Every flight, from beginning to end, owes its successful management to a series of systematic procedures conducted by the pilot. Checklists are the backbone of each aerial journey, the lifeblood that streams a flight towards a safe and secure completion. Given time and repetition, many pilots can perform checklist tasks from memory, but for some of lesser experience, their memory may not recall why they’re doing a particular item from the list. One task that every piston-engined pilot must perform is the magneto check, but not everyone may remember why they were taught to do it. Here’s an overview of what it’s about.
Aircraft engines generally have two spark plugs per cylinder. They also have two magnetos with one positioned on the left of the engine and the other positioned on its right. Each magneto fires one of those two spark plugs in each cylinder. Your pre-flight magneto check is, therefore, verifying each magneto’s operation to see that all plugs are firing evenly and regularly on each magneto.
The procedure is standard across all piston aircraft models: position the aircraft into the wind (for maximum cooling effect), run up the engine to a recommended rpm, move the ignition key from “Both” to “Left” back to “Both”, then from “Both” to “Right” back to “Both”, and check for the drop in rpm during each of these latter two sequences. This procedure allows you to check that the engine is running smoothly on both magnetos simultaneously, as well as on each of the magnetos individually. Moving the ignition key back to “Both” during the sequence actually plays a role in the drill: it allows the engine to regain its normal rpm and clears the inoperative set of plugs in case they were fouled with oil while their magneto switch was “Off” during the check. When you’re verifying the functionality of the magnetos, you’re testing the ignition system in its entirety. Typically, the drop in rpm of each individual magneto should not exceed 75 rpm relative to the engine’s operation on both magnetos. However, a loss that is greater than 75 rpm – or outside of limits prescribed by the manufacturer – may be a sign that a serious condition exists in one or the other magneto in which the excessive drop was observed. If each magneto’s individual rpm drop is roughly the same as the other, and still within limits, that would usually be a reflection of nothing more than normal wear and tear in each magneto. One dropping significantly more than the other, however, is telling you that something is wrong in that magneto’s area of the system.
A large, beyond-limit, drop on each magneto – with the engine still running smoothly – is worthy of a check into the timing on both magnetos. Small rpm drops are something to which the pilot should be alert too. It doesn’t mean the system is working better; rather, it could be an indication that the engine’s timing is too advanced and that current is arriving too early in the firing sequence of the plugs. No rpm drop could also be telling you something: a magneto may not be grounded. This “Hot” or “Live” magneto means that, even with the ignition switch turned off, the slightest movement of the propeller could kick the engine into life. An after-flight check of the magneto – at idle throttle, switch the magneto “Off” for a split second – will inform you whether your magneto is “Hot” or not.
An engine that is rough on one magneto but smooth on both could be an indication of a fouled plug or a bad ignition lead. Checking for a cool cylinder after running the engine on the “bad” magneto for a minute should allow you to isolate a problematic plug or lead. An engine that runs rough on each magneto individually is a symptom of a problem not related to the ignition system but is more likely an induction or valve train concern. Any irregular roughness that isn’t traceable to a cylinder can be a symptom of internal problems within a magneto itself. You can fly your aircraft on one magneto, but a take-off with one that’s not working correctly is asking for trouble while en route. Pay careful attention in your run-up: there’s a lot to be told from that methodical check of your magnetos.