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I had been flying a PA-28 Archer III for quite a few months before I sat down to properly read through the POH. Something stood out to me about the starting procedure (emphasis mine)

Open the throttle lever approximately 1/4 inch. Turn ON the battery master switch, alternator switch, left magneto switch and the electric fuel pump.

Move the mixture control to full RICH, verify the propeller area is clear and engage the starter. When the engine fires, release the starter switch, and move the throttle to the desired setting. Turn ON the right magneto switch. Check the oil pressure for a positive indication.

Having previously flown aircraft where a key operates the starter, and that key has to travel past left, then right magneto to the start position I was starting with both Mags in the ON position (For those unaware, the Archer III varies in that the left & right magneto are operated by a rocker switch).

The obvious answer to the question of "Why start the engine with only 1 magneto live?" is "Because that is what the POH says to do!" - I am nonetheless interested in the electrical or mechanical reason why? Or perhaps there is some safety aspect at work?

Does anyone know the details?

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The reasons seems to be that only the left magneto is equipped with an impulse coupling which delays and intensifies the spark during engine starting:

Many opposed reciprocating engines are equipped with an impulse coupling as the auxiliary starting system. An impulse coupling gives one of the magnetos attached to the engine, generally the left, a brief acceleration, that produces an intense spark for starting. This device consists of a cam and flyweight assembly, spring, and a body assembly.

[...]

The impulse coupling has performed two functions: rotating the magneto fast enough to produce a good spark and retarding the timing of the spark during the start cycle. After the engine is started and the magneto reaches a speed at which it furnishes sufficient current, the flyweights in the impulse coupling fly outward due to centrifugal force or rapid rotation. This action prevents the two flyweight coupling members from contact with the stop pin. That makes it a solid unit, returning the magneto to a normal timing position relative to the engine.

(flight-mechanic.com on Impulse Coupling, emphasis mine)

On an aircraft with key operated ignition (where the start position comes after both), the right magneto is grounded while you hold the key in start:

Most airplanes have an impulse coupling on the left mag only. During engine start, the ignition key grounds out the right mag, so only the left mag provides spark. The timing of the spark from the left mag is retarded (later than usual) during engine start so the engine will start easier. Once the ignition key is released from the start position, the timing of the spark is restored to normal, and the right mag starts firing.

(askacfi.com)

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    $\begingroup$ The POH of the Archer III I fly one states that both mags should be on for startup. Maybe newer versions ground the right mag when the starter is engaged ? $\endgroup$ – Quentin H Sep 8 at 8:21
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    $\begingroup$ Either that or the right mag also has an impulse coupling. But more likely the right mag is grounded by the starter. The system description in the POH is probably not detailed enough to include this information, is it? $\endgroup$ – Bianfable Sep 8 at 8:32
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    $\begingroup$ @QuentinH this is interesting. The checklist which is taken from the one I fly just specifies "Mags On" but the POH I was looking at was just one I found online for an Archer III. I shall have to check the actual POH of my aircraft next time I fly. $\endgroup$ – Jamiec Sep 8 at 12:15
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Magnetos perform badly at low speed, and their timing will be too advanced during start, making the spark too early. Usually the left magneto has an impulse coupler, which is a coil spring driven cam, to spin up the magneto and retard the ignition so it is at top dead center, otherwise the engine may spin backwards which is bad. The right magneto doesn't typically have an impulse coupler, so you leave it off. In a key system the start setting grounds the right mag for you.

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  • $\begingroup$ This raises the follow-on question of how much, if any, damage or extra wear "quite a few months" of turning on both magnetos on this particular aircraft this might have done. $\endgroup$ – JdeBP Sep 10 at 7:47
  • $\begingroup$ Probably very little @JdeBP. $\endgroup$ – GdD Sep 10 at 10:10

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