The Truth About Your Magnetos
July 15, 2018
Jasper De Hertog

The Truth about your Magnetos, finally revealed!

Despite its rudimental design, the Lycoming IO-360-L2A has come a long way in general aviation history and proves to be both a compact and extremely reliable powerplant.

1. Engine volume explained

Being unfamiliar with the imperial system of units an engine volume of 360 cubicle inches might not trigger one’s imagination. However if we look at this value the metric way, this little powerhorse proves to encompass a cylinder capacity of 5.9 liter. Have you ever felt the excitation of throttling up a 5.9 liter automobile? …Right, so maybe that’s a first good reason to treat this engine with respect!

2. Yes, it's made by hand

Another difference compared to the engine in most common cars is their manufacturing process. Instead of being built on high volume automated production lines, Lycoming engines are entirely hand built to particular airframe manufacturer’s specifications on a single assembly line in Williamsport, Pennsylvania. Furthermore due to their specific application, production volumes are much lower. It shouldn’t be a surprise that a new engine comes in at a cost of about $55.000. …Another good reason to show some dignity when putting your hand on the throttle.

3. Sources

A lot of information on the construction and sound operation of the IO-360-L2A can be found in both the Cessna and Lycoming IO-360 operator’s manuals. In addition, the Lycoming website contains an extensive database on both technical and operational recommendations on treating your engine the way it should be treated.

4. Facts of your IO-360-L2A engine

  • Rated horsepower:  180 HP
  • Rated speed:   2700 RPM
  • Bore 5.12 inches / 13,00 cm
  • Stroke 4.38 inches / 11,12 cm
  • Displacement 361 cubic inches / 5.9 liter
  • Compression ratio 8.5 : 1
  • Firing order 1-3-2-4
  • Ignition advance 25° before TDC
  • Crankshaft Rotation clockwise
  • Mass 278 lbs / 126 kg

5. The truth about your Magnetos revealed

Do you still remember the limitations on engine speed drop when checking the magneto’s? …and what are we actually looking for when when performing the magneto check?

Firing Order

6. Back to basics

In order to initiate the combustion process in a gasoline engine there is a need for fuel, oxygen and an ignition source. Hence a cylinder needs to contain at least one working spark plug which ignites the combustible fuel and air mixture which is drawn into the cylinder during the induction stroke. It’s pretty simple: no ignition means no combustion …and no combustion means the affected cylinder is not driving the crankshaft hence propeller.

In order to increase engine reliability, gasoline engines have been equipped with 2 spark plugs per cylinder for decades. In case 1 plug fails, proper combustion is ensured by the second redundant plug. To improve reliability even further, those spark plugs are powered by two independent engine driven electrical generators, which are called magnetos. Each individual magneto serves all cylinders by a discrete set of ignition leads and sprak plugs. This is clearly shown in the wiring diagram above.

Nowadays most gasoline engines are fitted with electronic ignition units and a single set of spark plugs. However, with an eye on reliability, the IO-360-L2A sticks to the mechanically timed Slick magnetos which keep on sparking the plugs as long as the engine is running.

Since ignition in the IO-360-L2A is initiated by 2 spark plugs, the fuel-air mixture contained in the cylinders is ignited at two different locations within the cylinder. This causes total combustion within the entire cylinder volume to be reached at a faster rate causing an increase in torque and hence power. Simply stated: for a particular throttle setting, the RPM will be higher.

No reason for cold sweat, we won’t describe the construction of the magneto! The only important thing to remember is that each magneto does 2 things:

  • It generates a high electrical power to provoke a spark at the spark plug to initiate combustion.
  • It is meticulously timed to cause the spark to occur at the right cylinder, at the right time. As described in the engine specifications, each magneto causes a spark to occur at a timing of 25° crankshaft rotation before the respective piston reaches top dead center.

7. The Magneto Check

So, what is the magneto check all about? Let’s start with Lycoming’s procedure which corresponds to the procedure stated in the Cessna POH.

  • Aircraft equipped with fixed pitch propellers may check magneto drop-off with engine operating at approximately 1800 RPM (2000 RPM maximum). 
  • Switch from both magnetos to one and note drop-off; return to both until engine regains speed and switch to the other magneto and note drop-off, then return to both. Drop-off must not exceed 175 RPM and must not exceed 50 RPM between magnetos. Smooth operation of the engine but with a drop-off that exceeds the normal specification of 175 RPM is usually a sign of propeller load condition at a rich mixture.
  • If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then retard the throttle to 1800 RPM for the magneto check and repeat the check. If the drop-off does not exceed 175 RPM, the difference between the magnetos does not exceed 50 RPM, and the engine is running smoothly, then the ignition system is operating properly. Return the mixture to full rich. 

Having this check procedure in mind you might wonder why the RPM is supposed to drop when running the engine on a single magneto. The answer is simple: as the air-fuel mixture is ignited from a single spark plug, total combustion within the entire cylinder volume will be reached at a slower rate. For a particular throttle setting this causes engine torque and power to decrease. Simply stated: the RPM will drop.

8. What can go wrong?

So what might go wrong during the magneto check? If the engine stops when running on a single magneto, this indicates that none of the cylinders are being fired by that particular magneto. Most likely this is caused by the magneto failing to generate electrical power to feed the plugs. This might be due to a failure of the magneto’s generator, or a deficiency in the magneto’s control wiring originating from the magneto switch. ‍

  • Whenever the engine stops during the magneto check, NEVER attempt to keep the engine running by switching to the ‘both’ setting. Why? As long as the crankshaft turns and pistons move up and down, air-fuel mixture is drawn into the cylinder. However, what happens when no ignition takes place? …Right! The non combusted mixture is expelled into the exhaust duct. Can you imagine what happens when the ignition is turned on? … At least you’ll hear a loud bang but in the worst case you might provoke a tailpipe fire. A better approach is to let the engine die, wait for some time to ensure no unburnt mixture is left in the exhaust system and restart the engine with the magnetos in the both setting. Have the aircraft serviced!
  • If the engine shows a significant vibration combined with an excessive drop in RPM when running on a single magneto, this indicates that the respective spark plug in one particular cylinder might not be igniting the mixture. This might be caused by spark plug fouling due to improper leaning during ground operations or by a loose ignition lead. Have the aircraft serviced.
  • If the RPM drop is exceeding 175 RPM without significant vibrations, this might indicate a discrepancy in ignition timing. Whenever the ignition takes place sooner or later than the specified ignition point, the power loss will be larger and hence the drop in RPM will increase. 
  • When the RPM drop between 2 magneto’s exceeds 50 RPM, this indicates a discrepancy between the timing of the two individual magneto’s.

Please SHARE your experiences with any Magneto Issues you have ever encountered throughout your aviation career, we'd love to hear from you!

Jasper De Hertog
Jasper started his aviation career at the age of 26 and is active as a full time flight instructor at EuroPilot and SoCal Pilot Center. He graduated from the Antwerp Maritime Academy in 2008 and has been working worldwide on various specialized ships. During his seagoing career he became passionate about aviation. In 2017 he docked for the last time and became fully dedicated to aviation training.