AVIATION SCIENCE
LESSON 5: SPARK PLUGS
Teacher:
Subject:
Grades:
Subjects:
Paul Ladegard, Alan Dick
Aviation Science
Secondary
Technology, Science
Learning Objectives:
Students can locate the sparkplugs on an aircraft engine, read the code, inspect
plugs for wear, clinkers, and other usage, demonstrate proper extraction, cleaning
of a plug, and describe how a spark plug works.
Student Activities:
• Students read ‘Student Reading’ below.
• Teacher reviews Student Reading with the class.
• Students learn to properly extract spark plugs, and put them back using the
proper sized wrench.
• Students are shown clean and contaminated plugs, and how to inspect them
for airworthiness.
• Students learn to compare the attributes of aircraft, snowmachine, truck,
spark plugs.
• Students practice cleaning plugs, using proper tools, safety glasses, etc.
• Visit local hangar and observe sand blasting and checking of spark plugs
under pressure.
• Students practice extracting plugs using proper wrench, and installing plugs
using proper torque wrench.
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Materials:
• Spark Plugs of various sizes and conditions
• Pick for cleaning
• Blue Gloves
• Q Tips
• Torque wrenches
• Socket wrenches
• Safety Glasses
• MEK cleaner
Student Reading:
Aircraft sparkplugs
Sparkplugs have been a very important component in aircraft engine improvement
and development since the very beginning of flight.
The aircraft engine depends on the ignition of the fuel/air mixture at exactly the
right time to drive the piston downward in the power stroke. The engine cannot be
more reliable than the sparkplugs. If the sparkplugs fail there will be no ignition of
the fuel, no power stroke, and probably no engine power.
In the early days of aviation, sparkplugs were a point of
vulnerability. The ceramic that surrounds the wire/electrode and
keeps it from grounding presented many problems. The ceramic
often broke, fractured or even disintegrated.
Improvements in ceramics have improved engine reliability tremendously. The
ceramic must be a good electrical insulator and able to withstand
high temperatures as well as thermal shock.
Keeping the high voltage from running to a
wrong place at a wrong time has been a
constant challenge for aircraft engineers.
Two sparkplugs per cylinder.
Modern aircraft engines have two sparkplugs per cylinder and two magnetos.
Each magneto separately fires one sparkplug in each cylinder, so if one magneto
fails, each cylinder is still supplied with a spark from the other magneto. Or, if
both mags are good but one sparkplug fails, that individual cylinder has another
working sparkplug. Each cylinder has one plug on the bottom and one on the top.
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While two plugs serve as a safety factor by providing
redundancy, there is a greater reason that prompted the
need for dual plugs.
An aircraft engine is working against air, and is best served
with a large diameter piston with a shorter stroke and fuel
with a much quicker flash point.
With the development of a wider diameter piston came the
problem of the flame front across a 5” cylinder.
The dual flame front provides much better combustion
across the wide cylinder.
As pilots do a run-up before takeoff, one magneto, and therefore one set of plugs,
is shut off at a time while the engine is held around 1800 rpm. When one mag is
shut off, there is a corresponding drop in rpm’s (50-100) that clearly demonstrates
the loss of power an engine would experience if there were only one set of plugs.
During preflight run-up, both mags are individually turned off while the other mag
is checked.
An aircraft engine can safely run on one magneto or one sparkplug per cylinder. The
purpose of the mag check before takeoff is to be certain all systems are
operational. In the air, two plugs provide a much smoother power pulse, and more
efficient fuel burn. This is one of the reasons for two magnetos and two plugs per
cylinder.
If one sparkplug seems to be running poorly, it is easy to find which cylinder has
the faulty plug. The engine is started on both magnetos then quickly switched to
the mag that is running ragged. It is run for a short time, then the engine turned
off. The exhaust manifold of each working cylinder will be sizzling, and the exhaust
manifold with the bad plug will be cool or cold.
Two kinds of plugs. There are basically two kinds of sparkplugs used in
manufactured aircraft today:
• Massive
• Finewire
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Massive plugs can have two, three or four grounding electrodes on the perimeter
and one electrode in the middle that the spark jumps from to the grounding
electrodes. Massive plugs can be easily cleaned and re-gapped and are less
expensive than finewire. ($25 each compared to $70-95 each) During the 100 hour
or annual inspection, the ceramic on the massive plugs is cleaned by sandblasting.
However, this does damage the ceramic, as the roughened surface will pick up
deposits more quickly than the smooth factory surface. The average life of a
massive plug is around 300-400 hours.
The gap is 0.019” set with a precise tool (not a pair of pliers!) and that gap is
consistent whether the engine is high or low compression. Low compression engines
are 7.0:1 and high compression are 8.5:1.
Finewire plugs resist buildup by clinkers and last longer than massives, but cannot
be cleaned by sandblasting as the sandblasting process would weaken the finewire
element to the point of breaking. Clinkers are cleaned with a pic that resembles a
dentist’s pic.
Plug Attributes:
The numbers and letters on a sparkplug tell their story. The number might be R H
B 37 E
•
•
•
•
•
The first letter, R indicates that there is a resistor in the plug to cut down
on radio interference.
The second letter, H indicates whether the barrel is shielded or not.
The third letter, B tells how long the threads are, and what size wrench
would remove the plug. Threads might be long or short depending on the
cylinder head design.
The forth number 37 indicates the heat range. A higher number indicates a
hotter plug.
The fifth letter E indicates the type of electrodes, whether massive or
finewire, and how many ground electrodes surround the inner
electrode.
This aircraft sparkplug system is comparable to the number/letter
system used on snowmachine, outboard and fourwheeler sparkplugs.
Electronic shielding: Every sparkplug can potentially cause radio
static, so each plug is shielded by a grounded metal jacket. The
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surrounding shield collects electronic interference that could threaten safe radio
communication and runs it to ground.
Cleaning Sparkplugs:
Cleaning sparkplugs involves:
• Wirebrushing the threads. The threads of both massive and finewire plugs
should be cleaned with a wire brush to prevent them from seizing in the
cylinder head. Each aluminum cylinder has an embedded helix coil to hold the
steel plug. If the plug grabs, loosens and twists the helix coil, an expensive
cylinder replacement is the next step.
• Cleaning clinkers with a pic.
• Sandblasting the plugs to remove carbon from electrodes and porcelain.
• Rinse in MEK (Methylethylketone) to remove any remaining carbon and
contaminants.
• Blast with compressed air to further clean any sand or other particles.
If a sparkplug is dropped on the ground or concrete, it must be discarded, as the
internal insulating ceramic could be fractured, resulting in grounding
of the spark.
Plug Condition:
A good sparkplug has no clinkers, the center electrode is round, and
is the color of good coffee.
A worn sparkplug has a center electrode that is shaped like a
football. Most massive plugs have an expected life of 300 hours,
with adjustments and cleanings done before discarding.
Sparkplugs can be cleaned, gapped and reused. However, cleaning
involves sandblasting. When a plug comes from the factory, the porcelain has a
smooth finish to resist carbon buildup. Once a plug is sandblasted, the protective
smooth finish of the porcelain has been removed. It then collects carbon more
easily and must be cleaned more often.
A fouled sparkplug has clinkers. Clinkers are rounded, almost
like shotgun shot. They might result from fuel problems and
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chemicals not scavenged from the cylinder. Lead additives have a greater density
than gasoline. If the additives settle in a tank, they can concentrate and, when
later burned, leave deposits on the plug. The clinkers must be cleaned with a pic.
The tank of a plane that has set for a long time would benefit from being stirred.
A carbon fouled sparkplug is black in color and is the result of a
fuel/air mixture that is too rich. This can be prevented by proper
leaning of the engine and monitoring of the EGT’s during
operation. A carbon fouled plug is undesirable as carbon is a
conductor of electricity, and the high voltage spark could migrate
from the center electrode and down the side of the porcelain
rather than jumping the gap to ignite the fuel.
An oil-fouled plug is black with a thick sticky film. This might be an indicator of oil
blowing by the piston rings and a low-compression problem, but it also could be an
indication of oil getting past the intake valve. Circulating oil serves to cool the
valves and bring the excessive heat back to the crankcase. Some oil might leak past
the valve into the cylinder. If oil leaks past the exhaust valve it will be coked, and
leave a sticky deposit.
Installing sparkplugs:
Anti-seize. In addition to cleaning the threads, anti-seize lubricant
must be applied to the threads, but sparingly. If excessive antiseize leaks into the cylinder, it can short the spark plug and ground
the spark.
Older planes had a copper helix coil embedded in the aluminum cylinder head, but
most newer engines have a steel helix coil. If that helix coil is backed out by a
seized plug, the repair will be well over $1000.
The plug should be inserted into the cylinder and turned at least three turns by
hand before putting a wrench to complete the tightening.
The plug wire is replaced and tightened with a 3/4” wrench.
Rotation: During the 100 hour or annual, plugs are rotated
in a pattern that changes their position from top to bottom
in each cylinder, changes cylinders and changes magnetos
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for each plug. The pattern follows the order shown on the right.
The bottom plugs tend to collect more clinkers than the top, and rotating helps
avert uneven wear of the electrodes. At the same time, sparkplugs are supposed to
fire from the center electrode to the outside grounding electrodes, but sometimes
this gets reversed. Rotating helps avert this as well.
Copper washer: The copper washer on a plug should be changed each time the plug
is extracted, as heat from the engine hardens the copper. A fresh soft copper
washer seals the plug from cylinder pressures. Some mechanics anneal used copper
washers by heating and plunging them into cold water, softening the copper.
Understanding and monitoring sparkplug condition are absolutely essential to
maintaining airworthiness.
Assessment: Students demonstrate cleaning and inspection of an aircraft plug and
evaluate for airworthiness use.
STANDARDS:
Technology C 1: A student…should use technology to observe, analyze, interpret,
and draw conclusions.
Science A: A student should understand and be able to apply the processes and
applications of scientific inquiry.
Science E: A student should understand the relationships among science,
technology, and society.
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