What to Do When a Magneto Fails?

A magneto is a self-contained generator that produces high-voltage electricity to fire an aircraft engine's spark plugs. It functions independently of the battery or electrical system, so although there are modern replacements like electronic ignition systems, magnetos are still a highly dependable ignition source for piston-powered aircraft, especially in emergency scenarios. However, they are still prone to damage and failure, so this blog will cover some common faults that may arise and troubleshooting techniques to ensure continued engine performance.

How an Aircraft Magneto Works

Before learning how to address magneto faults, it is imperative to understand their operation and design.

Basic Operation

1. As an aircraft engine turns, permanent magnets mounted on a rotating rotor spin past a coil of wire, producing a magnetic field.
2. When the contact points open at a precise moment, the magnetic field rapidly collapses. This change in flux induces a high-voltage current in the secondary coil through electromagnetic induction.
3. The induced current is then directed to a spark plug, igniting the air-fuel mixture in the corresponding engine cylinder.

Key Components

• Permanent Magnets: These magnets create the primary magnetic field needed for induction.
• Coils: The primary coil stores the initial magnetic energy, while the secondary coil transforms it into the high voltage required for spark generation.
• Contact Points: These mechanically-timed switch contacts open and close in coordination with engine rotation to control when the magnetic field collapses.
• Capacitor: Connected in parallel with the contact points, the capacitor absorbs the voltage spike when the points open, preventing arcing and ensuring efficient energy transfer.

Common Magneto Failures

1. Capacitor Failure

• Cause: Dielectric degradation (short circuit) or conductor breakage (open circuit).
• Effect: When a short circuit occurs, all power is absorbed with no spark output. Meanwhile, in an open circuit, voltage dissipates prematurely.

2. Coil Breakdown

• Cause: Insulation failure or wire breakage due to heat, vibration, or mechanical stress.
• Effect: Decreased efficiency or complete coil failure from arcing across breaks.

3. Contact Point Wear

• Cause: High resistance from capacitor failure.
• Effect: Reduced current flow in the primary coil and weakened output in the secondary coil.

4. Magnet Weakening

• Causes: Mechanical shock or improper handling.
• Effect: When magnets lose their magnetism, the system’s voltage is decreased.

Other Signs of a Failing Magneto

In addition to the above faults, keep an eye out for:

• Loss of engine power
• Failed ignition of spark plugs
• Timing irregularities

Troubleshooting a Faulty Magneto

Perform a Cold Cylinder Test

The first course of action is typically to carry out a cold cylinder test, which begins with shutting down the engine and allowing it to cool. Then, restart the engine and operate it at low RPM for about two minutes. After shutting it down again, carefully check the temperature of each cylinder, preferably using an infrared thermometer. A noticeably cooler cylinder may indicate a misfiring plug, weak spark, or another ignition-related issue.

Inspect Spark Plugs

If a cylinder is suspected to be underperforming, remove and examine the spark plug, as they are susceptible to the following issues:

• Lead fouling: Caused by the accumulation of lead from aviation fuel additives, often appearing as grayish or glazed deposits.
• Oil fouling: Results from oil leaking past worn piston rings or valve guides, leaving wet, black residue.
• Fuel fouling: Occurs when the air-fuel mixture is too rich, leaving dry, sooty carbon deposits.
• Resistor degradation: The internal resistor may break down over time, increasing resistance and weakening spark output.

Clean and test each plug with the proper tools, checking for correct gap, resistance, and general condition, and then reinstall serviceable plugs that remain within specifications. Meanwhile, replace any that show signs of excessive wear.

Evaluate the P-Lead

The P-lead is the wire that connects the magneto to the aircraft’s ignition switch and grounds the magneto to stop it from firing when switched off. To test functionality, the P-lead from the suspect magneto can be disconnected, effectively ungrounding it. With the lead removed, restart the engine to see if the magneto fires independently.

However, one must use extreme caution and follow safety procedures when doing this, as a magneto becomes “hot” when ungrounded, meaning it can produce a spark even if the engine is off.

Rule Out the Ignition Switch

Ignition switch failures are relatively uncommon, but a compromised ground path through the switch can lead to intermittent or incomplete magneto shutdown, resulting in unreliable ignition behavior. If both the spark plugs and magneto appear to be operating correctly, inspect the ignition switch for proper continuity and secure electrical connections.

Preventative Maintenance Tips

• Regularly inspect all magneto components, especially before and after extended use.
• Replace aging or worn parts based on manufacturer guidance.
• Use a magneto test bench or magnet strength tester to verify that the rotating magnets produce adequate magnetic flux for consistent spark generation.
• Document and track any abnormal engine behavior, especially during magneto checks in pre-flight runups, to detect gradual performance decline early.

Secure Magnetos from a Dependable Source

In summary, familiarity with a magneto’s components, functionality, and typical malfunctions can help operators swiftly identify problems and perform repairs. Another crucial step in upholding optimal combustion efficiency is having reliable access to high-quality magnetos, spark plugs, coils, capacitors, and other aircraft ignition system parts. ASAP Semiconductor is a leading distributor for such items, offering an extensive selection from leading manufacturers and suppliers through its website, ASAP Parts Services.

With a plethora of industry-standard options, an organized database, and dedicated customer service, this platform reduces the complexity of sourcing. We also prioritize providing the most convenient purchasing options for those who shop with us, where they can always benefit from fair pricing, timely fulfillment, and solutions catered to their procurement needs. Taking this into account, we encourage you to check out our inventory and connect with a team member for service at your convenience.