Shock Cooling - Myth Busted!!
 

Here is a little story I thought I would share with you.


One of the great Old Wives Tales, Shock Cooling!

Well I am not sure how easy it would be to do this, apart from spear into a lake, that would do it for sure, but even in an extreme descent with 2000FPM plus ROD I can't seem to do it.

So here is the story, last Saturday returning from St George Qld to Caboolture (Brisbane) I climbed to FL130 and was enjoying 45 knots on the tail, so a GS of 210 knots. As I was about 20 minutes out, I advised BNE CEN that my TOD point was 46 miles from YCAB and that was 13 minutes, to allow for a 12000' descent to circuit area altitude. I do this so it helps them with all the crossing traffic I have going against the flow. My tracks are often a pain I am sure.

Anyway 13 minutes comes and goes, and I think any second now........10 minutes, hmmm I wonder if my mate is on BNE APP this afternoon. Anyway they are busy with jet traffic and RFDS etc into YBBN, so I figure I can fit in with whatever keeps everyone happy. At 6.5 minutes to run, 22NM to destination.....the following occurs;
BNE CEN: Lima November Lima, Descend 6000 and contact BNE APP on 124.7 , I do the read back and over I go to 124.7,
LNL: Brisbane Approach, Lima November Lima FL130 cleared 6000 and Visual.
BNE APP: Lima November Lima, cleared to leave on descent, Brisbane QNH 1014, good afternoon Brownie!
LNL: Cleared to leave on descent, 1014 LNL....and gooday to you NP and did you have something to do with the late descent?
BNE APP: I have NO Idea what you are talking about <laughing>
LNL: WATHCH THIS <laughing>

Now 6 minutes and 20 miles

Now not exceeding VNE (by TAS), and then as the bumpy bits over the mountains (ok hills) staying out of the yellow arc and not wanting to lose the game, I had a job to do. At times the ROD was around and over 2000 FPM, and I watched happily the CHT's very slowly decline. The Throttle was pulled right back and the RPM about half way down was increased to 2550 to help the cause. Pitch went from +1 in cruise to -8.5 at times and mostly -5 to -7 and this was a very different view of the Brisbane region.

So I have downloaded the data, and done a Delta T on CHT and the graph shows a flat line. The cumulative Delta T shows a greater ramp for the taxi and take off than at any point in the descent. Unfortunately the Delta T will not upload to the SAVVY site, however you can see the slope on the curves.

I have uploaded the file to the Savvy site, you can click on the extra displayed data to show Vert Speed or other things.

https://my.savvyanalysis.com/public/...1-b50788c2095a

No doubt some flat earthers will tell me that the cylinders will fall apart due cracks in a few hundred hours.

It is pretty hard to shock cool and engine that is already cool!

Shock Cooling - BUSTED! :}

Oh goodness what have you done?

Awesome graph..

If you zoom in on the descent you can see the CHT is steadily dropping at 10*/min
If only the data continued after shut down.. How long does it take the remaining 270* to dissipate??

I've got another one - is it the rate of cooling/heating, or the actual temp.

My understanding is that hotter temps, above 190-200degC result in significant cylinder wear.

But if you slowly cool the engine to 100-110 deg and then glide descend at high rates of descent, and the CHT remains stable, does the fact that the engine is turning over cooler (and tighter seals I guess?) result in significantly more wear, even at minimal power & RPM?

[I realise a go-around at low CHT will be where a lot of wear occurs, but like the op, I'm more interested in if running at low CHT in a glide actually does any damage]

Would like to see the same level of analysis on a parachuting aircraft Jabba.

Oh goodness what have you done?

Haha should be good.

Hope you have not jinxed yourself Jaba;)

I would have to agree, after probably 12000+ descents from 12000ft to SL in the Cessna 180/185, I have never experienced a cracked cylinder. I would always leave RPM at 2200 or 2300 and pin the MP at that as well when it caught up. Descent rate would be around 3000fpm and airspeed quite high in smooth air. In later years I had an engine monitor etc.
http://i303.photobucket.com/albums/n...s/DSC01500.jpg

http://i303.photobucket.com/albums/n...s/IMG_6836.jpg

What happened at 14:10 Jabba? (you didn't fart did you?)

Doggies \\ //

hehehhehehe :}

yeah I deliberately did all the bad things, throttle closed, increased RPM to increase ROD, left the mixture alone from what was 37-38 LPH in the cruise.

Ohh yes the graph after shut down!

Taxi in the temps went from 256 to 276 and then after shut down they went up! Then they went down at 1Deg F every 6 seconds, then the monitor was shut down.

So shut down is just as nasty! :} And maybe worse! No oils moving around :}:eek::} :E

Ohhh dear me...... seems those rednecks Braly Deakin and Atkinson were not talking out their data ports after all! :D

Jack you trouble maker!! Doggies!! \\ // from Jnr Jaba to ya!:ok: And happy fathers day for next Sunday!:ok:

I was just doing some experiments on how far LOP I could go and see what the speeds did.

Lucky I did not include a high altitude WOTLOP mag check in that lot! :E

hey whatever you do, do not post a link to this in the Private Flying Forum, you may start a war......and besides, I may want to save this little gem for a later posting!

A kiwi or two might know what I mean :E

Do you think that perhaps you have answered your own query??

Shock cooling can and will damage a piston engine very rapidly under the right conditions, or wrong conditions.

Try carting a ton of fertiliser for a climb of maybe 2,000ft, max continuous power, relatively slow speed, sow the load, pull the pwer and descend rapidly back to the strip. Do this maybe 10 times an hour, for maybe 10 hour flight time day and you will soon find that shock cooling is not an old wives tale.

When I first started in Aerial Ag work, as loader driver, heard an FU24 with 300hp flat six coming up the strip sounding like an old chaff cutter,
Stopped the ops till the cowls were lifted and engine inspected, three blown plug inserts, big cracks, engineers put it down to shock cooling.

This was fifty years ago, no where near the engine instrumentation available then as is now, but if there was it would no doubt have shown just how hard these engines were worked, and would have shown, as most of us learned quite rapidly, leave a bit of power on during the descent.

So my question would be... sure, the engineers may have put it down to shock cooling... but what ACTUALLY caused it?

Back in the day, with no hard data, it was pretty normal to blame failures on mis-handling, whether it actually was or not. Many engine shops would blame any failure of one of their engines on the pilot, and "shock-cooling" was a pretty common charge.

Back when I was younger I used to race motocross bikes, and it always used to make me wonder why my two stroke motor, operating pretty much on the edge of seizing for most of the race, could endure being occasionally dunked in freezing water without ill effect... because that really IS shock-cooling...

Would be pretty interesting to see something like this done on a C206 or C182 doing parachute ops, only obvious difference I can see if on a real hot day with the engine struggling to take a full load all the way to say 14,000ft then dropping and descending say 2,500 - 3,000 fpm, i'd imagine this would be somewhat different!! Not to mention, as prospector said, doing it up to a dozen times a day.

BUT, IMHO, you're probably right about it being hard to shock cool an already cool engine!! Most of the decent drop pilots i've known had taught me the real trick is to start letting those temps come down as soon as you're at top of climb before you start descending, that means getting the power back to just above what you need, start getting a bit of extra mixture in, cowl flaps open etc...etc...

If memory serves me right, we were told to never exceed -13 on descent (Thats 13degrees per minute on the CHT, determined using a JPI Gauge), but my memory on those numbers is a little fuzzy now!!

Shock cooling caused it, from a hard climb with temps on top limit, to a high speed descent with no power does cause shock cooling.

Engineers who were responsible for fleets of up to 20 aircraft or more sould see a pattern Th aircxraft operating in locations with a long hard climb to sowing area and then a high speed descent saw a lot more cracked cylinders, plug inserts than those aircraft operating in kinder country.

Perhaps because they are two entirely different engine types, different metal masses etc.

Hmmm \, interesting one this, there is no doubt (is there?) that if you heated a cylinder to, say 350- 400 degrees and shoved it in a bucket of cold water, cracking would be possible, if not likely, however, if you shove it into air, what then? My experience leads me to believe that not much will happen except the cylinder will get cool at a fairly even rate. On the other hand, I have heard of many glider tugs that have cracked cylinders from descending too rapidly, or indeed, is this the reason that glider tugs seem to suffer an unusually high incidence of cracked cylinders? Hmmmm?:confused:

Actually Jabba what you have shown us with your graph is that your cylinders were not shock cooled, had you done that let me assure you that they would have cracked.

They are... the two-stroke engine being far more powerful for a given displacement, and, when in racing tune, running on the ragged edge of destruction every race... with very tight tolerances when operating like that. Throw less than perfect lubrication into the mix... you would have thought that the racing engine, particularly with it's low mass and therefore greater susceptibility to temperature change by sudden immersion in cold water, would be far more prone to failure... but they aren't.

Anyway, hard to argue with traditional wisdom... lol

And back at ya :ok:

Say hello to the bigger Jnr!

Quite an interesting bit of data from some pretty nifty monitoring gear (gotta love technology!), but to play a bit of devils advocate here, statistically speaking the data means nothing as it isn't repeated and we have only one sample. One could easily say that the engine held up well because it was handled properly over time so held up well with a little bit of "tough love".

I would really like to see how this graph (sample 1 if you may) holds up if you repeat this over a period of time.

I'd also love to see how the graphs would compare to super or turbocharged engines.

I'm by no means discounting (or rubbishing your theory for that matter), your monitoring equipment provides some some fantastic insights in to what is happening inside the engine and gives us possible insights in to debunking a wives tale, but lets not get ahead of ourselves, I'm sure you would not get a great reception from any of your mates who own a radial engined plane if you pulled the throttle back to the stops and pointed the nose down at the ground with the assurance of "I tried this once and the graph showed it was fine" :E

Again, just playing devils advocate.

Lots of old wives tales in aviation. Continental's seem more prone to cracking and warming up issues than Lycoming.
The more lives a cylinder has the more chance of cracking. Cylinder cracks seem less prevalent these days, people tend to put new cylinders on and I think that is part of it. The old days it wasn't unusual to have 4 or 5 lives of a cylinder before it was junked. The days Prospector was talking about the Flitchers used to have Continentals in and they had to run flat stick to get anywhere. I'm sure it was put down to shock cooling when a cylinder finally gave up the ghost after 5 or 6 thousand hours.
To talk radial folklore you have to be a deaf, dried up old fart with piles. Plenty on here like that so lets hear the old wives tales about radials. :8

I have a bit of experience with this.

At my last 100 hourly check on my plane with a Lycoming O-320-E1A engine three of the cylinders had fine cracks near the exhaust valve.

The fourth cylinder had a crack aluminium welded about 400 hours ago.

As I do a lot of aerobatics in the plane I thought the problem was shock cooling

When I checked the maintenance documentation I found that the cylinders manufactured in 1971) had 3,100 hours on them, including a 10 thou overbore at 2,000 hours done about 20 years ago.

Interestingly, the Lycoming documentation said the cylinders are good for 3,000, which means they made their estimated service life.

The most economical fix was to buy four new complete Lycoming cylinder kits, which include new cylinders, pistons, valves etc. It cost the same to switch to higher compression pistons, so these were installed under an Engineering Order and I picked up an extra 10 HP. Each cylinder kit cost about $1,300.

Bottom line is that cylinders do fail, but think that it is possibly people worry too much about shock cooling and that the problem is over stated.