FAA regulations for magneto checks and ignition systems in pistons

Maintaining aircraft readiness and safety hinges on a thorough understanding and consistent application of FAA regulations concerning magneto checks and ignition systems in piston aircraft. The ignition system is the heart of a piston engine, responsible for igniting the fuel-air mixture that produces power.¹ A robust and reliable ignition system, meticulously maintained, is paramount for every flight.

THE DUAL IGNITION SYSTEM: FAA’S FOUNDATION FOR RELIABILITY

The Federal Aviation Administration (FAA) mandates that all spark-ignition piston aircraft engines be equipped with a dual ignition system.² This requirement, outlined in 14 CFR § 33.37, isn’t merely a suggestion; it’s a critical safety measure. Each cylinder must have at least two spark plugs, powered by two separate electric circuits with independent sources of electrical energy.³ In most piston aircraft, this is achieved through the use of two independent magnetos – one on the left side of the engine and one on the right.⁴

This dual system provides a layer of redundancy that dramatically increases safety. If one magneto or its associated circuit fails, the other can continue to provide ignition, allowing the engine to run, albeit often with reduced performance. This design ensures that a single point of failure in the ignition system won’t lead to a complete loss of engine power, a testament to the FAA’s commitment to mitigating risk in aviation.⁵

MAGNETOS: THE SELF-SUFFICIENT SPARK GENERATORS

Unlike an automobile’s ignition system, which relies on the vehicle’s battery, aircraft magnetos are self-contained, engine-driven units that generate their own electricity for the spark plugs.⁶ This independence from the aircraft’s electrical system means that even in the event of a total electrical failure, the engine’s ignition will continue to function.⁷ This autonomy is another critical aspect of aviation safety, ensuring engine operation under various conditions.

Each magneto typically serves one of the two spark plugs in each cylinder. For example, the left magneto might fire the top spark plugs on the left bank of cylinders and the lower plugs on the right bank, while the right magneto fires the top plugs on the right bank and the lower plugs on the left bank. This intricate wiring ensures that each cylinder receives a spark from both systems when the ignition switch is in the “Both” position.

THE CRUCIAL MAGNETO CHECK: A PRE-FLIGHT RITUAL

The magneto check, a standard procedure during the pre-takeoff run-up, is more than just a routine step on a checklist; it’s a vital diagnostic tool. This check allows pilots to assess the health of each magneto individually and identify potential issues before takeoff.

The procedure typically involves running the engine at a specified RPM (usually between 1700-2000 RPM, depending on the engine and aircraft manufacturer’s recommendations).⁸ The pilot then switches the ignition from “Both” to one magneto (e.g., “Left”), observes the RPM drop, returns to “Both,” and then switches to the other magneto (e.g., “Right”), again observing the RPM drop.⁹

Key observations during a magneto check:

• RPM Drop: A slight, smooth drop in RPM is expected when switching from “Both” to a single magneto. This indicates that one ignition system is being temporarily disabled, and the engine is operating on a single magneto. The acceptable RPM drop limits are specified in the aircraft’s Pilot’s Operating Handbook (POH) or the engine manufacturer’s service instructions. Typically, a drop of no more than 150-175 RPM on one mag and a difference of no more than 50 RPM between the two magnetos is considered acceptable. A drop exceeding these limits, or one accompanied by engine roughness, warrants further investigation by a qualified mechanic.
• Engine Smoothness: The engine should run smoothly on each individual magneto. Roughness or excessive vibration is a strong indicator of a problem, such as a fouled spark plug, a faulty magneto, or improper timing.¹⁰
• No RPM Drop (“Hot Mag”): If there is little or no RPM drop when switching to a single magneto, it could indicate a “hot magneto.”¹¹ This is a dangerous condition where the magneto is not properly grounded when selected to the “off” or “single” position, meaning it continues to fire even when it shouldn’t.¹² A “hot mag” poses a significant safety risk, as the engine could inadvertently start with even slight propeller movement when the ignition is supposedly off.¹³ This requires immediate attention from a mechanic.
• Idle Cut-Off Check: Before shutdown, a quick “idle cut-off” check is performed by momentarily switching the ignition to “Off” from idle RPM. The engine should stumble and begin to quit, confirming that both magnetos are properly grounding. If the engine continues to run, it’s another sign of a “hot mag” and a serious safety concern.

These checks are not just about compliance; they are about applying the principle of social proof in aviation safety. Every pilot, through generations of training and practice, performs these checks, reinforcing their importance and efficacy. When you perform a magneto check, you’re not just following a checklist; you’re participating in a universally accepted standard of safety validated by thousands of hours of flight operations.

IGNITION SYSTEM MAINTENANCE: BEYOND THE CHECKLIST

While pilots perform pre-flight magneto checks, the deeper maintenance of ignition systems falls to certified aviation mechanics. Regular maintenance, as per FAA-approved schedules and manufacturer guidelines, is essential for the continued airworthiness of piston aircraft.¹⁴ This includes:

• Spark Plug Inspection and Replacement: Spark plugs are critical components and are prone to fouling from lead deposits or oil, especially in certain operating conditions.¹⁵ Regular inspection, cleaning, gapping, and replacement are vital.
• Magneto Inspection and Overhaul: Magnetos have internal components like breaker points, coils, and distributors that wear over time.¹⁶ Periodically, magnetos require inspection, repair, or overhaul by specialized shops.¹⁷
• Ignition Harness Inspection: The ignition harness, which carries the high-voltage current from the magnetos to the spark plugs, can suffer from wear, cracking, or insulation breakdown.¹⁸ Regular inspection for chafing, damage, or arcing is crucial.
• Ignition Timing: Proper magneto timing is essential for optimal engine performance and efficiency.¹⁹ Incorrect timing can lead to excessive RPM drops during checks, engine roughness, or even engine damage.²⁰

Adhering to these maintenance protocols demonstrates consistency in ensuring the longevity and safety of the aircraft. It’s not about one-time fixes but a continuous commitment to the highest standards of aviation maintenance.

THE AERO CENTER: YOUR PARTNER IN PISTON AIRCRAFT MAINTENANCE

At The Aero Center, we understand the critical role of the ignition system in your piston aircraft’s safety and performance. Our highly skilled and FAA-certified mechanics specialize in single-engine and twin-piston aircraft, providing comprehensive magneto checks, ignition system diagnostics, and expert maintenance. We pride ourselves on our meticulous adherence to FAA regulations and manufacturer specifications, giving you the authority of knowing your aircraft is in the most capable hands.

Serving the aviation community across California, Arizona, and Nevada, The Aero Center is uniquely positioned to minimize your aircraft’s downtime. As the only 24/7 maintenance center in the area, we are dedicated to getting you back in the air quickly and safely, without compromising on quality or regulatory compliance.

Footnotes:

1. Federal Aviation Administration. 14 CFR § 33.37 – Ignition system.
2. AOPA. The Magneto Check.
3. Lycoming Engines. Service Instruction No. 1014M – Magneto Check.
4. Aviation Safety Magazine. Magneto Checks.

The Aero Center is located at William J. Fox Airfield KWJF | Lancaster, CA. Contact us at 209.885.6950 for questions or appointments.