Aircraft Maintenance: Managing magnetos

Magneto technology has been around since the 1890s as a simple yet reliable method to create a self-sustaining ignition source for internal combustion engines. The principle is fairly simple: A rotating magnet induces a current in a coil of wire, amplified to a very high voltage through primary and secondary coils. This voltage is then interrupted and released through a spark plug inside the cylinder where it ignites the fuel/air mixture. A spinning rotor and distributor assembly inside the magneto directs the energy to the various spark plugs in sequence according to the cylinder firing order. The internal timing of the energy release is adjusted inside the magneto using the position of the breaker points, which open to interrupt the current flow to the coil. Externally, the position of the magneto can be rotated to control when each spark plug fires, relative to the position of the piston in the cylinder.

• Rotor: The permanent magnet attached to the drive shaft of the magneto.
• Coil: The stationary coils of primary and secondary windings where the current is induced by the spinning magnet and the voltage is amplified.
• Breaker points: The contact points used to interrupt the flow of current in the primary circuit at the right time, causing a voltage spike in the secondary winding.
• Condenser/capacitor: An electrical component connected across the breaker points to prevent arcing and ensure a clean voltage spike.
• Distributor block and gear: A rotating assembly that distributes each voltage spike and release of high-voltage electricity to the correct spark plug at the correct time.
• Impulse coupling: A spring-loaded assembly between the magneto and the engine that helps an engine start at low speeds by delaying spark timing and accelerating the speed of the rotor when released.
• P-lead: An external contact on the magneto that, when connected by a wire to the magneto switch, can be used to ground the primary coil and disable the magneto.

Mechanical magnetos are simple and reliable, but they do have several weaknesses. As electromechanical devices, magnetos are subject to electrical, mechanical, and environmental issues. Coils and condensers can become weak or fail over time because of vibration, heat, and cycles. In addition, the electrical arcing that occurs in the breaker points and within the distributor causes erosion of the contact surfaces, increased resistance, and timing drift.

The distributor block and gears are made of a non-conductive material that is capable of withstanding high voltage, but fairly fragile when compared to the metal shaft and drive gears in the engine. This means that kickbacks or other sudden stoppages of the engine can easily damage the magneto distributor and gears. The distributor can also degrade over time because of heat and environmental factors, and there have been airworthiness directives in the past pertaining to the security of the molded-in electrodes coming loose. Finally, the springs and other moving parts inside the impulse coupling are subject to wear and damage. These have also been the subject of ADs. All of this adds up to an accessory that is very reliable when maintained properly, but is guaranteed to fail if grossly neglected.

Although most aircraft engines have two separate magnetos to provide redundancy, they should never be simply run until failure. The failure of a magneto can be far more consequential than simply losing half of the spark in the engine. Because magnetos control the timing of the ignition spark, a magneto failure can sometimes alter that timing in a way that is damaging to the engine. For example, there are cases where the loss of the fragile teeth in the distributor gear can lead to gross mistiming of the spark. This can cause one of the spark plugs to fire far in advance of the proper timing, risking severe detonation. Your best defense against this type of failure is proactive maintenance, coupled with swift action in the cockpit if a failure should occur. Here are some key recommendations:

• Magneto external timing (adjusted by the position of the magneto) should be checked as part of every annual. If the timing has drifted more than a couple of degrees, it is advisable that your mechanic open up the magneto and check the internal timing (controlled by the position of the points) and inspect the internal components for damage or excessive wear.
• Magnetos should have a full inspection (with repairs as necessary) or overhaul every 500 hours, including testing of all components and replacement of key wear items including the points, seals, capacitor, and distributor components. Alternatively, you can exchange your magnetos for overhauled ones from a certified magneto repair station such as Quality Aircraft Accessories.
• Following any sudden engine stoppage or kickback, the magnetos should be opened up and inspected for damage.
• If your engine runs rough in flight, check the magnetos to see if you can isolate the issue and avoid running for a prolonged period on a bad magneto. (Always use the emergency procedures in your pilot's operating handbook as your primary guidance).

Although the basic design of mechanical magnetos has not evolved dramatically over the past century, there have been some recent developments that address some of the weak points. I met with Hartzell Engine Technologies recently to learn more about the company’s PowerUp Aircraft Ignition Systems magnetos. Under the PowerUp brand, Hartzell created parts manufacturer approval versions of existing magneto designs with a focus on making significant upgrades to address traditional weak points. These are drop-in replacements for most existing magneto types, with more designs being added all the time. When I met with Hartzell, the company had just announced PMA availability for the PowerUp versions of the Bendix S-20/S-200 series, with additional magneto model approvals expected later this year. Knowing how magnetos often fail, these new designs include the following improvements and advancements:

• New impulse coupling design using a single-piece hub with no rivets.
• Improved capacitor that is internally grounded and less prone to failure caused by rotation.
• More powerful coil with higher spark energy and a stronger physical design to resist physical failures.
• Strengthened points assembly with more clamping force and a stronger spring.
• Redesigned distributor block assembly with arc suppression.
• Improved distributor gear with a Monel electrode to resist erosion, heat-staked to improve security and stability.

I always applaud informed, incremental improvements to a time-tested design—especially when changes improve reliability. I’ve been impressed enough with the PowerUp improvements to order a set for my Beechcraft Bonanza as it approaches its own 500-hour inspection interval, and I’ll report back with my experience. I’ll cover the pros and cons of electronic ignition system options in a future article. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.