If you’re learning to work on reciprocating aircraft engines, the magneto is one of the most important components of the ignition system to understand. Magnetos have been used on aircraft engines for decades because they are simple, reliable, and—most importantly—do not depend on the aircraft’s electrical system to produce ignition.

Understanding magnetos is also important for anyone pursuing an FAA Airframe and Powerplant (A&P) certificate. Magneto operation, inspection, and timing are fundamental Powerplant subjects.

In this article, we’ll look at what an aircraft magneto is, how it generates the high voltage required to fire a spark plug, and finally walk through the basic FAA-described procedure for timing a magneto to an aircraft engine.

Important: This article is educational and is based primarily on the FAA Aviation Maintenance Technician Handbook—Powerplant (FAA-H-8083-32B). Actual maintenance must be performed using the current maintenance data for the specific engine, magneto, and aircraft being serviced.

What Is an Aircraft Magneto?

A magneto is a self-contained electrical generator used to provide high-voltage electrical current to the spark plugs of a reciprocating aircraft engine.

Unlike an automotive ignition system, an aircraft magneto does not normally require power from the aircraft battery or alternator to operate.

Instead, the engine mechanically rotates the magneto. The magneto converts this mechanical energy into electrical energy and produces the high voltage needed to jump the gap of the spark plug.

Once the engine is turning, the magneto can therefore continue producing ignition even if the aircraft’s electrical system fails.

This independence is one of the major reasons magnetos have traditionally been used in aircraft ignition systems.

Why Do Aircraft Engines Usually Have Two Magnetos?

Most conventional reciprocating aircraft engines use two independent magnetos.

Each magneto supplies a separate set of spark plugs. A cylinder commonly has two spark plugs, with one plug supplied by the left magneto and the other supplied by the right magneto.

This provides two important benefits.

Redundancy

If one magneto or ignition circuit fails, the other magneto can continue providing ignition.

Improved Combustion

Igniting the fuel-air mixture at two locations in the combustion chamber allows the flame fronts to travel through the combustion chamber more efficiently.

This is why pilots perform a magneto check during an engine run-up. Selecting LEFT or RIGHT grounds one magneto and allows engine operation on the other magneto to be checked.

How Does a Magneto Work?

At its heart, a magneto combines a rotating magnetic field, an ignition coil, breaker points, a condenser, and a distributor.

The process can be broken into several steps.

1. The Engine Turns the Magneto

The magneto is mechanically driven by the engine.

As the engine rotates, the magneto’s rotor also rotates. The rotor contains permanent magnets that create a changing magnetic field through the magneto’s magnetic circuit.

2. Current Is Induced in the Primary Coil

The ignition coil contains two windings:

  • Primary winding — relatively few turns of heavier wire
  • Secondary winding — many thousands of turns of fine wire

As the magnet rotates, changing magnetic flux induces current in the primary winding.

With the breaker points closed, current can flow through the primary circuit. Energy builds in the magnetic field surrounding the coil.

3. The Breaker Points Open

This is one of the most important events in magneto operation.

At precisely the correct point in magneto rotation, the breaker points begin to open.

Opening the points interrupts current flow through the primary winding. The magnetic field around the coil then collapses rapidly.

That rapid collapse is essential because it induces a very high voltage in the secondary winding.

The E-Gap

The position where the breaker points open is closely related to something called the E-gap, or efficiency gap.

The magneto produces its strongest ignition output when the breaker points open at a specific rotor position following the magnet’s neutral position.

The angular distance between the neutral position and the position where the points open is known as the E-gap angle.

Correct internal magneto timing therefore ensures that the breaker points open at the proper E-gap position.

Key distinction: E-gap concerns the internal timing of the magneto. It is different from timing the entire magneto to the engine.

What Does the Condenser Do?

A condenser, which electrically is a capacitor, is connected across the breaker points.

When the points open, the condenser helps prevent excessive arcing across them. It also allows the magnetic field in the ignition coil to collapse more rapidly.

A faster collapse of the magnetic field produces a stronger induced voltage in the secondary winding.

Producing High Voltage

The rapidly collapsing magnetic field induces voltage in both windings of the ignition coil.

Because the secondary winding contains far more turns of wire than the primary winding, the voltage induced in the secondary winding is dramatically increased.

This produces the high voltage required to jump the spark plug gap. The result is the spark that ignites the compressed fuel-air mixture inside the cylinder.

The Distributor

Producing high voltage isn’t enough. The magneto also has to send that voltage to the correct cylinder at the correct time.

That is the job of the distributor.

As the magneto rotates, the distributor routes the high-voltage output from the coil to each spark plug lead according to the engine’s firing order.

The basic sequence is therefore:

Engine turns magneto → magnetic field changes → primary current builds → breaker points open → magnetic field collapses → high voltage is induced → distributor sends voltage to the correct spark plug → spark occurs.

The P-Lead and Magneto Switch

One characteristic of a magneto can initially seem backward:

You don’t normally turn a magneto on by supplying electricity to it.

Instead, the magneto is stopped by grounding its primary circuit.

A wire called the P-lead connects the magneto primary circuit to the ignition switch.

When the magneto is grounded, it cannot produce normal ignition. When the magneto is ungrounded, it is capable of producing a spark whenever it is rotated.

This creates an extremely important safety rule:

Always treat an aircraft propeller as though the ignition system is live.

A broken or disconnected P-lead can leave a magneto “hot” even though the cockpit ignition switch appears to be OFF. Moving the propeller could then cause a spark and potentially start or kick the engine.

Why Magneto Timing Matters

A spark must occur at the correct point during the compression stroke.

The spark normally occurs before top dead center (BTDC).

Why before TDC?

Combustion is not instantaneous. The flame front requires time to propagate through the compressed fuel-air mixture. Therefore, ignition begins before the piston reaches the top of its compression stroke so that combustion pressure develops at the appropriate time as the piston begins its power stroke.

The exact ignition timing is specified by the engine manufacturer.

For example, an engine might specify ignition at a certain number of degrees BTDC.

Never assume the timing specification. Always use the applicable manufacturer’s maintenance data.

Two Different Types of Magneto Timing

This distinction is extremely important for an A&P student.

There are two timing relationships to consider.

1. Internal Magneto Timing

This establishes the relationship between the magneto rotor and the opening of the breaker points.

The breaker points must begin opening at the proper E-gap position.

This is commonly referred to as bench timing the magneto.

2. Magneto-to-Engine Timing

This establishes the relationship between the magneto and the engine crankshaft.

The magneto must begin firing when the engine crankshaft reaches the manufacturer’s specified number of degrees before top dead center on the compression stroke.

A magneto can therefore be internally timed correctly but still be incorrectly timed to the engine.

Both relationships must be correct.

Timing a Magneto to the Engine

The following is a general explanation based on the procedure described in the FAA Aviation Maintenance Technician Handbook—Powerplant.

Important: The exact procedure, timing specification, torque values, and required tooling for a particular aircraft must come from the applicable manufacturer’s maintenance instructions.

Step 1 — Determine the Correct Ignition Timing

Before touching the magneto, determine the ignition timing specified by the engine manufacturer.

The specification will normally be expressed in degrees before top dead center, such as:

XX° BTDC

Do not simply use a timing value remembered from another engine. Similar-looking engines can have different timing requirements.

Step 2 — Remove the Appropriate Spark Plug

Remove the most accessible spark plug from the No. 1 cylinder.

This allows you to determine when the No. 1 piston is approaching top dead center on its compression stroke.

Remember that the piston reaches top dead center twice during a four-stroke cycle:

  • Once between the compression and power strokes
  • Once between the exhaust and intake strokes

For ignition timing, we need the compression stroke.

Step 3 — Find the Compression Stroke

Rotate the engine in its normal direction of rotation.

Place a thumb over the No. 1 cylinder’s open spark plug hole.

As the piston rises on the compression stroke, pressure will build in the cylinder and air will push outward against your thumb.

That tells you the piston is approaching TDC on the compression stroke.

Do we stop at TDC?

Not necessarily.

TDC is useful as a reference, but the goal when timing the magneto is to position the crankshaft at the manufacturer’s specified ignition timing position, which is normally a specified number of degrees before TDC.

The sequence is essentially:

Find compression → establish the correct crankshaft position → set the engine to the specified BTDC timing mark.

Step 4 — Position the Engine at the Specified BTDC Setting

Continue positioning the crankshaft until the manufacturer’s specified ignition timing mark aligns with the engine’s timing reference.

Depending on the engine, this may be accomplished using:

  • Built-in timing marks
  • A timing disc
  • A piston-position indicator
  • Another manufacturer-approved timing method

The final movement into the timing position should be made in the engine’s normal direction of rotation. This helps eliminate errors caused by gear backlash.

The engine is now positioned where the magneto points should begin opening and the spark should occur.

Step 5 — Verify the Magneto’s Internal Timing

Before installing or timing the magneto to the engine, its internal timing must be correct.

The magneto should be positioned to fire the No. 1 cylinder.

Internally, the breaker points should begin opening at the proper E-gap position.

Many magnetos provide timing marks, timing windows, or reference marks to establish this position.

This is the magneto’s internal timing.

Step 6 — Install the Magneto in the Firing Position

With the engine positioned at the specified BTDC timing point and the magneto positioned to fire No. 1, install the magneto into the engine accessory drive.

The magneto should initially be installed approximately in the middle of its slotted mounting flange, where applicable. This provides room to rotate the magneto slightly in either direction during final adjustment.

Install the mounting hardware, but do not fully tighten it yet. The magneto must still be able to rotate slightly for final adjustment.

Step 7 — Connect the Magneto Timing Light

Connect a magneto timing light, sometimes called a magneto synchronizer, according to the timing equipment and magneto manufacturer’s instructions.

When timing a two-magneto installation, the timing device allows you to determine exactly when the breaker points in each magneto open.

The goal is for the magnetos to reach their specified firing position at the correct crankshaft position.

Step 8 — Rotate the Magneto Until the Points Just Open

With the engine positioned at the specified BTDC mark, slowly rotate the magneto in its mounting slots.

Watch the timing light.

Adjust the magneto until the breaker points just begin to open at the specified crankshaft position.

This is the critical timing event.

The magneto should not simply be timed to TDC. It must be timed to the manufacturer’s specified crankshaft position.

Step 9 — Synchronize Both Magnetos

On an engine using two magnetos, both normally need to fire at the same crankshaft position unless the applicable engine configuration specifically uses different timing.

Connect the timing device as specified and adjust the magnetos so that the breaker points open at the specified engine timing position.

In practical terms, at the specified BTDC position:

  • Left magneto → points open
  • Right magneto → points open

They should occur according to the tolerances and specifications in the applicable maintenance data.

Step 10 — Tighten the Magneto

Once the timing is correct, tighten the magneto mounting hardware to the manufacturer’s specified torque.

Be careful not to allow the magneto to rotate while tightening the hardware.

Even a small amount of movement can change ignition timing.

Step 11 — Recheck the Timing

Never assume the timing remained correct after tightening the magneto.

Back the engine away from the timing position, then rotate it again in the normal direction of rotation.

Watch the timing light as the crankshaft approaches the specified timing mark.

The breaker points should begin opening at the specified position. For a synchronized two-magneto installation, both timing indications should occur according to the manufacturer’s specification.

If they don’t, adjust the timing again.

A Simple Way to Remember the Procedure

For an A&P practical or oral examination, I like to think of the process this way:

  1. Find No. 1.
  2. Find compression.
  3. Set the engine to the specified BTDC position.
  4. Put the magneto in the No. 1 firing position.
  5. Install the magneto.
  6. Connect the timing light.
  7. Rotate the magneto until the points just open.
  8. Synchronize both magnetos as applicable.
  9. Tighten the magneto mounting hardware.
  10. Back the engine away and recheck while rotating in the normal direction.

That captures the basic logic of the procedure.

TDC vs. BTDC — An Important Distinction

This is an easy point to confuse when first learning magneto timing.

You may hear:

“Find top dead center.”

But finding TDC and establishing the compression stroke are not necessarily the final timing position.

The ignition event normally occurs before TDC.

Therefore:

Compression stroke → locate/reference TDC → position crankshaft at specified BTDC → time magneto.

The manufacturer’s specified ignition position is what ultimately matters.

E-Gap vs. Engine Timing

These two terms describe different relationships.

E-Gap

The relationship between the magneto’s rotating magnet and the instant the breaker points open.

Think:

Inside the magneto.

Ignition Timing

The relationship between the engine crankshaft position and the instant the magneto fires.

Think:

Magneto to engine.

A useful memory aid is:

E-gap = internal magneto timing.
BTDC = magneto timing relative to the engine.

Why Final Rotation Direction Matters

When checking ignition timing, the final movement of the crankshaft into the timing position should be in the engine’s normal direction of rotation.

Why?

Because the gears driving the magneto contain a small amount of clearance, or backlash.

Approaching the timing mark from the wrong direction can place that clearance on the opposite side of the gear teeth and create an inaccurate timing indication.

If you pass the timing mark, don’t simply rotate backward a tiny amount and call it good. Back the engine farther away from the timing position and approach the mark again in the normal direction of rotation.

The Big Picture

A magneto is fundamentally a mechanical electrical generator.

It converts rotation from the engine into the electrical energy necessary to create a spark.

The sequence can be summarized as:

Engine rotation
↓
Magneto rotor turns
↓
Magnetic flux changes
↓
Current builds in the primary winding
↓
Breaker points open at the E-gap
↓
Magnetic field collapses rapidly
↓
High voltage is induced in the secondary winding
↓
Distributor routes the voltage
↓
Spark plug fires
↓
Fuel-air mixture ignites

When we time the magneto to the engine, we’re making sure that final spark occurs at exactly the crankshaft position specified by the engine manufacturer.

FAA Study Takeaways

If you’re studying magnetos for the Powerplant written, oral, or practical examination, remember these concepts:

  • Magnetos are self-contained. They do not normally depend on the aircraft battery or alternator for ignition.
  • Aircraft engines commonly use two magnetos. This provides redundancy and improves combustion.
  • The P-lead grounds the magneto. An ungrounded magneto can be capable of producing ignition.
  • The breaker points open at the E-gap position. This establishes the proper internal timing relationship.
  • The condenser reduces arcing and assists rapid collapse of the magnetic field.
  • The secondary winding produces the high voltage needed by the spark plugs.
  • The distributor sends that voltage to the correct cylinder.
  • Internal magneto timing and magneto-to-engine timing are different.
  • The No. 1 cylinder is commonly used when establishing engine timing.
  • The No. 1 cylinder must be on its compression stroke when establishing the appropriate timing reference.
  • Ignition timing is normally specified in degrees before top dead center.
  • Final movement into the timing position should be in the normal direction of engine rotation.
  • After tightening the magneto, always recheck the timing.

And perhaps the most important maintenance rule:

Always use the timing specification and procedure provided by the applicable engine, magneto, and aircraft manufacturer.

References

Relevant FAA subjects include magneto ignition systems, magneto timing devices, engine timing reference marks, timing discs, piston-position indicators, timing lights, internal magneto timing, E-gap setting, timing a high-tension magneto to the engine, and ignition-system checks.