Mag Checks — There for a Reason

This is one you don’t hear about every day, and it concerns something that all piston engine pilots do and take for granted. The activity is called the mag check, and it verifies proper performance of the magnetos that provide the high voltage to our spark plugs before we leave the ground.

BACKGROUND: Most airplanes flying with piston engines these days have 2 sets of spark plugs and 2 magnetos. Each magneto is attached to one of two spark plugs installed in each cylinder — for a four cylinder engine, this translates to one magneto controlling the top set of spark plugs on one side and the bottom set on the other. In this way, if a single magneto should fail, the engine will continue to run from ignition provided by the second magneto and the second set of spark plugs — and the pilot will be able to make a safe and controlled landing.

In this case, our pilot had just finished a long leg, and had refueled both himself and his airplane during a pit stop. After he ate, he walked over to the plane and performed a fairly good preflight. By fairly good, we mean he didn’t just check the fuel caps and drain the sumps as many pilots would — he walked around the plane, and checked everything on the checklist.

From there, he jumped in the plane and started the engine. In accordance with his checklist, he made sure that his oil pressure was good, and that his oil temperature was acceptable. With the engine running well, and all the parameters showing ‘in the green‘, he taxied out to the active runway for departure.

Following his checklist, he advanced the engine to 1700 rpm and checked his magnetos. This check actually sets your plane up to run on only one magneto, so you can look for roughness that would indicate a fouled spark plug or bad connection on a specific magneto. Running on one plug per cylinder provides a less efficient spark to the fuel/air mixture in the cylinder and results in a slight drop in engine rpm (usually between 80 and 150 rpm).

The left mag check was a little odd — he really didn’t see or hear a drop in the engine rpm. He checked it several times with the same results. Not knowing what the lack of a drop implied, he checked the right magneto next. In this case, he did see a drop — the engine dropped to 0 rpm and shut down!

After getting the plane started back up again, he was able to taxi it back to the local FBO for a checkout. They repeated the test, and obtained the same results — the engine quit while testing the right mag. The FBO pulled the right magneto out of the airplane, and found that it had failed, with a sheared shaft. Had he taken off in this configuration, he may have been able to make it to his destination without event, but if the second mag failed, he would have been flying a glider without a very good lift to weight ratio!

Our pilot stuck to the checklist and found the gremlin before it found him.

WE DON’T KNOW WHEN THE MAG FAILED, and neither did the pilot. It may have failed in the air on the trip out, or it may have failed on the ground as the engine was started or shutdown. But it hardly matters. One way or another, it did fail, and deprived the plane of one of its two high-voltage electrical systems.

BOTTOM LINE: This can happen to anyone… that means you! The items on your airplane’s checklist are there to protect you. Don’t take shortcuts on the checklist, even in the case of a short stopover. Since you can never tell when something will fail, you can never skip a step or action in a checklist.