MAG Checks Demystified

One of the first things a new flight student learns on their very first day of flight training is the procedure for performing magneto checks. Basically, you are told to flip the key switch back and fourth and look for a loss in engine RPM on the tach. Because this is introduced so early in training, almost every pilot knows to accomplish these checks before flight. That being said, I rarely find a pilot with past flying experience who knows what they are actually checking for when they perform these checks.

So, let me clear things up:

First, here is an explaination of what a magneto actually is and what it does:

A magneto is a self-contained ignition system which is driven by the aircraft’s engine. Magnetos are used to generate the high voltage electricity required to fire the aircraft’s spark plugs and ignite the fuel inside the engine’s cylinders. Because magnetos are self generating, they do not require power from the aircraft’s batteries to operate. This means that if a total electrical failure is encountered, the aircraft’s ignition system will still function.

The FAA requires all aircraft engines to be equipped with a dual ignition system. This ensures that if one system were to fail, the engine would continue to run. Aircraft manufacturers meet this regulation by equipping each aircraft engine with two magnetos, one on the left side of the engine, and one on the right side of the engine (hence L and R key positions). Each magneto is geared directly to the engine and will function as long as the engine is turning.

Each magneto has five wires leading out of it (assuming its stuck to a four cylinder engine). The four big wires are the ignition leads. These wires carry the high voltage electricity generated by the magneto to one spark plug on each of the engine’s cylinders. The fifth wire is called the P-lead. This tiny, delicate wire is used to shut off the magneto and is connected to the key switched. If the P-lead becomes connected to ground, the magneto will stop producing ignition energy.

When you place the key switch in the ON position, the P-leads of both magnetos are opened, and both magnetos function normally. If the key switch is placed in the OFF position, the P-leads of both magnetos are connected to ground and the engine shuts off. If the key switch is placed in the L position, the P-lead from the right magneto is connected to ground. This will shut off the right magneto and cause the engine to be driven only by the left magneto. The opposite would occur if the key were placed in the R position.

Aircraft engines are designed to operate using two spark plugs per cylinder. Each cylinder has one spark plug fired by the left magneto, and one spark plug fired by the right magneto. If one of the magnetos is not firing (like what occurs when you place the key in the L or R position), the engine will produce less power. This will result is a drop in engine RPM.

The Three magneto checks:

1- After starting the engine

After you start the engine, it is recommended that you perform a magneto check to verify  that the key switch is wired correctly and that both mags are operating properly. To do this:

  • From both, go to left, verify a drop, then back to both
  • From both, go to right, verify a drop, then back to both

By obtaining a drop, you ensure that the key switch is grounding each magneto when it should. You also verify the engine is operating on both magnetos when on both. If there is no drop in RPM, something is wrong. You shouldn’t fly and should consult a mechanic. If there is a large drop in RPM accompanied by engine vibration, the engine likely has a fouled spark plug from starting. This is somewhat normal but should be watched. Be sure to lean the mixture out on taxi to keep the plugs clear and check it again on the second check.

2- Before Flight

The run-up magneto check primarily helps you determine the condition of the spark plugs. If you have a fouled spark plug, decreased engine performance will be encountered on takeoff. This will result in increased takeoff distance and a decreased climb rate. When you bring up the engine to the specified run-up RPM and you ground out a magneto by selecting the L or R position, you should notice a smooth drop in RPM. This drop lets you know that one magneto grounded as normal and all four plugs are firing. If the drop is large and the engine vibrates, the most likely cause is a carbon fouled plug.

If you encounter a fouled plug, don’t worry, it is a somewhat common problem. A old trick to remedy this is to attempt a procedure called “burning off the mags”. To perform this, run the engine up to a high power setting and lean the mixture until you receive a drop in RPM. Then enrichen the mixture until the RPM climbs back to its original value. leave the engine like this for one minute and monitor your T’s and P’s. This will heat up the cylinders, and will hopefully remove any soot that may be stuck to the plugs.  The name of this procedure is somewhat misleading as the magnetos themselves are not cleaned or burned. If this procedure does not fix the problem, taxi the aircraft back and have a mechanic look at it.

3- Before shutdown

Before you shut down the engine, you should perform a magneto safety check. To do this, turn the ignition switch briefly from both to off, and then back to both. when you do this, the engine should briefly shut off. This procedure assures that  the engine will not start when the key is in the off position. Performing this is necessary because it verifies that the engine will not start if the propeller is turned for maintenance, or if you need to move the prop to get the tow bar in.

Hope this clears things up


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