, run on each mag for at least 20 seconds. Return to two-mag operation for a similar length of time between single-mag runs.

If your ignition system is healthy, when you switch to single-magneto operation you should see all EGT bars rise by 50°F to 100°F. They may not all rise the same amount; in fact, it’s perfectly normal to see even-numbered cylinders rise more than odd-numbered ones (or vice versa). What’s important is that all EGT bars rise, and that all remain fairly stable at their elevated values. You will feel a small but perceptible loss of power during single-mag operation, but the engine should continue to run smoothly without uncomfortable roughness. (The engine will always run slightly less smoothly on one magneto than on two, but roughness sufficient to get your nonpilot passengers to ask “What’s wrong with the engine?” is definitely too much!)

Once on the ground, dump the engine monitor data and analyze it. You will be able to see the test results much more clearly by graphing the engine monitor data than by looking at the instrument during the test flight. Figure 1 shows a good example of an ignition stress test— note how No. 5 EGT goes unstable when the engine is running on the left mag only, indicating that the bottom spark plug in cylinder No. 5 has a problem. Also notice that all EGTs rise more during left-mag operation than during right-mag operation, suggesting that the two mags are timed differently.

I recommend performing the ignition stress test on a regular basis—I do it on most every flight, generally at the end of the cruise phase just before starting my descent. It should always be performed anytime any sort of engine anomaly is suspected.