Hot Props and Proper Magneto Checks
By Max Trescott, CFI, Glass Cockpit Specialist, Platinum CSIP, 2008 National
CFI of the Year, Aviation News Talk podcast host at
www.aviationnewstalk.com.
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:
- Shut off the engine correctly.
- 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 SB912-0361 suggests 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.
Magnetos
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!
1https://www.gpo.gov/fdsys/pkg/FR-2004-08-12/pdf/04-18440.pdf (page 2) and http://www.aviasport.com/Rotax/Documentacion_4T/Boletines/Ingles/SB_912_036.pdf.
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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 the
www.aviationnewstalk.com/ podcast, 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 www.maxtrescott.com/
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