This post focus on why spark plugs foul and how you can keep them clean in X-Plane with the Reality Expansion Pack.

It’s an easy topic but you better know it well so you can avoid surprises during magnetos check.

What is a spark plug?

Spark Plug
Quoting wikipedia:

A Spark Plug is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air mixture by an electric spark, while containing combustion pressure within the engine. A spark plug has a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator.

Basically, a spark plug just ignites the fuel by generating an electric spark at every combustion cycle, thousands of times every minute when the engine is at full RPMs.

An aircraft piston engine has two spark plugs for each cylinder, one connected to the left magneto, the other connected to the right magneto.

The following image shows the most important parts of the spark plug: the electrodes.

Spark Plug Electrodes

There are two electrodes on the spark plug. It’s between them that the spark ignites. If the electrodes are not in shape, the spark can’t take place.

Why do spark plugs foul?

The previous post introduced the stoichiometric ratio. It is the ratio between air and fuel mixed in the engine.

The stoichiometric ratio is fifteen parts of air for each of fuel (15:1). At peak EGT you’re right there.
But when the engine is idling, let’s say below 1000RPMs, mixture full rich, that ratio is way lower, that is, there’s much more fuel than needed.

The lower the air fuel ratio, the colder the combustion and the higher the amount of fuel unburned. As you may know, a cold combustion rich of fuel leads to one thing: carbon.

Some of the carbon is ejected out of the exhaust pipe but a part of it deposits over the spark plugs, fouling them.
A fouled spark plug is unable to make the spark that starts the combustion, because the carbon deposits are isolating the two electrodes, thus preventing the spark to take place.

How can you keep them clean?

Keepin the spark plugs clean is easy. You should make sure that the combustion is not generating carbon deposits, that is, the stoichiometric ratio is relatively high.

To do that you must:

  1. Raise the RPMs: stay above 1000RPMs
  2. Lean the mixture: on ground lean aggressively, in flight lean as stated here

But there’s also another trick!

With REP, you can choose which kind of spark plugs your engine will mount:

  1. Default: normal spark plugs
  2. Fine wire: special spark plugs with finer elecrodes. A thinner elecrode provides less surface to carbon to form deposits. It will then foul less.

To change the spark plugs type, open the Hangar window and, in the “Engine” tab, change the type using the provided interface.

How can you check if the spark plugs are fouling?

During the engine ground check, before takeoff, you should check the magnetos as per checklists.

If the RPM drop over a magneto is higher that 125-150 RPM, then one or more spark plugs attached to that magneto are fouling.

How can I clean the spark plugs once they fouled?

The trick is easy:

  1. Apply full brakes
  2. Apply half to full power
  3. Lean mixture aggressively
  4. Wait for 5-6 seconds
  5. Reduce the power and reset mixture full rich (preparing for takeoff)
  6. Re-do the magnetos check

You still have questions about spark plugs?

Join our Facebook group ( and discuss your doubts with the other REP pilots!

How-to: keep your spark plugs clean in X-Plane