Ever notice that whenever a piston aircraft engine has low compression or some other cylinder problem, chances are the mechanic will blame it on
August 20, 2001
|About the Author ...
John Deakin is a 35,000-hour pilot who worked his way up the aviation food chain
via charter, corporate, and cargo flying; spent five years in Southeast Asia
with Air America; 33 years with Japan Airlines, mostly as a 747 captain; and
now flies the Gulfstream IV for a West Coast operator.
He also flies his own
V35 Bonanza (N1BE) and is very active in the warbird and vintage aircraft
scene, flying the C-46, M-404, DC-3, F8F Bearcat, Constellation, B-29, and
others. He is also a National Designated Pilot Examiner (NDPER), able to give
type ratings and check rides on 43 different aircraft types.
as a result of these columns, I am privileged to get a lot of email about
engines and the basic principles, and I monitor a number of forums and mailing
lists where pilots and owners hang out online. GAMI gets even more mail and
phone calls of course, with over 8,000 sets of GAMIjectors "out
there" and more going out the door every day. George Braly is the
founding genius at GAMI, and we share some information in both directions
when I get a question I can't handle, I "ask George."
I feel the need to insert my usual disclaimer here: I am in no way
financially connected with GAMI, or Tornado Alley Turbo, or any other such
entity. The folks there are trusted and valued longtime friends, and we
have long shared data and ideas. I have their products in my airplane
because I believe in them, and I paid for them.
I think it's fair to say that I get to see a pretty representative sampling
of the common squawks and problems (and sometimes the uncommon ones!) in the
One constantly recurring theme we hear from owners and pilots is, "But
my mechanic says
There usually follows some statement that seems downright silly, given what
we now know and use every day in the fast-growing WOTLOPSOP community
("Wide Open Throttle, Lean of Peak, Standard Operating Procedure").
Even very good mechanics and highly experienced pilots often buy into what can
only be called "Old Wives' Tales" (OWTs). When asked to produce some
data to back up their statements, the answers will range from a dismissive
"Everyone knows that," to "I saw it somewhere, don't remember
where, too busy to look it up," to "That's what I learned in
business of running lean of peak (LOP) is but one example. Mike Busch my
distinguished, wise, handsome, erudite and world-famous editor
finally got serious about getting his A&P certificate. He's now passed
the three FAA knowledge tests (AMT General, AMT Airframe, and AMT Powerplant)
with near-perfect scores, and is scheduled to take his oral and practical exam
later this month.
Mike sent me a few of the FAA knowledge test questions he found silliest,
plus a couple others for comment. He also made a most interesting connection
between some of the common OWTs, and this particular test. (I think Mike knew
that what he sent would light a fire under me to do a column on it. He's
sneaky that way.)
In Mike's words:
I've long noticed and I'm sure you have, too that whenever an
aircraft owner brings his plane into the shop with a top-end problem of any
sort (and particularly a valve-related problem), the mechanic always blames
the problem on "running too lean." Burned valve? Obviously leaning
too aggressively. Stuck valve? You must've been running too lean. High oil
consumption? Low compression? No choke? Your fault, you leaned too much. (So
far, I've yet to hear a mechanic blame fouled plugs on leaning too much, but
I'm sure it's happened. )
Where the heck do they get this? Well, let's explore that a bit. Most OWTs
have some basis that got them started, and this one is no exception. As it
happens, some of them come straight from the FAA, and the questions on the
various "knowledge tests" those computerized tests some still
beaten the subject to death previously, but to review briefly, very few
"flat" engines will run LOP at all, the mixture varies too much
between cylinders. This has long given us only an "available
spectrum" of mixtures somewhere between full rich, and somewhere around
peak EGT. We know that the CHTs are at their hottest when the mixture is set
to roughly 50 ROP EGT, and if any more than about 65% of rated power is set,
those CHTs will become "too hot" at 400º F and above, even if the
engine manufacturers do set the usual redline at 460º or so.
A lot of practical experience with these engines over the years has
forcefully driven the point home (through top overhauls) to most people that
anything above 65% is "pushing the engine," and when you're pulling
65% or more, it's best to run a bit richer, for cooling. This is all true and
correct, when applied to an engine that can operate only in "the rich
half" of the mixture spectrum. This was hard-won knowledge, gained over
50 years of experience in the industry, and the knowledge was passed on to the
younger mechanics and pilots. It became an absolute, with anything else simply
dismissed, or, as my teen son might say, "dissed."
A very few engines (as on the Piper Malibu) were carefully tuned to be able
to run LOP, but even on these, pilots and mechanics were so nervous at this
"new" mixture setting, they'd often richen the mixture "just a
bit, for safety," thus putting the engines right back in the hottest part
of the mixture curve! It's no wonder those engines had so many problems! If
only someone had known (a few did) to simply lean the engine a bit, instead of
enrich it, things would have been a lot better, and MUCH cooler.
We now know that if the fuel and air are properly balanced between all
cylinders, all these engines can run much cooler and cleaner when LOP, and of
course, they can also still run the same old sorry, dirty ROP we've always
used. It's still clear that one should generally avoid only the 50 ROP area at
high power. But 50 years of experience is hard to overcome, and the idea that
"Leaner is always hotter" sticks tight. It is, until it ain't, then
really old hands with radial experience are no problem at all, one
demonstration of LOP in the airplane, watching the control movement, and they
instantly flashback and say, "Why, that's exactly what we used to do with
the big radials!"
The ONLY difference is terminology, and instrumentation. The radials use an
accurate power measurement device ("Torque," or "BMEP") to
lean to peak power, and then they lean further by some increment of power,
depending on what is desired. Modern engines do not have any way to accurately
display power, but we use engine monitors that show EGT and CHT, and we've
long known the relationships. The end results are exactly the same.
Mike also pointed out to me that the current A&P knowledge tests not
only don't address any of these modern issues, but are mostly based on the
ancient radial engines now operated only by flying museums, and a few
wonderfully anachronistic folks in Alaska who still run them. As Mike wrote:
Another fascinating aspect of the AMT Powerplant exam is that the
overwhelming emphasis of the recip-oriented questions is radial engines,
pressure carburetors, and Hamilton Standard hydramatic propellers. Out of
the 1000+ questions in the question bank, I think there are only a handful
about opposed engines, perhaps two or three about continuous-flow injection,
only one about modern Hartzell compact-hub props, and none at all about
McCauleys. It would be too kind to say that the test is anachronistic in the
extreme. (OTOH, if you ever have a question about adjusting valve clearances
in an R2800 or trimming the fuel control on a DC-8 engine, I'm your man!
I think we need to get Mike interested in the Confederate Air Force, where
that knowledge can be put to good use!
He also sent me a few sample questions from the FAA's current "AMT
Powerplant" knowledge test, and I'd like to kick a few of them around
|8072. Which fuel/air mixture will result in the
highest engine temperature (all other factors remaining constant)?
AA mixture leaner than a rich best-power mixture of .085.
BA mixture richer than a full-rich mixture of .087.
CA mixture leaner than a manual lean mixture of .060.
FAA-approved answer: C.
Mike Busch's comment: If memory serves, stoichiometric is
around 15:1 or .067, so answer C ("leaner than .060" or about
17:1) would be VERY lean-of-peak and leaner than most engines can run
smoothly. I'd imagine that the closest-to-correct answer is probably A.
Mike is absolutely correct on this one, of course. That mixture setting
would be very cool, on the lean side. However, engines equipped with
well-tuned GAMIjectors will run quite smoothly that lean, and leaner, perhaps
out to about 18:1.
But think of the generations of A&Ps who have studied their hearts out,
and learned this? Study guides probably have it, and while I haven't checked,
I'll bet the "official FAA-approved manuals" leading to the A&P
probably have this, too.
(Say, am I the only dinosaur who still has difficulty typing
"A&P?" Throughout my youth, it was "A&E," for
"Aircraft and Engine." WHY did the FAA change it? Now we're moving
on to "AMT," I guess, but I don't think I'll live long enough to
adapt to that. Similarly, when I was 23, I got my "ATR," now it's
Mike is also quite correct that most of the questions are from the radial
engine era, and may not apply directly to flat engines, even if the combustion
characteristics and metallurgy are more alike than different. If nothing else,
the language is very different, as I shall point out.
"Rich best-power mixture," and "Lean best-power
mixture" are straight out of the old radial manuals, and to my knowledge,
are never used with the flat engines. They certainly could be, of course, and
perhaps somewhere they are, but radial music plays in my head when I see those
We need to stop here, and get something straight. We throw a ton of numbers
around in these discussions, tests, and textbooks, so let me take a stab at
some clear definitions.
"BSFC" is for "Brake Specific Fuel Consumption." Pay no
attention to the techno-speak, it's simply the pounds of fuel used to make one
horsepower (HP) for one hour. On our big-bore flat engines with 8.5:1
compression ratios, you'll see figures like 0.40 when "pretty lean."
The lower the number, the more efficient the engine is at making HP. If you're
using 100 HP, with a mixture set fairly lean, you'll burn about 40 pounds per
hour, about 6.8 GPH. All kinds of things affect BSFC, including fuel, spark
timing, compression, piston size/shape, mixture, manifold pressure, RPM, the
list goes on forever.
"Fuel/Air Ratio," so common in the old manuals, is simply the
ratio of the weight of fuel to air. At a fairly lean mixture setting, you'll
see a number of around 0.062, or sixty two thousandths of a pound of fuel, to
one pound of air. In more normal numbers, that would be 6.3 pounds of gasoline
(roughly one gallon) to 100 pounds of air. Yes, 100 pounds of air is a lot of
air! That's 1,300 cubic feet, or about 9,700 gallons of air. Picture a room full
of air 13' long, 10' wide, and 10' high and a gallon of fuel, and you'll have
a mental picture of this fuel/air ratio. We can further simplify those
dreadful decimal numbers to whole numbers, and call "0.062" by its
(In other words, 0.062 pounds of fuel to 1 pound of air is exactly the same
mixture as 16 pounds of air to one pound of fuel. Engineers seem to always use
those pesky decimal numbers, instead of nice, whole round numbers I can relate
Don't confuse these two very different measurements. BSFC is pounds per
horsepower, the other is the fuel/air mix, by weight.
The charts for the old engines tend to use fuel/air ratio, the modern
charts for the flat engines tend to use temperatures rich and lean of peak EGT.
The results are the same.
Here's another question from the FAA's current AMT Powerplant knowledge
|8094. Which of the following would most likely cause a
reciprocating engine to backfire through the induction system at low RPM
AIdle mixture too rich.
BClogged derichment valve.
FAA-approved answer: C.
AMT study guide explanation: A lean fuel-air mixture
burns slower than either a rich or a chemically-correct mixture.
Well, sorta. The truth is that a mixture just slightly richer (maybe 25-50F
ROP) than a chemically-correct mixture burns the fastest, while the flame
front slows down if the flame front is EITHER richer or leaner from that
point. That's precisely why we use an excessively rich mixture at very high
power, to SLOW combustion, and put the peak pressure far enough after TDC to
prevent detonation and keep CHTs down to reasonable levels. The AMT study
There is a possibility that a lean mixture will still be burning as it
is pushed out through the exhaust valve.
This is true in theory, but it requires a mixture that is so lean
that it is not a useful mixture setting, even when normally operating lean of
During the time of valve overlap, when both the intake and the exhaust
valves are open, the burning exhaust gases can ignite the fresh fuel-air
charge being taken into the cylinder through the intake valve. This can
cause a backfire through the induction system.
Mike Busch's comment: At low RPM, it's difficult enough to keep
combustion going past TDC. By the time the intake valve opens more than 360
degrees of crankshaft rotation after ignition (and that's a loooooong time
at low RPM), there's not a snowball's chance in hell that combustion would
still be going on.
Mike gets that one right, too, but the truth is hiding in there somewhere,
because many old radials WILL backfire, and it's ALWAYS because the mixture is
too lean. There is one key point that the FAA misses, perhaps in an attempt to
Many of the old radials have large exhaust collectors, leading to one or
two big outlets. Like almost all internal combustion engines, there is
"valve overlap," where BOTH the intake and exhaust valves are open
at the same time, "overlapping" the end of the exhaust stroke, and
the beginning of the intake stroke.
In most of the big radials, they are both partly open for a full 45 degrees
of crank rotation! Long ago, engineers found this helps the bad stuff leave
the combustion chamber, and the good stuff to enter. One pushes, the other
pulls, and the valves snap closed and chop 'em off at the best point, so
almost all the bad stuff is gone, and very little of the good stuff sneaks out
early. The engineers speak of "volumetric efficiency," but I stick
to "bad stuff" and "good stuff."
During the time both valves are open, there is a direct channel open
between the intake manifold and the exhaust manifold. Any "fire" in
the exhaust can light off the mix in the intake. During normal operation, the
pressures and high flows prevent "backflow," and the fire can't get
to the intake. Good thing, too!
However, at very low RPM, just barely above cranking RPM (about 50 RPM),
and below normal idle RPM (about 600 on most), the pressure in the intake
manifold is very, very low (i.e., high suction), with the pistons trying to pull the
"good stuff" in against the nearly-closed throttle plate. This might
show up as 15" to 20" of MP, with an ambient pressure of 30".
What happens? Anything in the exhaust manifold gets sucked back into the
combustion chamber, and back into the intake manifold during
Sometimes, during the start, raw fuel, or a partial charge of fuel and air,
can get through the combustion chamber without being lit off. A spark plug
might be oily, or perhaps that shot of mixture wasn't quite combustible as it
went through the combustion chamber, either too "wet" (rich), or too
"dry" (lean). But once in the exhaust manifold, it will get lit off
by the fire coming from other cylinders. If there's enough raw fuel, it can
cause a "torching," up to 20 feet of slow-burning flame that burns
until the fuel is gone. That's generally spectacular, but usually harmless. It
may scorch paint a bit. Of course, if the engine is all greasy and oily, more
serious external fires can develop, a good reason to wipe the old birds down
when done. One the other hand, if there is a combustible mixture lurking in
the exhaust manifold, and enough of it, it can burn much more quickly, and
make a big bang, an "Afterfire." Very hard on exhaust manifolds.
Finally, if any fire in the exhaust manifold gets sucked back
into the intake through ANY cylinder when both valves are open, you've now got
a "fire in the hole," so to speak.
How to prevent this? The engine manufacturers were very clever. During a
normal start, at 50 RPM or so, with normal priming, the mixture in the intake
manifold is MUCH too rich to burn. If you could stick a lit match in there, it
would be snuffed out. No problem when fire blows backwards into that chamber,
it simply can't burn, and whatever fire there is is snuffed out for lack of
Once the exhaust valve closes, the piston is able to suck in a charge, it
is compressed (and heated), and mixed with the air that came in from the
exhaust manifold, becomes combustible, and lights off.
All this can make starting a big radial pretty tricky. To get the backfire,
you must first have "fire" in the exhaust, probably from a mixture
that was momentarily too rich. You must then create a "too lean"
mixture in the intakes, probably with too much throttle, or too little prime.
You'll see this happen when the engine coughs, tries to start, then
"BANG," when the pilot either let go of the primer, or had too much
throttle, just as the engine started.
Mercifully, this is one characteristic that did NOT carry over to flat
engines, where you really have to work at it to get a "bang."
|8107. One cause of afterfiring in an aircraft engine
Asticking intake valves.
Ban excessively lean mixture.
Can excessively rich mixture.
FAA-approved answer: C
AMT study guide explanation: Afterfiring, or torching,
is the burning of the fuel-air mixture in the exhaust manifold after the
mixture has passed through the exhaust valve. After-firing is usually caused
by operation with an excessively rich mixture, such as would be caused by
over priming, improper use of the mixture control when starting, or by poor
This one is correct, I think. Afterfire happens when the charge makes it
all the way through the cycle without being lit off at all DURING THE START.
It gets squirted out into the exhaust, where there just might be enough
heat/fire to light it off. During start, there will be more than enough air
(and oxygen) in the exhaust pipes to mix with it, and make it flammable, or
|8678. Why must a float-type carburetor supply a rich
mixture during idle?
AEngine operation at idle results in higher than normal
BBecause at idling speeds the engine may not have enough airflow
around the cylinder to provide proper cooling.
CBecause of reduced mechanical efficiency during idle.
FAA-approved answer: B
I can't believe even the FAA could go this far wrong! Tell me it isn't so!
See the previous question and answer for a better look at this. Mike offers an
Mike Busch's comment: Which is presumably why most pilots taxi
around at full-rich and foul the crap out of their spark plugs. The actual
answer to this question is "because a very rich mixture is required for
cold-starting, and aircraft carburetors don't have a choke to provide such a
rich mixture, so the idle mixture has to be set extremely rich ... which is
why as soon as the engine starts to warm up, you need to come back on the
mixture control." Of course, that answer isn't one of the choices
|8773. Carburetor icing is most severe at
Aair temperatures between 30 and 40 degrees F.
Clow engine temperatures.
FAA-approved answer: A
Mike Busch's comment: Oh really? At OATs in the 30-40F range,
the air seldom contains enough moisture to create severe carb icing. (Maybe
the air in OKC is frequently supersaturated at those temperatures, but not
here in California.) The risk of carb icing is greatest on warm moist summer
days. The jerk who wrote this question was probably a non-pilot.
I couldn't have said it better myself! Can you spell "Carbureted
Cessna 182, in Florida?"
Remember, to pass the A&P exam, you have to memorize this and other
wrong answers, or risk failing!
|8808. In addition to causing accelerated wear, dust or
sand ingested by a reciprocating engine may also cause
Asilicon fouling of spark plugs
FAA-approved answer: A
AMT study guide explanation: Sand that gets into an
engine acts as an abrasive and causes accelerated wear of the cylinder
walls. Silica in the sand also forms a silicon glaze on the nose core
insulators of the spark plugs. This form of contamination is an insulator at
low temperature, but becomes a conductor when it is heated.
Mike Busch's comment: Has ANYONE ever heard of silicon fouling
of plugs? I've never heard of it in 35 years of flying and 14 years of
swinging wrenches. The Champion Spark Plug manual certainly doesn't mention
it, although it covers both oil fouling and lead fouling in detail. On the
other hand, "sludge" is caused by particulate contamination that
comes out of suspension from the oil, so my vote for the correct answer is
"B" ... but the FAA didn't consult me.
This one's a total mystery to me, too. I'm willing to learn, so can any
reader come up with anything on this? Silicon is an element of sand, so if it
shows up in the oil analysis, it's an indicator that you've got a dirty filter
(bypassing it), have been operating in dusty conditions too much with the carb
heat on (bypasses the filter on most engines) or your mechanic used too
much DC-4 silicon grease when he installed your oil filter. Any dirt and sand
that comes in the intake gets pulverized, a bit of it blows by the rings into
the case, that mixes with the oil, and the oil becomes a bit abrasive.
|8829. Which of the following defects would likely
cause a hot spot on a reciprocating engine cylinder?
AToo much cooling fin area broken off.
BA cracked cylinder baffle.
CCowling air seal leakage.
FAA-approved answer: A
Mike Busch's comment: In my experience, uneven cylinder cooling
(hot spots) are most often caused by defective or misaligned baffle seals
(answer C) and, less frequently, missing or cracked baffles (answer B).
Broken cooling fins are down at the noise level as a cause.
Let's see here, the FAA makes up three answers, and says the "wrongest"
is right. Mike is right again. (Durn, that boy is good, he ought to have an
A&P certificate!) I suppose if you broke off enough fins, A could be
correct, but baffling is so very important, it's far more likely to be either
B or C.
|8982. If a flanged propeller shaft has dowel pins
Ainstall the propeller so that the blades are positioned for hand
Bthe propeller can be installed in only one position.
Ccheck carefully for front cone bottoming against the pins.
FAA-approved answer: B
Mike Busch's comment: Modern opposed engines use flanged
crankshafts with holes for dowel pins that are an interference fit into the
propeller hub and mate with registration holes drilled in the crankshaft
mounting flange. There are two identical pins spaced 180-degrees apart,
which permits the propeller to be installed in either of two orientations.
If the prop is three-bladed, one of these orientations results in the
vertical blade pointing down when the engine stops, and the other
orientation results in the vertical blade pointing up. Only one of these two
orientations is correct, and which is correct depends on the particular
installation, so you have to check the service manual. Installing the prop
in the opposite orientation often results in serious vibration problems.
Answer B is just plain wrong (as are A and C).
If this were an internet forum, I'd be ROF,L (rolling on floor, laughing)!
Hand-propping? A current FAA test, talking about hand-propping? But I do seem
to have a very dim memory of wooden props on the very old birds being
installed so that it was easy (or hard) to hand-prop them, for that was the
only way to get them started.
Mike is too young, but this old phart can remember the days when you could
holler at almost anyone on the ramp, "Hey, gimme a prop, willya?"
The old litany would ensue, "Off and closed," a few flips, and then
"Make it hot," or (more formally), "Brakes, and contact!"
We used to have contests to see who could make a Champ (N1943E, why do I
remember that?) prop spin the most turns, or who could prop a Bonanza, or a
Then there was a period when that first call across the ramp might be,
"Hey, do you know how to hand prop an airplane?"
Now? Well, I guess it's safer now, but damn, it ain't near as much fun.
But, I digress
Mike's case, the test is a pain, but relatively harmless. But what of the
person who doesn't have the vast background he does? Mike can learn the bogus
answers, regurgitate them (an appropriate simile), and then (as he says)
"dump them into the bit bucket where they belong." But what of the
person learning this junk from the FAA manuals, at the FAA-approved schools,
who takes the tests, gets his A&P, and goes out to practice. (Oh, and what
of the turbine questions, where Mike is not an expert? Are there as many bad
Now, with all this, you bring your favorite flying flivver in, your
mechanic finds a cracked cylinder, and next think you know, "Yep, I tole
ya, you're running too lean!"
Or, the other scenario we're seeing/hearing a lot. Read the old ads in
Trade-A-Plane, you'll find a HUGE percentage of the big-bore flat six engines
mentioned there were "topped" or overhauled at 500, or 600 hours.
This is almost the rule, rather than the exception. That was LONG before
anyone was running LOP, because they couldn't.
Today, someone runs the same old engine to 500 or 600 hours, decides to put
in GAMIjectors, runs LOP a few times, and suddenly finds his compression
dropping. Our mechanic nods wisely and, "Yep, I told ya running LOP would
do you in, it ain't natcheral!"
it time someone updated some of these tests? Shouldn't the tests cover modern
engines to a greater extent, and the radials a bit less, more in proportion to
the engine count? From my personal viewpoint, loving radials, I hope they
don't do away with the radials entirely, the lack of qualified mechanics is
one factor killing the old airplane, but at least the questions and answers on
radials ought to identify them, and be right.
Couldn't we make all FAA knowledge tests so that instead of three
ridiculous and sometimes equally wrong answers, we could have two that are
wrong logically, and one that is RIGHT, 100% right, instead of one that is
Be careful up there!