Shock Cooling a myth?

[PostmasterGeneral]

With regards to running an engine "over square" what's the logic behind this? I can't find any concrete evidence that suggests doing so will cause additional wear on an engine. In fact, leaving the throttle wide open the entire flight and pulling the prop RPM back further would in theory increase the volumetric efficiency of the engine, by having as little restriction as possible within the intake tract, would it not?

I wonder where the notion that it was verboten first came from?

[photofly]

There's no real logic. Some Continental engines are not to be run at low RPM due to detuning of the crankshaft counterweights, that is believed to have led to a spate of engine failures: http://www.tcmlink.com/pdf2/CSB09-11A.pdf

[fleetcanuck]

Cpn Crunch... I've never seen any shock cooling warnings when leveling off and reducing power with an O-360.

I can assure you that with an O-200 you can get shock cooling (>30F/min) coming back from climb to cruise in the circuit. I have an EI monitor and have noted it on occasion. As a result I have changed procedures.

[CpnCrunch]

Cpn Crunch... I've never seen any shock cooling warnings when leveling off and reducing power with an O-360.

I can assure you that with an O-200 you can get shock cooling (>30F/min) coming back from climb to cruise in the circuit. I have an EI monitor and have noted it on occasion. As a result I have changed procedures.

I have an EDM 700, and the only time it has shown me warnings of >50F/min is when going straight to idle at high speed. I don't do that any more. Levelling off to cruise power seems fine.

[ahramin]

I can assure you that with an O-200 you can get shock cooling (>30F/min)

30°F / min? Where did that number come from?

[photofly]

Shock cooling is when I toss my aluminum welding piece into a bucket of cold water and it cools from 1200° to 90 in about 0.25 seconds

[vcollazo]

I used to be unsure about this until 2 yrs ago. In 8 years of flying our Navajo we had never had a cracked cylinder. 2years ago #2 and#3 cylinders on our right engine (our hotter running engine) came up with low compression due to cracks running from the intake valves towards the top plugs. I decided to check the engine monitor data and found that our newest partner who had lots of turbine time, but little piston time was going to idle on descent while maintaining cruise airspeed, and then going immediately to cruise power on level off. There was a temperature drop pf about 42F in less than a minute and then an increase of that much very quickly on level off. I counseled him and we have had no more issues. Turbines also have issues with extreme heat cycles. The F111s that I flew used to need engine overhauls every 800 or so hours while the DC9s that I flew at NWA would go well over 3000hrs before needing a hot section (Both airplanes had basically the same engines PW JT8Ds though the fighters had afterburners which did not affect main engine temps). In the 111 we would go from full military power to dead idle a dozen times in the dive bombing pattern while maintaining 450 to 500 KIAS.

[fleetcanuck]

@ahramin;
As i recall that was an example number in the EI literature for the installation of the monitor and seemed like a conservative figure to use.

[Therewewere]

Shock Cooling: Myth or Reality?
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.
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.
Kas lives in Wilton, Connecticut, with his wife Rita and their two children Justin and Mallory.
Not 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 probably 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 there before."
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"? Just kidding.)
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," page 26.)
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 cylinder-head cracking.
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?

[goldeneagle]

When I was at FliteSafety this issue was discussed in detail with regard to the C-421, a rather slippery airplane with a reputation for cracking cylinders due to 'shock cooling'. Some time after the aircraft went into service and this issue started to become well known, apparently Cessna went back to the engine manufacturer to find out why the GTSIO-520 tended to have a lot of cracked cylinders on a 421 when it was typically used for long flights compared to the IO-520 on a 185 floatplane, where lots of short flights at low altitude with much more abuse and many more heat/cooling cycles, and not so many cylinder cracks. Early thoughts were that it revolved around the gearing and turbo on what was otherwise the same engine core. After some investigation the answer came back like this.

185 tends to have shorter flights at much lower altitudes. The 421 typically does longer flights at much higher altitudes. During those longer flights up high, the fuel in the tanks will cold soak, so the fuel in the 421 will be MUCH colder than that in the 185. Then the 421 comes barreling down from altitude, and on short final the checklist says 'mixture rich'. Suddenly you are splashing a whole bunch of cold soaked fuel on the hottest / weakest part of the cylinder head, that small piece of metal between the injector and the spark plugs. Not co-incidentally, that's where most of the cracks occur. Shock cooling is not from the power reduction during descent, it's from the extra cold fuel when you suddenly go mixture rich with engines at / near idle.

The reccommendation at that time from engine manufacturer was to move 'mixtures rich' from the pre-land to the 'on go around' checklist, but many regulatory authorities were not happy about that change, so alternatively they reccommended that mixture be advanced slowly during descent, so on short final when checklist says 'mixture rich', the answer is 'already there', and all that extra cold soaked fuel has been introduced slowly to the small hot chunk of metal prone to cracking.

Bottom line is, every installation is somewhat different, they all have their own set of quirks, some of which come from mechanical reasons, others from operational reasons. I've got on the order of a thousand hours in the 421 and always used 'one click a minute' on the mixture during descent. I suspect the cost of the course at FliteSafety paid for itself on that one tidbit around the cylinder cracking history and reputation of the airplane.

[Heliian]

Shock Cooling: Myth or Reality?

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?

Shock cooling is not a myth but it might not cause instant cracking. We can move past just cylinder damage, it's not good for the exhaust system either.

I think it's good to heed the warnings if you're trying to get the maximum life from your engine and these practices will translate to other aircraft.

[photofly]

on short final the checklist says 'mixture rich'.

Why does it say that?

[ahramin]

on short final the checklist says 'mixture rich'.

Why does it say that?

To have full power available for a go-around.

[photofly]

And you can’t move the mixtures to rich at the time when you decide to go around because...?
I see the comment above about “regulatory authorities didn’t like it”, but why?

As to whether going mixture rich with the throttle at idle would make any difference to temperature, regardless of how cold the fuel is, there’s hardly any of it flowing. Mixture rich with significant power? Sure. But with the throttles closed? Who would have thought.

[ahramin]

And you can’t move the mixtures to rich at the time when you decide to go around because...?

You can, and many of us do. But I doubt that's what you did in primary training. The manufacturers know that this technique would be better for the engine, but they also know that it is not what is being taught.

I see the comment above about “regulatory authorities didn’t like it”, but why?

The regulators may or may not know which technique is better but they certainly know what is being taught. Pretty standard stuff really: before conducting a landing, the aircraft is configured as much as possible for a go-around. Makes the go-around as easy as possible. We know that you and I can do it, but when some idiot forgets the mixture and kills the engines applying power for a go-around, the manufacturer is going to look pretty stupid requiring a technique that caused an accident and could easily have been avoided.

As to whether going mixture rich with the throttle at idle would make any difference to temperature, regardless of how cold the fuel is, there’s hardly any of it flowing. Mixture rich with significant power? Sure. But with the throttles closed? Who would have thought.

No point guessing, I'll collect some data on my next flight and post it.

[photofly]

But I doubt that's what you did in primary training. The manufacturers know that this technique would be better for the engine, but they also know that it is not what is being taught.

I see your point, but really we ought to teach best practice, not advocate poor practice just because poor practice is what is taught. Tail wagging the dog, and all that.

[goldeneagle]

And you can’t move the mixtures to rich at the time when you decide to go around because...?
I see the comment above about “regulatory authorities didn’t like it”, but why?

The manufacture checklist in the 'pre landing' section says 'mixture rich'. Regulatory folks these days are pretty adamant, we can add items to various checklists, but, that listed in the POH must still be there. When I initially re-worked checklists for the airplane while we were applying for the 703 certificate, I was forced to move the 'mixtures rich' back to pre-land before the transport inspectors would approve things.

As to whether going mixture rich with the throttle at idle would make any difference to temperature, regardless of how cold the fuel is, there’s hardly any of it flowing. Mixture rich with significant power? Sure. But with the throttles closed? Who would have thought.

Thing is, those throttles will not be at idle when completing the pre-land checklist. Dunno about other airplanes, but I can say with some certainty for the 421, with the gear down and flaps out, you will require 22 inches of manifold pressure to hold the glideslope, possibly a bit more if you have a heavy airplane with seats full. In this day and age of stabilized approaches, those throttles wont come back to idle until the flare. So yes, from pre-land to flare the engine will sit at significant power for at least a minute or two, longer if you are doing an approach in IMC.

[ahramin]

But I doubt that's what you did in primary training. The manufacturers know that this technique would be better for the engine, but they also know that it is not what is being taught.

I see your point, but really we ought to teach best practice, not advocate poor practice just because poor practice is what is taught. Tail wagging the dog, and all that.

I'm not so sure. Not everyone is always going to be current, competent, and confident at all times. When I transition a pilot for the first time to an aircraft with a constant speed prop, one of the first things we go over is that the blue knob can stay all the way in as much as they want. It can stay there all day long if that's what it takes. Basically they are not to touch it unless everything else it done and they have time to think about what they are doing. This lets them get used to everything else new with their aircraft without stressing about a system that they do not yet understand instinctively.

It's not ideal, it's certainly not best practice, it's definitely not efficient. It's safe, and easy. Similarly, training pilots with procedures for the red knob that are easy and reduce the workload during a go around is a skill that may come in useful at some point down the road when they haven't flown in 6 months or are in a new aircraft for the first time. Unless we're going to require every pilot to have 10 hours dual on every aircraft they fly and then 10 hours PIC a month to maintain currency.

Keep in mind some of us are barely hanging on for dear life, we're not all professional pilots.

[photofly]

I just can’t in my head classify a go-around as a difficult and stressful manoeuvre that needs workload-reducing but suboptimal suboptimal hacks to make it accessible to an average pilot.

And if mixture rich on final damages engines, it doesn’t meet the “safe and easy” test, does it?

I’m a lazy pilot, I’m afraid. I don’t touch a lever until it needs to be touched. It reduces wear to the finish on the knobs and handles, and keeps the plane in tip top condition.

Now, if you want to talk about shock cooling a cylinder head by the rapid introduction of lots of very cold fuel, let’s talk about go-arounds themselves.

[PilotDAR]

I've never seen a benefit in using an uncertain outcome to attempt to justify not being easy going on an engine, and airplane, or any machine, for that matter. Plan ahead, and be gentle on the machine. For those [hopefully]rare occasions when things surprise you, do what you have to do to assure a safe flight.

I recently sat right seat to a well experienced pilot while stall testing a turbine airplane. He was rough with the power lever. He'd pull it rapidly from cruise power to idle (and I'd test my shoulder harness), and then open it up again quickly enough the I was seeing 100 RPM propeller exceedances, as the governor tried to keep up. There was zero urgency in the maneuver, nor operational need for haste. I commented a couple of times about this during the flight. In the post flight company debrief, he asked if there had been any safety concerns during the last flight. In the presence of everyone, I said that I had no safety concerns, though I would appreciate a more gentle touch on the power lever. He acknowledged, and the following flights were better.

If you can be gentle to the plane, be gentle to the plane, don't try to argue why you shouldn't have to be....

[CpnCrunch]

The problem with not putting the mixture full rich on final is that if you forget to richen it on a go-around you could destroy the engine. See today's savvy aviation newsletter where a guy did exactly this with an IO-540, and his #5 CHT went up to 600F and was over redline for 6 minutes! Luckily there was only minor damage to the spark plugs, perhaps thanks to Lycoming cylinders being more tolerant of high temperatures.

[photofly]

The problem with not putting the mixture full rich on final is that if you forget to richen it on a go-around you could destroy the engine. See today's savvy aviation newsletter where a guy did exactly this with an IO-540, and his #5 CHT went up to 600F and was over redline for 6 minutes! Luckily there was only minor damage to the spark plugs, perhaps thanks to Lycoming cylinders being more tolerant of high temperatures.

There are a million "if you forget to do X then you'll screw up the plane, crash, die, etc." items involved in flying. Training involves learning to not forget to do any of them.

Leaving the mixture too lean in a full power climb for six whole minutes is a defect in procedure way, way, way beyond simply "forgetting to put the mixture rich on the go around". It involves that, yet, and but way more significantly in involves failing for six minutes (count them, yes all six of them) to check the configuration of the aircraft or the engine indicators.

Frankly, if you can learn to put the mixture rich on short final then you can learn to put the mixture rich at any given time, including when you increase power for a go-around. If you are a pilot who is likely to forget to put the mixture rich on a go-around then you're likely to forget it at any other time, too.

[CpnCrunch]

Frankly, if you can learn to put the mixture rich on short final then you can learn to put the mixture rich at any given time, including when you increase power for a go-around. If you are a pilot who is likely to forget to put the mixture rich on a go-around then you're likely to forget it at any other time, too.

The problem is that if you don't do a go around on every flight, you're not training yourself to do that. I don't think I've done an actual go-around in years, other than practice ones. The last time I did an actual one I forgot to turn the carb heat off. I've also forgotten to put the flaps up after takeoff a few time, so I practice takeoffs with flaps more often now. If you put the mixture rich on every flight at the same point, you are much less likely to forget it than if you only do it once every 5 years when you do a go-around.

[photofly]

There’s nothing special about adding full power for a go around compared to adding full power for any other reason.

I think you train to put the mixture rich as a prelude to applying full power, at any time you apply full power. I do that, several times on any flight.

I have no greater concern that I’ll forget to enrichen the mixture(s) on a go around than I do on any other occasion I apply full power.

I also think checking the aircraft is correctly configured for whatever phase of flight you intend, is wise.

If you find you don’t get the go around correct, perhaps practicing it more often is the answer.

[RedAndWhiteBaron]

Let me ask a naïve student question here.

Assume I'm flying a trainer. Assume I have correctly configured to land, with the mixture correctly leaned as per however the PoH tells me is correct. What is the risk inherent in forgetting to go full rich? Is it a loss of power upon applying full throttle, or is it the risk of overheating? Or is it something I have not considered?