It really only takes three things to make our piston engines go: air, fuel and, of course, a spark to set things burning and those pistons churning. We, the pilots, are responsible for making sure our engines have enough fuel to mix with air (and enough air, for that matter) to make it all burn. We pretty much know where the air and fuel come from, but what about that spark? The answer is an engineering marvel, but an ancient one.
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As pilots in the 21st century, more than 100 (nearly 110!) years after the invention of the first flying aircraft engine, we rely, amazingly, on nearly the same technology to generate spark today as did the Wright Brothers. Yes, the magneto is older than the Wright Flyer, and its relatively simple and nearly bulletproof technology has carried humans aloft for decades. For the most part, properly maintained and serviced, magnetos just work. But things can go wrong, and when it does, some knowledge about the system is invaluable.
Magneto Basics
Inside the unremarkable black casing of a magneto is a rotating two-pole magnet mounted on a shaft that is driven by the aircraft engine. As you might imagine, just as in a compass, iron plays a role in getting the magnetism going. Surrounding the magnet are “pole shoes” of soft iron, creating a U-shape with open upturned ends onto which the wire-wrapped coil sits. The rotating magnet keeps swapping the polarity of the iron U, creating magnetic pulses of alternating current. Add a condenser, a contact breaker, a distribution system (opening and closing points) and a wiring harness to carry the pulses to the proper spark plugs, where the carefully timed and measured pulses can be released as sparks, and the engine runs.
Engineers, being conservative types, prescribed early on that aircraft engines carry two magnetos. One is enough to power the engine, but two is better. With two magnetos powering a top and bottom set of spark plugs, generating two sparks per cylinder, the air/fuel mixture burns more completely, generating more power. That’s always a good thing. Even better is that if one magneto quits, there’s a second one to keep the engine running, although at a slightly reduced power output, to help you get you home.
Of course, you’ve got to get the engine going first. Fortunately, the magnetos are basically charged and ready for business from the time they are magnetized on the work bench or the factory floor. Yep, they are always on. Only a P-lead grounding each magneto keeps it from sending pulses of AC to the spark plugs until commanded to do so by the control switch.
The starter, or in the case of an airplane with no electrical system, the human arm, is required to swing the propeller and crankshaft, engaging an impulse coupling typically built into the left magneto. This mechanical device generates the energy for the magneto to produce the first hot spark necessary to get the engine going (you can hear it click and feel the resistance of its spring winding up if you pull a duly equipped propeller through an arc, slowly please, and only after verifying the magneto switches are off and the P-leads are grounding the mags). Impulse-coupled magnetos actually fire a “retarded” spark, timed to get the engine started. As soon as the rotating engine comes to speed, the impulse coupling decouples with centrifugal force and the timing advances to normal, which is typically 20-30 degrees before a cylinder’s top dead center (TDC) position. If the impulse coupling does not decouple, you’ll know it by the misfiring. Shut down the engine before damage occurs.
Some aircraft forgo impulse couplings for a “shower of sparks” or “retard-breaker” that can be heard functioning by a buzzing sound with the master switch on and the starter engaged, just before start. These aircraft cannot be hand-propped and need a good battery, which sends a pulse of direct current to a vibrator that generates the sparks to get going. Finally, the SlickStart system arrived on the scene in the late 1990s, providing a solid-state replacement for the “shower of sparks” mechanism and the hottest spark yet.
Timing is Everything
For most pilots of single-engine aircraft, all they know about magnetos they learn on the ground. They know the start sequence for their engine, be it on one magneto (typically the left, with its impulse coupling or retard-breaker) or two. Turning the ignition key energizes the starter and rotates the magnetos, providing the necessary spark until the engine is shut down or the mags are switched off.
Of course, some of us fly aircraft where the magnetos are not tied into a keyed ignition switch. On these aircraft—some very old, some very sporty, and many piston twins—the mags are on individual toggle or rocker switches independent from their engine’s starter switch. I can’t tell you how embarrassing it is to grind along on the starter of an engine only to realize that you’ve missed the critical “mags-on” step in the pre-start checklist. (Many a nervous multiengine pilot candidate has flunked a checkride this way during an in-flight engine restart!) The cure to this hard-start is easy: read the checklist!
More complicated engine starting issues are often caused by a low or dead battery, that simply cannot generate enough power to the retard-breaker or to spin the starter. And then there is timing.
Have you ever heard an engine running with magnetos timed incorrectly? The hope is that you haven’t, because most of them will refuse to start. Sometimes the problem is as simple as a new, lightweight starter was installed, but it is now spinning the engine faster than the magneto’s impulse-coupling is timed for. You may experience a kickback, where the propeller actually throws itself, and the crankshaft it is attached to, backwards in the process of trying to start. This is a sign that the mags could be miswired, or mistimed. Heed it! Don’t start the engine until it’s been looked at. On aircraft engines with Bendix type starter drives, this condition has the potential to break the drive gear or even the starter housing.
And if the engine does miraculously start? Look out. The cracking-slamming-banging you hear next should be your cue to immediately shut it down before the whole thing tears itself apart.
More subtle mistiming issues are likely only to show up during the pre-flight run-up, which should be mandatory on each flight.
Know Before You Go
The pre-flight run up is the next best place to discover any issues with your magneto system. A medium-power run up is typically prescribed by most aircraft checklists. Once the engine is turning at about 1800 rpm, 1700 for Continental engines, or generating 20 inches of manifold pressure, the checklist asks the pilot to select one, then the other magneto, and check to see if:
• the engine continues to run with only that magneto selected;
• it runs smoothly, powering its one bank of spark plugs;
• there is only a slight (100 rpm or so) loss of engine power with a single magneto selected; and,
• the magnetos, when selected individually, ground out the other magneto (guaranteeing you are only running on the one selected magneto).
If the engine runs rough on either magneto, you might suspect that lead is fouling your spark plugs. To correct the issue, set the ignition on both mags, run the engine up to full cruise of about 2300 rpm and lean the mixture to best power for about a minute to see if you can melt the lead contamination off the spark plugs, restoring the air gap that electricity must jump to spark properly. Reduce the power and try the normal magneto check again. If the engine runs smoothly, you’ve fixed your problem. If it does not, run up the engine again and watch your EGTs on your engine analyzer. They should rise in sequence when one mag is selected, and fall in sequence when both mags are selected. If that does not happen, it’s time to return the airplane to the maintenance hangar. You may have a spark plug or ignition harness problem. It could also be a mis-timed magneto. You won’t know until maintenance has had a chance to check it out. Some additional thoughts on mag checks are in the sidebar at right.
Never Leave on One
No matter what you do at this point, don’t be tempted to fly on only one magneto. Sure, the airplane will probably fly that way. You’d have to tweak the mixture, and accept less power from the engine, but it would run—as it is designed to when a magneto fails.
But, wait! As long as you are safely on the ground with your airplane and your one functioning magneto, it is NOT an emergency. So, why suddenly create one? Remember, when the second mag goes, the engine stops running. Are you sure you want to risk a flight where 100 percent of your redundant power systems is now inoperative? Doesn’t sound like sane risk management to me.
When It Stumbles
Now that we’ve established that you are not crazy enough to ever fly on one magneto, let’s talk about troubleshooting an ignition problem that develops in flight. First and foremost, you’ve got a wonderfully redundant system here, so don’t get too distracted by any of its issues. If you are at altitude and your engine starts to run rough, keep your focus on flying, bring out your aircraft checklist and turn to the page on in-flight engine issues.
Run through the checklist. Somewhere on there, probably after it asks you to address any possible fuel flow issues, it will ask you to switch the ignition for one mag off, to see how the engine runs. If the engine quits, well, switch on the other magneto, which should get the prop powered up again immediately.
Now you know. Keep the offending mag off, adjust the mixture for the new power setting, and anticipate landing where you can get maintenance. Tell the mechanic the sequence of events. If the mag ran well until your engine was warmed up, there’s a chance the problem is in the coil. That’s a fairly simple fix, presuming the part is in stock.
If the broken mag happens to have an impulse-coupling system, you might want to land sooner than later, just in case a problem with the impulse coupling itself caused the failure. Pieces of that system rattling around loose can make a mess of your engine accessories in short time.
Finally, if you’ve got an EGT/CHT display, watch it carefully as you switch from both mags to one. You may discover one EGT is higher than the others on both mags, and drops off precipitously when you select the “rough” mag. That’s a sign that you’ve got a bad spark plug or ignition harness, not a bad mag. You’ll probably also notice that cylinder’s CHT is cooler than the others. The diagnosis may be different, but the “cure” is the same. Limp to your next stop on the mag with the good spark plugs and the smooth function, and plan on maintenance when you get there.
Flight Testing Worth Doing
That engine analyzer can even help you diagnose mistimed mags in flight, too. High EGTs could be caused by the timing of the spark being late, causing the burn in the cylinders to go on for a bit too long, flushing burning fuel/air out through the exhaust, which is not good for the life of that valve, or your exhaust system. It also results in reduced engine power.
But you don’t have to have a multi-probe engine analyzer to test for ignition issues in flight. The best test of all is to simply do a magneto check during cruise, at a safe altitude, and note whether the engine behaves well. You can tell right away whether the mags are grounding out each other properly, and running smoothly without undue power loss. And you are putting them through their paces at the temperatures and pressures where they should be operating at their best. If they are not, then, it’s time to do some maintenance.
No, it’s not a prescribed check on the aircraft’s in-flight checklist, but go out and do some research. You’ll find that some of the best mechanical minds in the industry prescribe it for diagnostic purposes. If you’ve got a two-mag system without any fancy electric ignition components an in-flight mag check should be no problem, and may just save you from big problems down the road.
