I have logged at least 1,500 hours flying behind one of those combined magnetos, and I have to say, I wondered about the concept, especially when the clouds were on the ground or it was a dark night. Are two magnetos in one box really enough?
The magneto I’m talking about has a single drive from the engine, but contains two totally independent mags in a single unit. The most popular of the series, as far as I know, is the Bendix 3000 family of dual mags.
The single drive dual mag is used mostly on the Lycoming four-cylinder 360 engines, but it is also used on others. The reason for the design is to overcome a shortage of accessory drive pads on the rear of the engine.
Aircraft piston engines need to power several accessories, including magnetos for ignition, sometimes fuel pumps for fuel injection, vacuum pumps for instruments and to inflate deice boots, and alternators.
Within the engine is a set of gears rotated by the crankshaft. The gears typically rotate the camshaft to open and close the exhaust and intake valves in the cylinder heads. And the same gear train in most engines powers the accessories, including the mags.
I understand why a magneto is called an accessory because it is not in the core of the engine like a crankshaft or piston. The mag is bolted onto the outside of the engine. But the word accessory implies that the mag is an option, like adding chrome wheels to your new car. Now that’s an accessory. But a mag is absolutely essential to operation of the engine. That’s why we have two mags because the engine won’t run without at least one providing the spark for ignition.
So, does putting both mags in a single unit and driving that unit with a single shaft from the engine offer the same redundancy as two independent mags? At first, the answer would appear to be no. If that common drive coupling to the engine fails, both mags cease to product spark and the engine quits.
But if you think about that single-point failure potential a little more, the single drive dual mag doesn’t seem so bad. After all, there is only one gear train in the engine to drive mags no matter how many mags there are. If those gears fail, all mags on the engine do too. And essentially all magneto problems involve the components inside the mag, not the drive coupling to the engine. The dual mag has independent points, rotor, coil and so on making the operating components redundant.
Most of my hours flying with the dual mag were in Mooney 201 airplanes powered by the Lycoming IO-360 fuel injected engine. The dual mag never let me down, though it seemed to be harder for the mechanics to time correctly, and it also seemed to need overhaul more frequently than conventional single mags. But those are only recollections with no real substantive data to back them up.
In looking at more than 30 years worth of NTSB accident reports I haven’t seen any pattern of more frequent failure of the dual mag leading to an accident. Of course, if the mag failed and the pilot landed successfully without a lot of damage the mag failure would never make it into an NTSB report because by NTSB definition there was no accident.
But the other day I came across the report of a fatal accident in a Mooney 201 where the NTSB determined that the dual mag had failed leading to a loss of engine power. The Mooney was flying over the inhospitable terrain of a dry lake bed with jagged salt deposits and deep crevasses that ripped into the airplane on landing making the crash unsurvivable.
NTSB investigators found that the dual mag on the Mooney engine performed normally when tested after the accident, but also found evidence that the mag rotated freely in its mounting to the rear of the engine. Magnetos are held to the engine case by clamps. Mechanics loosen the clamps to rotate the magneto so its spark is timed to the correct angle of the crankshaft. Once the timing is set the clamps are tightened to hold the mag in place.
The NTSB could not determine why the mag in the crashed Mooney was loose because all of the proper hardware was still in place. But when the mag rotated out of position it would have changed the spark timing disrupting engine operation, making the engine run rough, and eventually lose power.
The clamps holding any mag could come loose so that situation is not unique to a dual mag. But if one of the two mags usually found on an engine were to come loose and rotate out of proper timing position the engine would run rough because of the mistimed spark. What step does every engine roughness checklist include? Check each mag, right? If one of two mags has come loose and slipped out of time the engine will run smoothly when you turn off that mag. With only one mag functioning the engine won’t produce quite full rated power, but will run perfectly fine to fly you to a runway.
So all my worry about gears grinding up, or that common single drive shaft failing in the dual mag were misplaced. It’s the possibility of the mag clamps coming loose and the mag rotating out of timing that can get you.