Every pilot performs a magneto check before takeoff, hoping to discover an ignition problem while still on the ground. It's always better to find a problem before becoming airborne than after. Unfortunately, some ignition system problems simply don’t show themselves during a run-up. They pass every ground check with flying colors, only to begin misfiring thousands of feet later.

Paschen’s Law is an interesting concept that applies to our ignition systems. An understanding of this simple principle can turn an ordinary in-flight mag check into very valuable diagnostic tool for piston aircraft owners.

What is Paschen’s Law?

In simple terms, Paschen’s Law describes how well air acts as an electrical insulator. At the altitudes where we fly piston airplanes, decreasing air density reduces air’s ability to keep high-voltage electricity where it belongs. As we climb, electrons find it easier to escape damaged insulation and arc to places they were never intended to go.

A healthy ignition system doesn’t care about altitude because its insulation is doing the work. A damaged ignition system, however, has been relying on dense sea-level air to make up for failing insulation. As altitude increases, that extra margin disappears.

That’s why ignition systems that appear perfectly healthy during a ground run-up can begin misfiring in cruise.

Ground mag checks

Most flight training still teaches pilots to judge ignition health by RPM drop alone. While that certainly has value, it’s a crude test. An RPM drop of around 250 RPM on a six-cylinder engine usually indicates one spark plug isn’t contributing. Unfortunately, RPM alone doesn’t tell us which cylinder is involved.

Often, what pilots describe as a “bad mag” turns out to be a single spark plug or ignition lead. Identifying the affected cylinder before calling your maintenance provider can save everyone considerable troubleshooting time.

A much better way to perform a mag check is by monitoring the EGTs. After stabilizing the engine at the desired check RPM, allow the EGTs to settle. Switch from BOTH to LEFT and watch every EGT. Each should rise approximately 80°F while the engine continues running smoothly. Return the switch to BOTH and verify the temperatures fall again. Repeat the process on the RIGHT magneto. If one cylinder’s EGT drops or doesn’t respond like the others, you’ve just identified where the problem lives.

Knowing which spark plug failed

Knowing which spark plugs are fired by which magneto on a Continental-powered Cirrus aircraft with standard magneto harnesses make diagnosis even easier.

The LEFT magneto fires:
 Top plugs on cylinders 2, 4, 6 and Bottom plugs on cylinders 1, 3, 5

The RIGHT magneto fires:
 Top plugs on cylinder 1, 3, 5 and Bottom plugs on cylinder 2, 4, 6

Suppose the #3 cylinder fails to show the expected EGT rise when operating on the LEFT magneto. You can now tell your mechanic that the lower spark plug on cylinder #3 is the likely culprit. That simple observation may eliminate a significant amount of troubleshooting.

Why perform mag checks in flight?

The ground run-up only tests the ignition system at relatively low power, low cylinder pressures and sea-level air density. It's sufficient for finding major failures but doesn’t place much stress on the system.

An in-flight mag check, on the other hand, evaluates the ignition system under the same conditions where it actually earns its keep.

At cruise power, simply switch from BOTH to LEFT, back to BOTH, then to RIGHT while monitoring the EGTs. Every cylinder should respond normally and the engine should continue running smoothly just as it would on a ground check.

One failed spark plug becomes much more noticeable in flight than during a run-up. In  other words, don’t do this check with family members or nonaviators on board.

 

Magneto pressurization

Turbocharged Cirrus aircraft add another wrinkle.

These airplanes use compressor discharge air from the turbocharger to pressurize both magnetos. That pressurized air helps prevent internal arcing inside the magneto as altitude increases.

Without it, internal arcing usually begins around 15,000 feet. Because the problem exists inside the magneto itself, the resulting misfires often appear random, affecting different cylinders with no obvious pattern. It will certainly get your attention.

Plug wires and Paschen’s Law

Ignition leads are also vulnerable.

Each lead is essentially a coaxial cable consisting of a center conductor surrounded by a grounded metallic shield. Between them is insulation designed to contain approximately 20,000 volts generated by the magneto.

Age, heat and improper handling, improper installation can damage that insulation.

Early in its life, the damage may go unnoticed because dense air at low altitude provides additional insulation. As altitude increases, however, Paschen’s Law takes away that safety margin. The spark may begin jumping through the damaged insulation instead of reaching the spark plug.

The engine may continue running normally during every ground run-up while quietly developing an ignition problem that only appears in at altitude.

Why a megger isn’t enough

Mechanics can evaluate ignition leads using a high-tension lead tester, commonly called a megger. It’s a useful tool, but it has limitations.

The tester typically applies only a couple of thousand volts to a stationary wire sitting at ground level. During normal cruise, the magneto is producing roughly 20,000 volts while the wire is vibrating on a hot engine in thin air.

There’s simply no better test than operating the ignition system under actual flight conditions.

A simple diagnostic test

One of the most useful tests you can perform is comparing mag checks at two different altitudes.

Conduct the first check at 10,000 feet or higher while operating rich of peak. If a cylinder misfires, repeat the same test at 3,000 feet or below.

If the low-altitude test is normal but the high-altitude test consistently shows a single-cylinder misfire, you can be reasonably confident the associated ignition lead has failed.

If random cylinders begin misfiring only at higher altitude, the problem is more likely inside a magneto or its pressurization system.

On turbocharged airplanes above approximately 15,000 feet, simultaneous misfires on both magnetos strongly suggest the magneto pressurization system has failed. Without pressurized air, electrons begin finding unintended paths inside both magnetos.

Problems don’t fix themselves

Just because an ignition problem only appears at altitude during a test doesn’t mean it isn’t there the rest of the time.

A deteriorating ignition lead may first begin leaking energy only at 12,000 feet. As the insulation continues to degrade, the same lead may begin failing at 8,000 feet, then 5,000 feet, and eventually during the run-up.

If you’re watching your engine monitor closely during a climb, you may notice one EGT begin rising as a single spark plug quits firing. Most of us, however, have more important things to monitor during departure than a slowly wandering EGT.

Running on both magnetos often masks these intermittent failures. The engine may continue operating well enough that the pilot never notices a problem, even though power is being lost and unnecessary stress is being placed on ignition components. Magneto internal arcing will eventually create permanent carbon tracks that invite arcing at even lower altitudes. Electronic ignition systems can be even less forgiving, sometimes allowing a missed spark in a wire to create combustion issues in the opposing cylinder.

The takeaway

An in-flight mag check takes less than a minute, costs nothing, and can reveal ignition problems months before they become obvious during a normal run-up. Paschen’s Law may sound like something from a college physics textbook, but understanding it gives every owner-pilot another practical maintenance tool. The next time you’re settled into cruise on a cross-country flight, spend sixty seconds checking your ignition system. Your mechanic, and your engine, will appreciate it.

© 2026 Paul New