Shock Cooling

[Martin H]

Really?   Says who, specifically?

 

   Based on what science?

 

   Why is this even a worry?

 

   Thank you,

 

   Martin

[NotarPilot]

My understanding is it's fairly difficult to shock cool an engine. You would have to see a cooling trend in excess of 30-50 F degrees of cooling per minute and that 30-50 range is depending on who you ask.

[orionflt]

here is a good artical on shock cooling from around 1996

 

http://www.avweb.com/news/maint/182883-1.html?redirected=1

[orionflt]

to follow up with that artical I looked up Lycomings engine operating manuals statement on shock cooling: Note that they say" consistantly make fast let downs" not the occasional.....

 

Sudden Cooling

Sudden cooling is detrimental to the good health of the piston aircraft engine. Textron

Lycoming Service Instruction 1094D recommends a maximum temperature change of

50oF per minute to avoid shock cooling of the cylinders.

Operations that tend to induce rapid engine cool down are often associated with a fast let

down 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 approximately 1000 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 50o 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.

[aviatoreb]

And why is shock heating not a problem?

[mooneyflyer]

And why is shock heating not a problem?

Try to keep your Lycomings below 380f and your TCMs below 400f.

Hottest CHTs often occur on departure.

[aviatoreb]

Try to keep your Lycomings below 380f and your TCMs below 400f.

Hottest CHTs often occur on departure.

 

That answer is beside the point of my question.

 

380 is or 400 is for the long term health of your engine.  Shock cooling is an issue that suggest cracks developing immediately in your engine.  If my engine can go from 280 to 380 (or 400 or yes 420 if I am not careful regarding long term health...yes I am careful) in about a minute, and survive...that is suggesting to me that shock cool is no worse.

[DaV8or]

And why is shock heating not a problem?

 

Don't know the scientific reason, but I can tell you that if you've done any welding of aluminum, that you can get cracks from cooling the weld too fast. It happens where the metal is thinnest. I have never heard of cracking due to applying heat, but it may be possible, don't know. I personally would not use this theory to debunk shock cooling.

[BigTex]

Try to keep your Lycomings below 380f and your TCMs below 400f. Hottest CHTs often occur on departure. That answer is beside the point of my question. 380 is or 400 is for the long term health of your engine. Shock cooling is an issue that suggest cracks developing immediately in your engine. If my engine can go from 280 to 380 (or 400 or yes 420 if I am not careful regarding long term health...yes I am careful) in about a minute, and survive...that is suggesting to me that shock cool is no worse.

Interesting... Maybe some of our Mechanical Engineers out there that understand strength of materials can comment on if there a difference between rapid cooling and rapid heating on the metals.

[orionflt]

The rapid heating and cooling of metal always produces fatigue in the metal, after awhile you are goung to see some kind of failure. The combination of reducing the number of cycles, lessening the delta T of the temp change and increasing the time it takes for the change to occur reduces the amount of fatigue placed on the metal. So simply stated if you love to shove the throttle in on take off, jockey it around during flight, then pull it all the way back from WOT then you hit the pattern your engine probably won't last as long as the person who smoothly applies power for take off, sets cruise power, and steps down during his descent.

[DonMuncy]

I agree that it is not good to ham-fistedly heat and cool your engine. But we really have no idea how this affects the engine in a practical way. Perhaps ill treatment will result in cylinder failure after 20,000 hours and nice treatment would stretch that to 40,000. But both numbers are so far past what we get from cylinders from other problems, it doesn't matter.

Not saying this is the case; we just don't know.

[Lood]

And on the other side of the argument, you have training airplanes that are notorious to reach their TBO on a very regular basis. They suffer from extreme power changes all day, every day...

[Shadrach]

The rapid heating and cooling of metal always produces fatigue in the metal, after awhile you are goung to see some kind of failure. The combination of reducing the number of cycles, lessening the delta T of the temp change and increasing the time it takes for the change to occur reduces the amount of fatigue placed on the metal. So simply stated if you love to shove the throttle in on take off, jockey it around during flight, then pull it all the way back from WOT then you hit the pattern your engine probably won't last as long as the person who smoothly applies power for take off, sets cruise power, and steps down during his descent.

This is true, but most of us really never heat up our cylinders, at least not in cruise.  Normal cruise temps run from 330 to 360 depending on OAT. 

[201er]

If my engine can go from 280 to 380 (or 400 or yes 420 if I am not careful regarding long term health...yes I am careful) in about a minute, and survive...that is suggesting to me that shock cool is no worse.

I don't know. But I can tell you that pouring boiling water into a cold glass bottle doesn't do anything but pouring cold water on a just boiled bottle cracks it. Shock heating vs shock cooling may have varying implications. That said, I still have yet to see immediate evidence of harm done by supposed "shock cooling" an engine. Still, it's not a bad idea to allow the engine to get warmer and cooler in as gradual a manner as reasonably possible and then not worry about it beyond that.

[jetdriven]

Lycoming says if the engine accepts full throttle without faltering its ready for takeoff. I think mine would do that right out the front of the hangar, but to be easy on it, I dont exceed 1200 RPM until I get 200 CHT and wont take off until I have 250-300 on the CHT.

[MARZ]

The only experience I had with shock cooling is when my buddy put his snow mobile through the ice - never started again...... 

[201er]

I dont exceed 1200 RPM until I get 200 CHT and wont take off until I have 250-300 on the CHT.

300?? Sometimes my front cylinders don't get to 300 in cruise!

 

What about Oil Temp? Mine never reaches "green" on the ground but I figure since there's no lower boundary red line, it's ok as long as it's warm.

[jetdriven]

mine wont move off the white dot until 2-3 mins after takeoff. new hoses, engine, rebuilt oil cooler. It just wont come up on the ground

[Sabremech]

I've never been a believer in shock cooling. Have never heard an engine shop able to confirm that there was damage as a result of it.

I don't find myself ever thinking that I should be careful moving my throttle or descending to fast as I fly. I just fly it as I think I should and how it feels and sounds. I have 1850 hours on my cylinders and when they do come off for overhaul, I doubt any of the wear can be directly linked to shock cooling.

David

[DS1980]

Martin,

If shock cooling existed, then we couldn't fly in the rain. Or cruise into a cold front.

 

Those that mention shock heating are dead on. Think about the immediate heat and pressure that occurs inside an engine when it is first started. This is much more dramatic than pulling the throttle  to descend, which is the most popular scenario for this argument.

 

Do not take Lycoming as the final answer to the operation of your engine. They know how to mechanically make a great running, reliable engine. They have no idea how to run them.

 

When does aluminum alloy loose it's strength? At high temps.

[PMcClure]

If I remember right from my mechanical engineering days, the ratio of elasticity to induced stress (Young's modulus) changes with temperature. At higher temperatures, the material is less likely to crack under stress as the Young's modulus is lower (the same force will induce less stress at higher temps than at lower temps). That explains why things crack when cooling not heating. Sharp edges, holes, inclusions and minor cracks greatly concentrate stress. Also if the surface cools faster than the rest of the material, it will induce internal stress (pouring cold water on hot glass for example). Uneven temp changes lead to internal stresses. The faster the change happens, the more likely it will be uneven.

 

As for heating, a material's strength is not necessarily linear. As it reaches a certain temps (Seems like +400 in engines if I believe everyone) it tensile strength weakens sharply. Starting an engine cold and heating up quickly won't lead to cracking in the material. But uneven heating could lead to problems including cracking. I used to have an old Pontiac that had aluminum block and a cast iron head. This caused uneven heating and cooling and lead to gasket failures. I hope TCM did a better job than GM!

 

I can say that shock cooling or shock heating can't be good for any machined part or parts. Especially considering gasket fitment, critical clearances and all. That said, I don't worry too much about shock cooling or heating but do try to make power changes as smooth as possible. I assume most of this has been engineered into the system. I don't freak out when I fly into heavy rains or if I have to cut power and drop a few 000 feet. 

[kgbpost]

I instructed at BDU in the early 90s and taught mountain flying. We required a mandatory mountain checkout for all renters regardless of experience. Shock cooling was one of the reasons for this. I'm not going to bore anyone with the details of some of my experiences with some of these checkouts, especially going into ASE via DBL, but i will say this... The reason for implementing our required mountain checkout regardless of experience was the result of numerous cylinder replacements on our rental aircraft, among other things.

 

So, I'm gonna have to go with Lycoming on this one 100%. If you wanna chop and drop, I would suggest you switch to burning kerosene. 

 

On a side, I flew from LMO to C08 and on to ALS today. The colors are amazing right now and the weather today and tomorrow is ideal for enjoyable mountain flying. Cheers. Brian

[aaronk25]

I've said it on here before the only thing we can do to really truly shorten the life of the engine is not run it. We don't wear them out we rust them out. Regular oil changes and keeping the heads under 400 go along way too and while were at it camguard don't hurt either

[201er]

So, I'm gonna have to go with Lycoming on this one 100%. If you wanna chop and drop, I would suggest you switch to burning kerosene.

Would you care to elaborate? What strategy do you recommend for those of us who don't have spoilers what to do when ATC demands you down and you don't want to exceed speeds? Or like you say coming into a valley where you need a steep descent. I realize you give a whole course about this but can you summarize the strategy for avoiding shock cooling/overspeeding and descending all at the same time?

[M016576]

I've said it on here before the only thing we can do to really truly shorten the life of the engine is not run it. We don't wear them out we rust them out. Regular oil changes and keeping the heads under 400 go along way too and while were at it camguard don't hurt either

Well said- this doesn't have to be complicated: keep them running, keep them cool. Avoid ring flutter (idle descents) and all's good!!!