MAGNETOS
In a spark ignition engine, a spark ignitions the fuel-air mixture in the cylinders which is made of a few different items:
Magnetos
Spark plugs
High-tension leads
Ignition switch
Magnetos are electromagnets that generate an electrical current when the engine is running. Each magneto operates independently to fire one of the two spark plugs in each cylinder (total of eight spark plugs for a four-cylinder engine). It provides a high voltage spark, up to 20,000 volts required to ignite the fuel/air mixture in the cylinder.
The magnetos are attached to the crank shaft at the back of the engine on the accessory drive.
The operation of the magnetos is controlled in the flight deck by the ignition switch. It controls either the RIGHT or LEFT magneto, or BOTH when selected.
The switch has five positions:
OFF
R (right)
L (left)
BOTH
START
Why a left and right magneto?
The magneto system is set up as a dual ignition system for redundancy.
Magneto failure
The magneto’s run completely independent from the aircraft’s electrical system. This means that if the aircraft’s electrical system fails the engine will still operate.
If one of the magnetos fails, the other magneto is not affected. The engine still operates normally, but with a slight decrease in engine power. If one of the two spark plugs fails in the cylinder, the same result is expected.
Losing both magnetos at the same time is very rare to almost never happens.
System Malfunctions
Any ignition system malfunctions may be identified while performing the pretakeoff checklist by observing the decrease in RPM. A small decrease in engine RPM is normal during the magneto check. The parameters for the decrease are listed in the aircrafts AFM/POH.
If there is “No Drop” in engine RPM the aircraft should not be flown.
If the engine stops running when switched to either the LEFT or RIGHT magneto or if the RPM drop exceeds the allowable limits as per the AFM/POH, do not fly the aircraft have a certified mechanic look at it.
There are a few things that could cause this problem:
Fouled spark plugs
Broken or shorted wires between the magneto and the plugs
Improperly timed firing of the plugs
Engine Shutdown
After engine shutdown turn the ignition switch to OFF along with the battery and master switch to OFF. If the ignition switch is left ON, even though the master and battery are OFF the engine can fire and turn over.
With the ignition switch in the OFF position, if the ground wire (P-Lead) between the magneto and the ignition switch becomes disconnected or broken, the engine could accidentally start if the propeller is moved.
References:
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