Powerplant management guru Kas Thomas of TBO ADVISOR examines the physics and metallurgy of "shock cooling" and concludes that, contrary to the conventional wisdom, it is not a major contributor to cylinder head cracking.
February 19, 1997
|About the Author ...
Kas Thomas is among the best-known aviation technical writers and a
world-recognized expert on piston aircraft engines. Kas is a frequent speaker at Oshkosh and
AOPA Expo, and has written hundreds of articles on technical topics for
Light Plane Maintenance (which
The Aviation Consumer, General Aviation News & Flyer,
Private Pilot, Plane & Pilot, and many others.
He is author of numerous aviation books and is the editor-in-chief of TBO Advisor
magazine. Thomas holds ASMEL, instrument and rotorcraft ratings and is the owner of a Cessna 310.
lives in Wilton, Connecticut, with his wife Rita and their two children Justin and Mallory.
long ago, a writer for a major aviation publication called to ask
my opinion(s) on the subject of shock cooling. It turns out the
caller had already written his article, but he wanted to run some
ideas by me to make sure he wasn't missing something. Since I get
a lot of calls on this subject, I had some ready answers for him.
Not necessarily correct answers—just ready answers.
|Photo: Fred Weick made many contributions to our knowledge
of engine cooling.
I don't think anybody has provably correct answers to
questions involving shock cooling of aircraft engines. To my way
of thinking, there is no scientific proof that shock cooling
plays a significant role in cylinder damage in aviation.
"Scientific proof" is perhaps a poor choice of words.
What I'm simply trying to say is, the hard evidence is scanty. I
know of no fleet studies on this subject. I know of no pilot who
can say "I went up and did this and this and this to the
engine, and then when I landed I found these cracks that weren't
Still, it's hard to argue with common sense, and common sense
says that if you thermal-cycle a piece of cast aluminum
(especially while beating on it!) you just might induce it to
crack. Pilots can perhaps be forgiven for harboring a strong gut
feeling that yanking the throttle back is a good way to bring on
cylinder cracking. Certainly, many millions of dollars' worth of
spoiler kits and CHT systems have been sold to pilots on this
basis over the years.
My own gut
tells me that shock cooling—while bound to induce dimensional
changes in the engine—is not a great contributor to cylinder
cracking. We know it induces dimensional changes, because (for
example) valve sticking has been induced in some engines by
sudden power reductions. (A Lycoming Flyer article once stated:
"Engineering tests have demonstrated that valves will stick
when a large amount of very cold air is directed over an engine
which has been quickly throttled back after operating at normal
running temperatures." See
101 Ways to Extend
the Life of Your Engine, page 96.) But it's a big jump
to go from that to saying you can make a cylinder head crack just
by pulling the throttle back too quickly.
To my knowledge, Bob Hoover has not experienced any problem
with cylinder-head cracking on his Shrike, despite his rather odd
predisposition to feather both engines while in a redline dive.
(Maybe this is what FAA meant by "cognitive defect"?
Besides which, I think any careful examination of the concept
of "cooling" (as it applies to current aircraft
engines) will leave one virtually empty-handed, because I think
it could be argued that cooling fins on aircraft cylinders are of
mainly ornamental value. I suspect that you could hacksaw much of
the finnage off, say, a TSIO-520's cylinders and not affect
inflight CHT readings by very much. As it happens, this is
exactly what Continental did when it created the
"lightweight" Crusader engine—the TSIO-520-AE used in
the Cessna T303. The cooling fins on this engine are fewer in
number, and about half the size of, those on a standard TSIO-520.
And yet, CHTs in the T303 are remarkably cool. (One of our
readers, in fact, reports a problem in getting CHTs to stay in
the green; see this month's "Questions and Answers,"
Various investigators have done "energy balance
sheets" on aircraft engines, and the result is always the
same: Only about 12% of the heat energy generated in combustion
goes out an "air-cooled" engine's cooling fins. The
biggest fraction (around 44%) goes right out the exhaust pipe, of
course. Another 8% or so finds its way into the oil—which is
quite interesting, because it means the oil plays almost as big a
role in cooling your engine as air does. The remaining energy
shows up as work at the crankshaft.
Throttle placement doesn't have nearly as direct an effect on
CHT as you might think. Back in 1983, there was an SAE paper (No.
830718) by three Texas A&M researchers who tried to correlate
OAT (outside air temp), CHT (cylinder head temp), EGT (exhaust
gas temp), power settings, air density, and cowl pressure drop in
Lycoming TIO-540 engines. Their work was partly based on the NACA
Cooling Correlation (NACA Report No. 683, published in 1940),
which in turn was based on pioneering work done by Fred Weick in
the late 1920s. The Texas A&M group merely extended NACA's
approach, verifying their results with inflight measurements
taken on a Piper Turbo Aztec and a Rockwell 700. One of their key
findings was that the difference between CHT and OAT is
proportional to the difference between EGT and CHT, which is (if
you dwell on it long enough) intuitive, since the difference
between the average exhaust temperature and CHT is what
"drives" CHT changes to begin with. (If this isn't
intuitive to you, you may want to go back and re-read Fourier's
classic Analytic Theory of Heat.) This portion of the group's
findings might be summarized by saying that the stored heat of
the cylinder head is proportional to the input heat, represented
by EGT minus CHT.
But there are two aspects to cylinder cooling. One is the
"supply side" aspect (which we have just been taking
about—all this business about EGT minus CHT), while the other is
the taking-away of heat, or "cooling" aspect. The Texas
A&M group accounted for this too. They found that the stored
heat is proportional to the input heat—proportional, that is, by
a factor y. The factor y, in turn, is made up of engine power
raised to a certain exponent, divided by cooling airflow delta-p
raised to a certain exponent. The engine-power exponent is
fractional; for the Rockwell 700 it turns out to be 0.33. (It
varies from plane to plane depending, apparently, on
peculiarities of engine installation and operating envelope.) The
air-cooling delta-p exponent is also fractional (0.29). In plain
English: CHT depends on the cube root of engine power, divided by
the cube root (roughly) of the cooling-airflow pressure drop.
After a few rough scratchpad calculations, you find that
cutting an engine's power by half (but leaving airspeed constant,
such as in a descent) results in a CHT drop of only 10% or so, or
about 80¡ F. (Recall that in calculations of this sort, you want
to use a Rankine temperature scale, which begins at absolute
zero, or minus-460°F.) Most of the time, a 50% power cut is
accompanied by some loss of indicated airspeed, which would tend
to offset the CHT drop, making it less than 80° F. The numbers
are within reason, evidently. But is this kind of CHT drop
capable of trashing a set of cylinders? I doubt it.
Of course, the rate of the drop is plainly an important factor
here (not just the magnitude of the drop). In this connection, I
am reminded of an experiment once done by John Schwaner (of
Sacramento Sky Ranch). It seems Schwaner, curious as to whether
he could "crack" a cylinder at will, in a shop
environment, one time took a cylinder that was heated to several
hundred degrees in an oven (I believe it was an O-320 jug,
although here I'm going from memory) and dunked it in a bucket of
cold acetone. The abruptly cooled cylinder was later examined,
and no abnormalities could be found in it.
And then there's ordinary rain. Every pilot flies through rain
at one time or another, and rain should be a very effective
coolant (more so than mere air, certainly)—yet no one, as far as
I can determine, ascribes cylinder damage to flying through too
much rain. In fact, most pilots (I think) consider just the
opposite to be true; namely, that flying through rain is good for
an engine, because of the extra cooling.
Let us assume that a moderate downpour contains one cubic
centimeter (one gram) of water per cubic meter, and let us
further assume a cooling airflow of 100 cubic meters per minute
for a high-performance engine. (David Thurston's Design for
Flying suggests 77 cubic meters per minute as typical for many
engines.) We might reasonably expect, therefore, that 100 grams
of water might enter the cowling per minute while flying in rain.
Considering that water has a heat of vaporization of about 540
cal/g, it's not impossible for 100 g/min of rain influx to give
about 54,000 cal/min of cooling, which is about 200 British
Thermal Units per minute.
The question is, how does this compare with the heat of
combustion? We can do a rough calculation this way: We know that
(by ASTM spec) avgas contains a minimum 18,720 BTU per pound or
about 112,320 BTU per gallon. If an O-470 burns 13 gal/hr in
cruise (or 78 lb/hr, roughly), the engine is capable of producing
24,336 BTU per minute of combustion heat—if combustion is 100%
efficient. In the real world of mixture maldistribution, rich
mixtures, and incomplete combustion, we can safely say that
probably no more than 21,000 BTU/min of heat is actually
liberated, of which 12%, or 2,520 BTU/min goes to the outside
world via the cylinder cooling fins. If rainwater cooling was
100% efficient (no droplets escaping between cooling fins; all of
the water 100% evaporated in contact with fins), we might expect
rain to reduce the cylinder fins' burden by about 8% (200 divided
by 2,520). If you could somehow translate this into a direct CHT
reduction, it might mean a reduction of 64°F (assuming your CHT
started out at 800° Rankine). That's a pretty sizable reduction
of CHT. In fact, it should qualify as shock cooling.
I think the fact that Navajos and 421s aren't raining engine
parts down on unsuspecting civilians while flying through precip
(I was going to say while penetrating virga—but decided against
it) is pretty good evidence that "sudden cooling" of an
air-cooled engine does not contribute in any dramatic way to
If shock cooling were a definite hazard, your engine should
fall apart when you bring the mixture into idle cutoff at the end
of a flight. CHTs fall at a rate of 100°F/min or more in the
first seconds of shutdown—triple the rate that starts the
typical "shock cooling" annunciator blinking. Does
anyone complain that repeated shutdowns are causing head
cracking? Of course not.
Then why are we worried about pulling the throttle back?