Magneto Services of St Louis
– 500 hour Inspections –
– Overhauls –
– Repairs –
– Installation –
Bendix / TCM S20 / S200 Series
Slick 4300 / 6300 Series
Call (636) 541-3927 for more details
The “What, Where, & How” of aircraft/helicopter magnetos
What is an aircraft magneto?
An aircraft magneto is a self-contained permanent magnet AC generator that operates on the principle of magnetic induction. The two-pole permanent magnet (north and south) rotates (engine driven) and produces alternating current (two sparks) for every 180 degrees of rotation. The magneto is independent of the aircraft electrical system and will continue to operate even if the aircraft’s primary electrical system should fail.
Where are the magnetos located on the aircraft engine?
The magnetos are located on the accessory case of the engine. There are typically two magnetos per engine and two spark plugs per cylinder (top and bottom) provides ignition spark redundancy and more efficient combustion of the fuel in the cylinder. For example, the left magneto is typically mounted on the left side of the engine accessory case. On a four cylinder engine, each magneto has four spark plug wires coming from the magneto. Two of the wires are connected to the top spark plugs on the left side of the engine and the bottom spark plugs on the right side of the engine. The right magneto, typically mounted on the right side of the engine accessory case, is connected similarly to the remaining spark plugs. If one magneto should fail then all cylinders will have a spark plug firing allowing the engine to continue to run.
How does the aircraft magneto work?
The magneto consists of a rotating magnet (rotor), coil, contact points, condenser, distributor gear, and distributor block. It also consists of three circuits; the magnetic, primary, and secondary circuits. The magnetic circuit is formed when the rotating magnets align with the pole shoes. The lines of flux created by the north and south magnets flow from north to south through the soft iron core located in the coil. If the rotor is rotated 180 degrees the lines of flux will flow in the opposite direction. A thicker wire of several hundred turns is wrapped around the soft iron ore of the coil and when the flux lines cross the windings at 90 degrees a voltage is created somewhere in the neighborhood of 200 volts as a result of magnetic induction, this constitutes the primary circuit. During this time the primary contact points are closed allowing the primary circuit to be complete. The voltage created by the primary circuit passes through the secondary windings that consist of several thousand turns of a finer wire. This in turn creates a magnetic field that opposes the field that created it. This is referred to as Lenz’s law The other end of the secondary winding is connected to the high tang on the coil. As the rotor rotates past the neutral position (point where the magnets are 90 degrees to the pole shoes) and just past the neutral position (called “E” gap) the primary contact points open. This in turn opens the primary circuit and the cotage supporting the secondary circuit stops and collapses the secondary magnetic field. The secondary magnetic field collapses 90 degrees to the secondary windings and creates a high voltage (somewhere in the neighborhood of 18,000 volts). The high voltage is transmitted from the secondary windings to the metal tang located on the ignition coil. The distributor gear contains a carbon brush that rests on the metal tang. From the carbon brush the high voltage passes to the distributor finger and rotates to each contact within the distributor block. From the distributor block the high voltage is transmitted to the spark plugs via the spark plug leads. The rotating magnet is driven by the magneto drives located on the engine accessory case.