Shock Cooling (was Flaps/Slowing Plane)

Some discussion came up in another thread about "shock" cooling of engines in descending and slowing for the pattern from cruise altitude and speed.

The best information I've seen about this is in this Avweb article. The bottom line (literally!):

"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?"

I'm thinking corrosion caused by underused engines is a lot more damaging than pulling power.

Fuel to the fire

I used to tow gliders for a club. We used Super Cubs and a Pawnee as tugs. The life of a glider tug is brutal! Slow extended climbs at slow airspeeds are the norm. When the glider releases, pull the power back and dive. We did try to minimize the shock cooling by extending the decent time and use a myriad of techniques but when gliders were waiting to get in the air, you had to get them in the air fast (**** the techniques). Basically the flight cycle was the perfect storm for shock cooling.

The mistreatment showed itself at annual in the form of cracked heads usually (maybe always) radiating out of the exhaust port. We always, always had cracked heads. The Super Cubs seemed more susceptible to it than the Pawnee. I know this is anecdotal but my personal experience is more is line with the old wives tales than with the ?experts?. I wonder, if the ?experts? drove a tug would they still come to the same conclusion?

You don't know if the cracked heads are due to shock cooling. It might as well be shock heating, or simply due to extended periods of running at max power with low speed (heating).

You don't know if the cracked heads are due to shock cooling. It might as well be shock heating, or simply due to extended periods of running at max power with low speed (heating).

Click to expand...

What is the difference between a take-off in a RV (or flight school plane if you like) and a Super Cub glider tug, shock heating wise? They both go to full power they both stay within recommended CHT temps. The tug would only climb to 2000 AGL so time in climb was 6-9 minutes typically. Not a whole lot of difference here.

The decent phase is drastically different. In the RV you most likely have stable relatively cool CHT?s and reduce the power gradually until you land. In the tug you are full power, high CHT?s and have an airspeed between (55-75 mph depending on the glider), chop the power to 50% or less while diving (diving!) and gradually reduce the power until you land. The entire flight in the tug was typically 8-12 minutes so the gradual power reduction was not really all that gradual.

I had thought that maybe the number of thermal cycles (for lack of a better term) was the culprit for the cracked heads but flight schools usually make TBO (that?s what I hear anyway) and they have a high number of thermal cycles too.

So yeah in my mind, shock cooling is the culprit

Flying slow at full power surely must be worse than flying faster at full power since there will be less cooling when flying slow. The number of thermal cycles can be indefinite or at least within TBO, if the amplitude (variation in temperature in this case) is below the threshold for fattigue. If you go above that threshold, cracking starts immediately at any minor fault in the material, propagates with encreasing velocity for each cycle and soon becomes a visible crack.

Anyway, shock cooling is not possible unless it is really hot to start with, so in my opinion shock cooling is not the only answer. "Shock warming" may just as well be the answer, and of course the cycling itself.

Nobody paid any attention to shock cooling till fancy engine monitors came out. Seaplanes are bad for cracks around the spark plugs. Thats not from shock cooling but a lack of cooling during water operations.

Yea thats the ticket

rfinch said:

Some discussion came up in another thread about "shock" cooling of engines in descending and slowing for the pattern from cruise altitude and speed.

The best information I've seen about this is in this Avweb article. The bottom line (literally!):

"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?"

I'm thinking corrosion caused by underused engines is a lot more damaging than pulling power.

Click to expand...

Sure you have the right attitude. Corrosion is worse. However a jump plane full power for 20 minutes and than chops and drops back to airport does seem to crack cylinders like a son of a gun. For us with carbs a long let down from altitude at low power (low heat) is a sure recipe for carb ice. You DO want to keep some power during LET DOWN. Back driving engines w/ low cyl pressure can cause ring flutter. Does not sound good to me.

Thermal cracks happen just fatigue or cycles. It's not just a one time deal, its 10,000's little cycles, it adds up. So if you go up full throttle to full temp than back to idle to min temp and repeat that over and over several times a flight, yea you will cause problems. I have no doubt.


This is what Lyc says:

A. GENERAL RULES
1. Without exception, observe the red-line temperature limits during takeoff, climb and high-performance cruise power operation.

a. Cylinder head temperature — maximum limit listed in the Lycoming Operator’s Manual.
b. Oil temperature limit — maximum limit listed in the Lycoming Operator’s Manual.
c. TIT - maximum allowable limit specified in the Lycoming Operator’s Manual.
2. Whenever mixture is adjusted, rich or lean, it should be done slowly.
3. Always return mixture slowly to full before increasing power setting.
4. At all times, caution must be taken not to shock-cool the cylinders. The maximum recommended temperature change should not exceed 50˚ F per minute.

AND


And finally, power-off letdowns should be avoided. This is especially applicable to cold-weather operations when shock-cooling of the cylinder heads is likely. It is recommended that cylinder head temperature change not exceed 50˚ F. per minute. Plan ahead, reduce power gradually and maintain some power throughout the descent. Also keep the fuel/air mixture leaned out during the descent. If an exhaust gas temperature gage is installed with a normally aspirated engine, keep (EGT) peaked to ensure the greatest possible engine heat for the power setting selected; for a turbocharged installation, lean to peak during descent unless otherwise specified in the Pilot’s Operating Handbook, or under conditions where the limiting turbine inlet temperature would be exceeded.

AND

Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock-cooling of the cylinders. Operations that tend to induce rapid engine cooldown are often associated with a fast letdown and return to the field after dropping parachutists or a glider tow. There are occasions when Air Traffic Control also calls for fast descents that may lead to sudden cooling. The engine problems that may be expected when pilots consistently
make fast letdowns with little or no power include:

1. Excessively worn ring grooves accompanied by broken rings.
2. Cracked cylinder heads.
3. Warped exhaust valves.
4. Bent pushrods.
5. Spark plug fouling.

Generally speaking, pilots hold the key to dodging these problems. They must avoid fast letdowns with very low power (high-cruise RPM and low manifold pressure), along with rich mixtures that contribute to sudden cooling. It is recommended that pilots maintain at least 15" MP or higher, and set the RPM at the lowest cruise position. This should prevent ring flutter and the problems associated with it. Letdown speed should not exceed high cruise speed or approx 1,000 feet per minute of descent. Keeping descent and airspeed within these limits will help to prevent the sudden cooling that may result in cracked cylinder heads, warped exhaust valves and bent pushrods.

The mixture setting also has an effect on engine cooling. To reduce spark plug fouling and keep the cylinder cooling within the recommended 50˚ per-minute limit, the mixture should be left at the lean setting used for cruise and then richened gradually during descent from altitude. The lean mixture, maintaining some power and using a sensible airspeed should achieve the most efficient engine temperatures possible. The operating techniques recommended in this article are worth consideration as they will be a positive step toward saving dollars that might be spent on maintenance. Whatever the circumstances, pilots must plan their flight operations so that the potential damage caused by sudden engine cooling can be avoided.

Sounds like good advice. The don't worry about it, you can do no harm per AvWeb way too Laissez-faire for me. It's just their opinion. I would definitely TRY to follow some of the above tips, especially since it does not impose any operational limits. You want in general to descend at no more than fast cruise speed & reasonable descent rate if possible (by starting down early). Common sense and good technique.

Personally I try to plan ahead & reduce MP an 1" or two at a time if possible. I try to never just pull it to idle, unless I'm just about to touch down. However sometimes you got to do, what you got to do, to fly the plane. Again corrosion from disuse is worse and as long as you don't make a habit of doing jump plane or glider tow plane missions it should be fine.

Shock Cooling - empirical experience.

I used to tow gliders at Lasham, probably the largest gliding club in the world. In the early'90s we were doing roughly 15-18000 tows per year with 5 tugs. When I first flew my -9a I did find the whole issue of slowing down quite difficult. It became much easier when I re-read Lasham's tug pilot handbook and remembered how I had operated as a tug pilot. I found that with the -9a I was worrying too much about the shock cooling issue.

When we were busy, the normal regime was to hook on a glider, tow it at full power to 2000', descend and repeat. At busy times with several tugs operating and a switched on ground crew, we could average 6 cycles an hour per tug.

The club had consumed 28 cylinders over 4 years and the financial impact was not inconsiderable. After a lot of careful monitoring a new operational regime was introduced; the one I trained on as a tug pilot.

By the time the glider has cleared and you had checked it has cleared, with the tug in level flight, it was already accelerating. This was followed by a very slight throttle closure, just 50rpm, and then a very slow count to ten. It is these 10 seconds primarily, that made all the difference in the consumption of cylinders. This was then followed by a progressive acceleration and gentle reduction in power to bring the rpm back to 2000 / 110Knts which was maintained for the descent. The rpm was held almost all the way to touchdown. From glider release to roll out at the launch point was still no more than 2 minutes. The points I take from this are 1) reduction from full power is key and needs to be managed carefully, though 10-15 seconds is enough, and 2) keeping the engine working during descent(admittedly harder in the RV) is key also. You just need to start several miles out. 3) You can over worry about the cooling when faced with the RV. The tugging regime is far more brutal.

The result of that operational change was a dramatic reduction in the number of cracked cyls..

I did see an article about this experience o the web at:
http://www.faasquadron.org.uk/gascosafety1.html
I think it was written before the operating handbook, but it only differs very slightly.

I hope this is useful to someone.

PS I should add that although I like instruments - I have an AFS on order - all this towing was done with the absolute minimum of instrumentation and attention to it. I think the tugs typically had a CHT gauge on one cyl and OT. With several other tugs in the air, 50 or 60 gliders waiting - in the air - for the thermals to get going properly and up to 4 winch cables running the training fleet, instruments are a very poor second for your attention. Fuel never left full rich with airfield asl of 610'

I do not pretend to be an expert about engines nor do I know for certain as to whether shock cooling is myth or reality, as it applies, to operation of general aviation engines. I suspect the true answer is somewhere in between myth and absolute gospel. I also suspect it depends, very much, on the use to which the aircraft is put.

There are though several factors that occur to me when deciding how to operate my engine and which expert to believe.

1. I do not think we can draw comparisons between routine RV operations and glider towing operations regarding engine wear, cooling and cylinder cracking.

2. Some of the "experts" we love to dis deserve dissing, as they are just journalists, publishing incomplete facts and conjecture mostly based on anecdotal evidence.

3. Some of the experts offer compelling thoughts based on data, experimentation, and documentable results.

4. I always keep in mind it is in Lycomings best interest to convince us that we as pilots are the major cause of engine damage by causing "shock cooling",
running LOP, and a myraid of other "bad habits".

I tend to not believe in shock cooling and believe LOP as it applies to my normal operations based upon the work done by George Braly and John Deakin as well as my own observations.

Over many years I have written multiple letters to Lycoming asking to see the data upon which they base their "rules" enumerated by gmc above. To date I have yet to recieve even acknowledgement that I asked.

The subjects of shock cooling and LOP became important to me when I owned a 685 Twin Commander running 2 Continental GTSIO 520K's each putting out 435HP. Combined they burned 52GPH at cruise and 90+ GPH in the climb.

The 685 is nothing more than a Turbine Commander airframe with piston engines. As you can imagine the plane is underpowered for its size and weight and the climb to the flight levels was long and hard on the engines.

Also a descent from 25,000' where it is really cold is a prime setup up for shock cooling (if it exists) during a rapid descent. Trying to do a prolonged controlled power reduction descent in a 685 is a difficult endeavor and really prolongs the flight (consuming lots of gas).

Because you are cranking out .83hp/cubic inch the stress on the engine especially the cylinders is extreme. Typically 685 engines would require 2 or 3 cylinder replacements at 900 hrs and the rest at TBO (1600hrs) and operating by the book mine was no different.

At major overhaul I decided to run LOP even in the climb and also quit worrying about shock cooling in my descents. Sold the plane at 1200 hrs without replacing any cylinders and it made it to TBO without needing any cylinders. The new owners ran it to 1860 hrs before reman and then just had the existing cylinders reworked.

My presumption ifrom this is the decreased ICPs and decreased cruise CHTs from LOP extended engine life and the more rapid cooling from greater power reductions and more rapid descents was not a huge factor.

Yes this is anecdotal but my point is following the advice of the avweb experts and ignoring the advice of the manufacturers did not lead to disaster. I also do not think you can draw conclusions about how to operate an RV based on this experience.
I suspect following the advice of Lycoming will also not lead to disaster.

To date I have not seen any compelling evidence that shock cooling (as it applies to the GA fleet) is the cause of engine damage.

The manufacturers who publish the rules certainly have not been forthcoming with the evidence upon which those rules are based.

I also think it wise to remember that when you follow another persons advice it is you and not them that suffer the consequences if the advice is wrong.

The glider towing mission is unquestionably hard on air cooled engines. A recent E-mail from a fellow in Oz stated that their gliding club was unable to continue operations economically with Lycoming engines in their Pawnees due to massive numbers of cylinders cracking even while always observing CHT limits.

Their solution was to re-engine the tow planes with Chevrolet LS-1 liquid cooled engines. The life cycle costs were already lower than the Lycoming with only 500 hours and they are able to do 2-3 more tows per hour because they go to flight idle after release (complete fuel shutoff with the OE ECU) and can come down really quick to hook up again. These have been so successful that they are planning to re-engine 23 more low planes with the liquid cooled engines.

My view is that shock cooling is real under extreme circumstances of max CHT then immediate prolonged descents at idle or very low power settings. I think in RVs, just maintaining some manifold pressure is probably enough to prevent damage. We usually don't make 2000-3000 fpm descents. Aluminum expands a lot with high temperature and as it cools, it contracts a lot. This is a fatigue cycle technically. Tow planes probably get more heat cycles per flight hour than any other type of operation.

Prolonged high CHTs may have just as much to do with head cracking and rapid descents. Aluminum loses about 1/2 its tensile strength at 400F. Do this long enough with the mechanical and thermal distortion stresses and it will fail. The local flying school where I used to rent from has many engines going to 2000 hours but also a fair number that need cylinders replaced well before that time so even in this somewhat more benign environment, cracking is still somewhat common.

I'm not trying to start another war here with air vs. liquid cooled engines, just posting the experiences of a large glider tow operation in Australia as reported to me.

too bad the liquid cooled re-engine would not be an option in the US. When I tow in the supercub i usually do a hard slipping turn once glider releases. this allows me to keep the power up in the 2000-2300 RPM range, gradually reducing to maintain Lycomings recommended 50 degF per minute cooldown. I also get descents of 1500-2000 fpm so i can get down for the next hookup. Our club has had no cylinder issues while operating this way.