Subscriber question:
"What’s an in-flight mag check and why would anyone do it?"
Martin:
“We all learned to check the mags during the run-up before flight. Magnetos, ignition harnesses and spark plugs degrade over time, and this wear and tear can and will over time lead to ignition problems. When these problems have gotten severe enough, the mag check will fail, and we will taxi back to parking to have a mechanic figure out what’s wrong. Needless to say, the flight is canceled.
Some pilots like to do an additional mag check in flight, typically at the end of the cruise segment before starting a descent. Why would one do that? Well, the idea is to detect a developing degradation of the ignition system before it starts to impact performance or the safety of flight, so that repairs can be made at a convenient time – as opposed to canceling a planned flight when the ground run-up is the first time we see any signs of concern.
So how does an in-flight mag-check provide that early indication? Do the mags work any harder in flight than on the ground? No, they don’t. What is different, though, is that a stronger spark is needed in order to ignite the fuel/air mixture at normal cruise power settings than at the very rich and barely-above-idle power setting we normally use for the mag check on the ground. That’s especially true when running lean of peak EGT. Meaning that a gradual decline of the spark will be noticeable in-flight at cruise power long before it shows any abnormal signs during a ground mag check at 1,700 RPM.
So, with that knowledge, we can perform an occasional mag check in cruise. A little roughness is normal when switching to just one magneto, but a very rough running engine or a complete EGT drop-out on one cylinder is a sign that it’s time for maintenance at the next convenient opportunity. Take note of which cylinder had the EGT drop-out, and whether it happened with the ignition in the RIGHT or LEFT position. This information will tell your mechanic exactly which spark plug had the issue, and it will greatly reduce the time needed to troubleshoot on the ground.”
