Hot Props and Proper Magneto Checks


November 2017

Most pilots know that under some circumstances, turning a prop by hand can lead to it accidentally starting and possible injury or death. But many pilots don’t know all of the things you can do to detect or prevent a hot prop. On a recent episode of the Aviation News Talk podcast, I talked about hot props and magnetos, and much of this content comes from that show.

Hot Props
For a propeller to be hot, two things have to happen. There has to be some small amount of fuel left in the engine, and a spark has to reach the spark plugs. Under normal conditions, having the magneto switches or key turned to the Off position will prevent any spark from reaching the spark plugs. However, if there’s a loose connection to the switch, or the switch is becoming old and intermittent, a spark can still reach a spark plug EVEN with a mag key in the Off position!

Therefore, it’s critical that you always shut down an engine correctly, so that there is no residual fuel left in it. That way, even if your mag switch is not operating properly in the Off position and a spark does reach the spark plugs, the engine won’t start accidentally when the propeller is moved.

Shutting Off an Engine Correctly
There are two things you can do to assure that a propeller never accidentally fires when you turn it:

  1. Shut off the engine correctly.
  2. Only turn the prop (e.g. to attach a tow bar) in the opposite direction of normal rotation.

To shut down an engine, you generally pull the mixture back to the idle cut off position, which starves the engine of fuel. After the propeller comes to a stop, and ONLY after it has come to a complete stop, turn the magnetos switches or the Mag key to the Off position. Doing this correctly assures there’s no unburned fuel left in the engine.

The reason it’s important to WAIT for the prop to stop before turning off the Mag switches or key is that while the switch is still on, the engine continues to burn all residual fuel. However, if you turn the magnetos off while the prop is still turning, you’ve just removed the spark and some unburned fuel can be left in the engine. Then if the magneto grounding system is faulty and you turn the prop even slightly, the engine can start for a few seconds, which is just long enough to kill you!

Safely Turning a Prop by Hand
You can also prevent an accidental prop start by only turning a propeller in the opposite direction from which it normally turns. When you do that, the magnetos cannot send a spark to the spark plugs, so even if there’s residual fuel in the engine, a spark cannot be generated.

You may have heard that one should not turn a prop backwards. People often say this about Rotax engines. However SB912-0361suggests it’s okay as long as you turn the prop less than one revolution in the reverse direction. Another reason often given is that this may shorten the life of the carbon vanes that rotate inside a vacuum pump. That might be true for some pumps, but others are designed to rotate in either direction. Regardless, what is the value of your life compared to potentially having to replace a vacuum pump a few hours sooner than it would’ve failed anyway?

Here is an easy way to remember how to rotate a prop backwards. When sitting in the cockpit looking forward, a propeller turns clockwise when the engine is running. Therefore, when standing in front of a propeller and looking at it from the opposite direction, turn it clockwise from your view, which is the opposite direction it was turning when viewed from the pilot seat.

To understand why it’s important to turn a propeller backwards, and why turning it in the normal direction can generate a spark, it’s important to understand how magnetos work. Most piston-powered aircraft have magnetos, unless they have electronic ignition, which is still relatively rare.

When a magneto is turned, it generates a spark that goes to the spark plugs. This occurs regardless of whether a running engine turns it, or you turn a propeller manually! They are widely used in piston-powered aircraft because of their reliability. That’s because even if you have a complete electrical failure, as long as the engine is turning, the magnetos will still generate a spark and the engine will keep running.

Most engines have two spark plugs per cylinder, so a four-cylinder engine has a total of eight spark plugs. And each engine has two magnetos, one that sequentially fires half of the spark plugs in all cylinders, and another that fires the remaining spark plugs.

Normally, both magnetos are operating, and all spark plugs fire at various times within all cylinders. Having two spark plugs in each cylinder provides redundancy, in case one of the spark plugs is not working. If both spark plugs in the cylinder are working, one plug fires a little before the other, which results in a more even burning of the fuel throughout the cylinder, leading to lower temperatures. But if only one spark plug is working, it takes longer for that single spark plug to burn all of the fuel in the cylinder, resulting in higher temperatures. So when doing a mag check, all EGT temperatures should rise in the L and R positions.

Checking the Magnetos
When doing a mag check with the magneto switches or key in either the L or R position, the RPM should drop. That’s because half the spark plugs are off, and the engine is putting out slightly less power. The RPM drops should not exceed the amount specified in your checklist, and the drops should be about the same in both the L and R positions.

In the L and R switch positions, a wire is connected to the output of one magneto, which shorts the spark to ground, so that the spark plugs connected to that magneto cannot fire. That ground wire is called the P– lead. In the L position, only the spark plugs connected to the left mag will fire, because a P-lead is grounding the output of the right mag. And in the R position, only the spark plugs connected to the right mag will fire, because a P-lead is grounding the output of the left mag. And as you’ve probably guessed, in the Off position both magnetos are grounded by P-leads, so no spark can reach any of the plugs.

Here are things to look for during a mag check. First, it is essential that you see some RPM drop when you switch to the L or R position. If there is no drop, one of the P-leads is loose or broken. The engine will work fine in this condition, but you now have a hot prop! With a hot prop, it is very important to shut down the engine properly, so no unburned fuel is left in the engine. If you rush the process and turn off the magneto switches or key before the prop stops turning, you are now at high risk of having an accidental prop start. If you later turn the prop by hand, at some position the points will open and a spark will be sent to a spark plug. If there is unburned fuel in that cylinder, the engine will probably start for a few seconds, which could kill you.

When checking mags in an airplane with a key, such as most Cessnas, I recommend you first check the magneto that's two clicks away from the Both position. This can help you avoid taking off on one magneto. Here’s why. When you turn the key one click to check one of the magnetos, you will always turn the key back one click to get back to the Both position. But sometimes when pilots check the magneto that’s two clicks away, they only turn the key one click as they try to return to the Both position. If that’s the last mag you check, you’re now set up to take off on one magneto! I’ve seen pilots do this a couple of times. The first time I didn't catch it, and we took off on one mag!

Many aircraft now have sophisticated engine monitors that show the exhaust gas temperature or EGT, and cylinder head temperature or CHT, for each cylinder. If you have an engine monitor, look at the EGTs when you turn the key to the L and R positions. All four or six EGTs should rise in the L or R position, and all of them should fall when you return the key to the Both position. If instead, the temperature for one of the cylinders falls in the L or R position, you now know that one of the spark plugs is not working in that cylinder. Note the cylinder number so that you can tell the mechanic which cylinder has a spark plug problem.

In Flight Problems
If an engine becomes rough during flight, it could be because one magneto has developed a timing problem. In that case, switch to the L and R positions to see if the engine runs better in one of those positions. If it does, leave the magneto switches or key in that position for the rest of the flight and report the problem to your mechanic.

Shutdown Mag Check
Some checklists say to switch the mag switches or key very briefly to the Off position, so you can hear if the engine is stopping. If it is, that verifies both mags are properly grounded. Then before the engine quits, switch back to the Both position and shut down the engine properly by pulling the mixture to idle. If there’s a mag key, it’s important that you do this check by turning all the way to the Off position, and not just turn to the L and R positions, as some CFIs insist. Turning to the L and R positions checks part of the grounding system, but it doesn’t check whether the switch itself works in the Off position! It's not unusual for an older switch to become corroded and fail to work in one position, so definitely do the shutdown mag check by switching briefly to the Off position.

Do the mag checks and engine shutdown procedures properly, and you will greatly enhance your safety when you move a prop. To learn even more about flying safely, join me each week by listening to the Aviation News Talk podcast, which can be found in the Apple Podcasts app, or wherever you listen to podcasts. Fly safely and keep the blue side up!

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Max Trescott, author and 2008 National CFI of the Year specializes in teaching in glass cockpit aircraft. He is best known for his Max Trescott's G1000 Glass Cockpit Handbook. He hosts, which focuses on General Aviation and safety and is a former magazine columnist for EAA Sport Aviation. He also authored a series of safety tips for FAAST, the FAA Safety Team. Max is a San Francisco area-based CFI, Gold Seal Flight Instructor Certificate, and Cirrus Platinum CSIP instructor who specializes in teaching in and publishing training materials for glass cockpit aircraft. In addition to being an FAA FAASTeam Representative, he gives teaches and gives safety presentations across the country. Read more of his work at

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