How to Check Magnetos on an Airplane

Magnetos are electrical generators that provide electrical current to the spark plugs in an internal-combustion engine.They are similar to the generator in a car, and were in fact once commonly used in cars. Airplanes usually have two of them, for redundancy's sake. Prior to flight, it is important that they be checked by the pilot, to verify that both of them are running properly. Without them the engine will quit.

Difficulty: Easy

Instructions

Things You'll Need:

• A propeller-driven airplane with magnetos

1. Run the engine for at least 10 minutes. Typically, pilots wait until they have taxied their aircraft from parking to a holding area near the end of the runway from which they will depart.

2. Position the plane so that the prop wash is not directed at other aircraft, vehicles or people.

3. Set the parking brake.

4. Gradually open the throttle until the engine RPM on the tachometer reaches the prescribed run-up speed. Typically, for a light single-engine plane, this should be about 1,800 RPM. Let the engine run for a few seconds and verify the the RPM is stable at the prescribed engine speed.

5. Verify that the oil pressure and temperature are normal (needles should be in the green area of each instrument).

6. Look at the ignition key. Right now, it should be set to "Both." This means that both magnetos are engaged. This is the normal setting used for flight.

7. Turn the key one notch left, to the position labled "Right." Now watch the tachometer closely. The needle should drop by a about 100 RPM. By doing this, you are shutting off the left magneto and verifying that the right magneto is working. The drop in RPM happens because one magneto produces slightly less electrical power than two magnetos working together. If you do not see a drop in RPM, this is an indication that the left magneto was not working to begin with.

8. Switch the ignition key back to "Both" and allow the RPM to stabilize.

9. Now switch the key two notches to the left, to the position labeled "Left." Repeat the check you just did in Step 7. This time, you are verifying that the Right magneto is working. The RPM drop should be about the same as in the previous check. If the drop you get with the Left magneto is significantly different than the one with the Right, this could indicate a problem.

10. Switch the key back to "Both" and let the engine stabilize.

Tips & Warnings

• Always consult the operating manual of the specific aircraft you are flying to determine its specific RPM settings and tollerances. This guide is a generic description intended only to give the reader a general understanding for how the procedure works.