What It Looks Like:

The pretakeoff magneto check is one of the rituals of flying piston-powered aircraft, but just what are you checking?

First, a quick explanation of the aircraft magneto. This venerable device is actually a self-contained AC (alternating current) generator. Magnetos (two per engine) are bolted to the engine block, and a large gear inside the engine turns a small gear on the magneto shaft.

An internal magnet on the shaft spins inside two layers of wire coils to generate an electrical charge. The initial charge generated by the primary coil is fed to a set of points inside the magneto. When the points open, the electrical field in the primary coil "collapses," inducing a high voltage into the secondary portion of the coil, which is then delivered through a spinning rotor to the distributor block to the ignition leads to the spark plugs. Because magnetos generate their own electricity for the plugs, the engine will continue to run just fine even if the aircraft electrical system fails completely.

But wait, there's more: A clever device in the magneto called the impulse coupling is mounted between the drive gear and the rotating magnet. When you turn the ignition key to start the engine, the impulse coupling momentarily delays the spinning of the magnet in the coil while it applies tension to a spring. When the spring releases, it causes the magnet to spin extra fast, which generates a stronger-than-normal charge. This gives extra oomph to the current that jumps the gap between the spark plug tip and electrode, which helps start the engine. In addition to producing a hot spark, the impulse coupling delays the spark, which helps prevent engine kick-back. If you hear a clicking sound as you slowly turn the prop by hand (with magneto, battery, and alternator switches off, please), that's the impulse coupling releasing to super-spin the magnet in the coil.

When you do your pretakeoff engine runup, you set the power at 1,700 rpm (or whatever rpm is specified in the pilot's operating handbook), and check the magnetos by alternately selecting each mag on the ignition switch. When you select the Left mag switch position, for example, it grounds out the right mag, which cuts off current to the spark plugs connected to the right mag. You should notice a drop in rpm because with just one plug firing inside each cylinder, the fuel-air mixture burns less efficiently.

The pilot's operating handbook will specify the maximum rpm drop permitted for each mag, as well as the maximum allowable difference in rpm drop between the two mags. If you do not see a drop in rpm, it could mean the ground connection is not working. This calls for an inspection by a mechanic.

If engine rpm drops when you switch to one mag, but the engine runs smoothly, it probably means the points inside that magneto are wearing, which changes the timing of the mag. This is normal. If the engine runs rough, it could mean the spark plugs have fouled from an overly rich mixture. Try increasing the amount of throttle, then leaning the mixture to burn off the deposits on the plug. If that doesn't work, it could mean there's a bad spark plug, or a problem in the wiring harness, or something else only a mechanic can troubleshoot. If the rpm drop exceeds the maximum allowable, it's time to taxi back to the ramp and let the shop have a look.