I remember a particular learning curve, when just trying to remember all the pre take-off checks, and their order, was the challenge ‘du jour’. Eventually, I was quite pleased with myself when I could impress my instructor with an almost seamless flow, and I was proudly loyal to the checklist. The only problem with all this, is that even though we conduct the checks to the best of our ability (after all, lives depend on these), we often don’t really know, or recall the reason why we are checking a particular item.The magneto check is one of these – we know it is one of the most important checks, and that we are looking for a particular drop, but what is the significance of the drop?
I am writing this for people like myself, who may be a bit mechanically challenged, and asking these questions usually yields an incomprehensible speech of technical jargon, which only serves as a deterrent for asking further questions. However, if we take out all the big words, the magneto system is actually quite interesting and amazingly simple to understand. Magneto ignition systems are used in aircraft because of the safety factor. Unlike a car, which relies solely on a battery/generator system, which, in the case of a failure would leave you with a dead engine, the magneto system is completely independent and your engine will continue running. Although one magneto can do the job, it is a requirement by law to have two per engine in an aircraft, for safety of course,
Although magnetos have been around for at least a century, and they have a multitude of applications, in piston engine aircraft, they produce impulses of high voltage for the spark plugs, which ignite the whole power generating system. In order to understand what we are checking, you need to know that each magneto is connected by a wire to it’s own spark plug, which in turn fires up a specific engine cylinder. When we select the switch to ‘Left’ we are in fact grounding the right magneto, not turning the left off, before returning to ‘Both’. The purpose of this check is to make sure that each ignition system is working on it’s own. So why do we see a drop in RPM?
The drop in RPM is due to less efficient fuel burn as only one spark plug is producing a spark which makes ignition less efficient. How much drop is acceptable is very specific to each aircraft, and it is imperative to know these numbers and check properly, not merely glancing over for a drop of any kind. The other important number that you want to be aware of, is the maximum allowable drop of the two magnetos combined, which will also be found in the aircraft manual.
What should you do if the drop on one magneto or the combined drop of both magnetos is too low? Well, simply redirect your aircraft to the parking bay and call an AMO. You will never be ridiculed for being attentive! Excessive magneto drops (or no drops) are never a good sign, and could be an indication of fouled spark plugs or loose wires. Hope that helps!
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