A magneto is a self-contained, engine-driven device that provides ignition by generating high voltage pulses independent of the aircraft battery. Each aircraft uses two magnetos - left and right - mounted on the accessory case; the left magneto fires one spark plug in each cylinder while the right magneto fires the other, so every cylinder receives dual sparks for safety. The magneto spins with the engine, its gear driving a primary coil, secondary coil, breaker points, capacitor, and distributor that together produce the hot spark the spark plugs need. Because the ignition system relies on the magneto, maintenance rules require that the magneto be inspected and a magneto check performed regularly. Overhaul is scheduled every 500 hours to guarantee the magneto remains reliable.
What does a magneto do in an aircraft?
A magneto powers spark plugs and operates separately from the aircraft electrical system, thereby guaranteeing continued operation in the event of electrical system failure. Two magnetos fire spark plugs in each cylinder, which makes the engine work. Magneto ignition systems are not attached to the battery and have remained a source of power in light aircraft.
Magnetos provide ignition to aircraft engines through spark plugs. Magnetos are used in piston aircraft engines for reliability. Magneto provides electrical power independent of the battery, allowing the engine to continue running even during a battery or electrical failure.
Magnetos provide a sufficient amount of electrical power to fire a spark plug at the right time. A magneto generates a high voltage pulse up to 20,000 volts sufficient to create the spark required by spark plugs. The left and the right magneto each fire one of two spark plugs on each cylinder.
Operators of small aircraft like Cessna planes rely on magnetos to fire the airplane's spark plugs. Two magnetos provide smoother and more complete combustion of the fuel mixture and boost power output. You can control the operation of your magnetos with your ignition switch; clicking to LEFT or RIGHT detents disables one magneto by grounding it.
A magneto is the self-contained power source of the ignition system, producing its personal high-tension flash exclusively through automatic spin, separate from the aircraft's electrical arrangement. This independence is a main cause for the dependability of aircraft engines, for the motor would keep working even if the rest of the electrical arrangement failed.
How does a piston engine aircraft magneto work?
A piston engine aircraft magneto is an engine-driven electrical generator that needs no external power from a battery as the spinning engine itself rotates the permanent magnet. The working principle is electromagnetic induction: as the magnet spins in close proximity to a coil of wire, it creates a magnetic field that repeatedly collapses and rebuilds, producing current.
The primary winding of the coil consists of about 200 turns of heavy gauge copper wire. One end of this primary coil is connected to a set of cam-operated tungsten breaker points that are normally closed, grounding both ends of the primary coil. When the breaker points close, they complete the primary electrical circuit, allowing current to flow and a magnetic field to build in the coil's core. Just before ignition, the cam opens breaker points at the proper piston position. Opening the breaker points interrupts the flow of current in the primary coil winding and causes the magnetic field in the coil's core to collapse suddenly. This collapse produces a 200-300 volt spike in the primary coil.
The secondary coil, wound with many thousands of turns of fine wire around the same core, transforms that spike: the voltage spike in the primary induces a voltage in the secondary winding that is about 100 times larger, so the secondary coil generates 20,000 to 30,000 volts. The high-tension lead of the coil is connected to a rotating wiper on a large distributor gear that turns at one-half crankshaft speed. As the wiper passes in close proximity to individual electrodes connected to spark plug lead wires, it routes the high-voltage pulses to fire spark plugs in the correct firing order.
Because each cylinder has two spark plugs, the left magnet fires one spark plug per cylinder and the right magnet fires the other spark plug per cylinder. The spark plugs ignite the fuel-air mixture, and the controlled explosion lines up with the piston reaching the top of the cylinder, firing the engine. An ignition switch determines whether electricity from the magnetos is forced to flow through the spark plugs or bypasses them and flows to ground through the engine block and airframe. Breaker points must be set with the recommended gap so that they are just opening on the No. 1 cam lobe. Erosion from arcing leads to unstable magnetic field collapse when the breaker points make contact, so magneto points are inspected regularly to guarantee reliable ignition.
What are the components of a magneto ignition system in aircraft?
The components of a magneto ignition system include a magneto, distributor, spark plug, and a capacitor. These key components work together to generate spark for an internal combustion engine. The magneto produces electrical current through rotational motion, a breaker interrupts the current flow, a condenser prevents arcing at the points, and high-tension wires lead to the spark plugs. Together, these components ensure reliable ignition in various engines, especially in older vehicles and small engines.
The distributor in the magneto ignition system regulates spark distribution in the correct sequence among the spark plugs. It uniformly distributes the ignition surge to all the spark plugs. The spark plug has two electrodes separated from each other. A high voltage flows through it, leading to the generation of a spark used to ignite the cylinder's combustion mixture, such as oil. The spark plug consists of a steel shell and an insulator. The central electrode is connected to the ignition coil's supply, while the outer steel shell is grounded, effectively insulating both. A small air gap between the central electrode and the steel shell generates the spark. The central electrode is constructed with a high nickel alloy to withstand high temperatures and resistances. The capacitor is a simple electrical capacitor consisting of two metal plates separated by an insulating material at a distance. Generally, air is used as the insulating material, but in some cases, a high-quality insulating material is employed for specific technical requirements.
When the engine starts, it initiates the rotation of the magneto, generating energy in the form of high voltage. One end of the magneto is grounded through a contact breaker, and the ignition capacitor is connected in parallel to it. The contact breaker is regulated by a cam, and when it opens, current flows through the condenser, charging it. As the condenser functions as a charger, the primary current flow reduces, leading to a decrease in the overall magnetic field generated in the system, thereby increasing the voltage in the condenser. The increased high voltage in the condenser acts as an EMF (Electromotive Force), producing the spark at the appropriate spark plug through the distributor. Initially, with a low engine speed, the voltage generated by the magneto is low, but as the engine's rotating speed increases, the voltage generated by the magneto and the current flow also increase. To facilitate engine kick-starting, an external source such as a battery can be used to avoid slow engine starts.
What are the types of aircraft magnetos?
The predominant aircraft magneto types are PowerUp Magnetos, Bendix dual magnetos, and Bendix single magnetos. High-tension system magnetos used on aircraft engines are either single or dual type magnetos. Low-tension magnetos are the majority used on general aviation engines and have a rotating magnet, cam-operated breaker points, and a capacitor.
A dual magneto contains left and right magnetos in a single housing, providing two complete magnetos in one magneto housing. Dual magnetos incorporate two distributors, two sets of breaker points, two coils, and two capacitors. Dual magnetos provide two independent ignition systems, offering redundancy when one magneto fails. Dual magnetos generally use one rotating magnet and a cam common to both magnetos. These magnetos provide two spark plugs per cylinder, producing smoother and more complete combustion, refining engine performance and efficiency.
Why is aircraft magneto inspection performed?
Aircraft magneto inspection is performed to safeguard lives and keep aircraft operational by catching ignition issues before they escalate. Faulty ignition causes twenty percent of engine failures, so magnetos are required to be inspected and overhauled at 500-hour intervals by many OEMs because beyond 500 hours, magnetos operate unreliably. The inspection includes checking for damaged or worn distributor gear, internal parts, incorrect timing, fuel delivery issues, cleaning, replacing parts, repainting housings, and functional tests. Internal timing involves setting the point gap and E-gap.
Magneto check is a vital diagnostic tool that proves each ignition system operates without help from the other and validates both ignition systems independently because the double ignition system provides higher safety. Pilots perform pre-takeoff magneto checks in the run-up area adjacent to the runway during the before-takeoff checklist. Pilots switch the left mag, then both, and then right mag and note any RPM drops. A mag check that shows no RPM drop indicates the mag is hot and reveals gross ignition issues, rough running points, dead magneto, fouled plugs, timing faults, or carbon tracking inside distributors. After landing, mechanics open the mags and inspect further for carbon tracking or pit and erode on breaker points, which causes magneto mortality and leads to loss of a tooth on the plastic distributor gear, causing spark plugs to fire randomly.
External timing is performed with the mags on the engine during annual inspection and 100-hour inspections. A certified technician can perform a magneto overhaul that involves completely disassembling, inspecting, replacing all required parts called out in the inspection manual, performing a complete functional test, replacing seals, and repairing every 500 hours. A magneto exchange is a service where a technician swaps a removed magneto for an overhauled unit to reduce downtime. Manufacturers recommend 500-hour inspections even though the FAA does not require a 500-hour inspection.
Magneto examination is a basic precaution against engine malfunction. During a regular 500-hour examination I performed on a high-time trainer aircraft, a dedicated inspection of each ignition system separately showed a substantial fall in RPM on the port front. The motor ran smoothly on both magnetos collectively, yet the ineffective discharge from that ignition system raised concerns. More disassembly exposed badly worn contact points and a fragile capacitor. The broken supplier block was within one of its ignition systems, so the ignition system misfired entirely and failed to give the flash required. This breakdown happened because the regular check had been delayed and the succeeding analysis traced the defect to the damaged supplier area. The issue could have caused a whole mechanical malfunction under high-demand circumstances, heightened gas use, and bumpy motor procedure. I observed the impacts of an uncared-for ignition system when a single-engine aircraft witnessed a full motor malfunction soon after departure. These examinations proactively determine and remedy potential flaws that can show in aviation, consolidating my realization that we conduct these checks to prevent such failures.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
