Shock Cooling/Cracked Cylinders, Fact or Fiction?

[Disco Stu]

So to all you current and ex-piston jockeys, I pose a question.

Flying a Continental 6 cylinder (B55) non-turbocharged.

In my training, taught to reduce power 1 inch a minute to prevent shock cooling and resulting cracked cylinders.

Now I have enough experience to know not to pull it from cruise to idle in an instant, but is 1 inch a minute really necessary? My understanding was stage cooling is only necessary for turbo-charged planes.

Never heard of doing it in a 185.

Was told that you don't do it in the Travelair because "it is only 4 cylinders". That doesn't make sense, or any difference I think.

Opionions? Thoughts? Experiences?

[flyinhigh]

I my opinion, your right I have never heard of stage cooling a non-turbo charged aircraft.
I think of it as your in training pulling power, adding power, same idea of engine, no chargers. However stage cooling would be a good thing to start to do so when you do hit,(if you haven't already)turbo and superchargers you already cool it as second nature cause your use to it.

[LastSamurai]

You dont use stage cooling on the Travelair not cuz it has "4 cylinders" but because its not turbo'd or supercharged and therefore the engines do not generate enough heat to utilise stage cooling.

I agree with flyinhigh tho, practice will only make it easier in the future when you end up on something turbo'd or supercharged.

Happy flying

[Disco Stu]

I think this was posted before

AvWeb Article

I won't be flying Turbo/Supercharged pistons after this. 3 more months then back to nice easy tubines.

[KAG]

Disco. I used the fly the BE55, still my favorite plane. We had to replace 2 jugs juring the winter because of new drivers comming off of turbines and forgetting about the whole shock cooling effect. So in my experience stage cooling is is necessarry but not at the rate you would a turbo charged. I'd do about 2 to 3 inches a minute, smoothly (slowly) reduceing the power in mid winter. In the summer I wouldn't really worry about it, but you still cant throw the power levers around.

Cheers.

[Panama Jack]

Interestingly, a PT6 operator I worked with preached stage cooling on their turboprops-- they ran their engines "on condition" and swore that it added hundreds of hours to the life of hot sections.

[Disco Stu]

Chubbs says
"I won't be flying Turbo/Supercharged pistons after this. 3 more months then back to nice easy tubines."

HAHAHAHAHAHAHAHA
Dreamer


PS- You crack a cylinder or blow a jug and you won't see your 'nice easy turbine', until I retire.

Anyone else have a mentally handicapped Chief Pilot?

Ours has the mental capacity of a 12 year old, and frequently finds himself roaming around bars without his shirt on.

[IMAC]

Does he also moonlight on Trailer Park Boys???? Just Kidding....

As for the cooling..., I like to follow the inch per minute rule. I was once told by an old Otter driver that engines are like light bulbs, they blow when you turn 'em on or off. Take that as you will.

I have only experienced a crack once, but that was a crank case at the base of a cylinder where it was bolted to a stud. It puked out a quart in just under an hour. Since it was an recently overhauled engine, our ame figured either over torqued stud, or bad casting. This was in the summer and the case vent was not frozen.....Has anyone else had a case crack, and why?

[tripod]

IMAC

Case failure on rear engine of C-337. One cylinder made the prop a lot shorter. Another cylinder was hanging on by a plug wire just about ready to leave the cowling. The engine quit suddenly with a bad sound.

A small crack in a case a few inches from a cylinder on a Lance caused an oil leak. Never found out what caused either failure.

[co-joe]

Those IO 470's on your B55 are definitely succeptible to shock cooling, even if you treat them like gold there's still not a great deal of chance they'll make TBO. IMHO An inch a minute or two inches every two minutes will help them last. If you crack front jugs, you are guilty of shock cooling.

BTW, Still one of my favorite planes of all time. What other six seat piston twin can actually take full mains with six sets of ass cheeks and climb at 1500 fpm on such low burns then cruise at 180 KTAS? (310 burns more and drives 10 kts slower) Those stacks sure do pound out the decibles too! Gggrrrrrrrr! cj

[Always Moving]

I use to do 2" a minute in a C-401 (turbo) and looking at the JPI monitoring it.
If it has 4 cylinders( I learned and taught in Pa-44) it doesn't SEEAM to matter(although I was not paying for maintenance) and if it has 6( I taught in pa23-250) it does matter( I was more involved in maintenance)
And think about it...... the longer the engine the more differencial tempeture betwen cylinders, besides if you plan ahead you shouldn't need to move the throttle much. Takeoff power( after this is all reductions), climbing, cruise, and reduce till you land.
I always fly like that no mater what I have in my hands, unless is a toy

[Wasn't Me]

It makes sense to slowly reduce or increase power on any piston. You are controlling a machine that is producing controlled explosions at a fast rate any change to that process is going to cause stress turbo charged or not. The turbo/supercharged machines just produce more heat and require extra care. Bottom line no Chop and drops, preplan.

[N2]

If you can get your hands on a copy of The Lycoming Key Reprint book you should really take the time to read it. Lots of good information on proper care and operation of piston engines. For instance I never knew that Lycoming suggest reducing power first during a decent not mixture or that leaning the mixture will not hurt the engine regardless of altitude as long as you are at 75% power or less. I remember being told not to lean below 5000'! Good reading for all who fly.

[hz2p]

It stands to reason that shock cooling - which we could define as an excessive rate of cylinder head temperature change per unit time - applies to departure as well as approach.

During approach, you're cooling on the outside, and hot on the inside. The cold aluminum on the outside contracts, and cracks result. I can understand that.

However, during departure, again you're cold on the outside, and quickly getting very hot on the inside - same as during approach. Given that the above argument applies, shouldn't one take off using only 1" of MP per minute? If so, what runways are you guys using? The trans-canada highway?

It would appear obvious to me that there are maximum rates (degrees per minute) of heating and cooling that are permissible, that the cylinder heads can withstand. With sufficient instrumentation (ie multi-probe CHT w/uprocessor) this should be easy to measure and display to the pilot. Such multi-probe systems are not very expensive compared to a top overhaul.

[Disco Stu]

Those IO 470's on your B55 are definitely succeptible to shock cooling, even if you treat them like gold there's still not a great deal of chance they'll make TBO. IMHO An inch a minute or two inches every two minutes will help them last. If you crack front jugs, you are guilty of shock cooling.

BTW, Still one of my favorite planes of all time. What other six seat piston twin can actually take full mains with six sets of ass cheeks and climb at 1500 fpm on such low burns then cruise at 180 KTAS? (310 burns more and drives 10 kts slower) Those stacks sure do pound out the decibles too! Gggrrrrrrrr! cj

Sure is a fun plane. Slippery little bugger too.

[KAG]

STU, you flying it at Perimeter? I loved EQK - the little yellow corvette, before it got its paint job. I miss that harley sound those stacks throw out. If I was to buy a twinn piston, it would be the barron.

[JigglyBus]

I've been a subscriber to the shock cooling theories every since I started flying. Seems to make sense, rapid temperature changes can't be good for anything.

However, over the last few years, I've began to question my faith.

That avweb article posted earlier is one of many that I've read which all seem to point to it being a myth. The best arguments I've heard being the rain cooling effect and the shutdown cooling effect.

If flying through rain will cool the engine faster than dropping it from 28 to 17 inches, than does that mean I should avoid all rain showers?

If shutting the engine down has the same rapid cooling effect, should I never shut it down?

All the arguments aside, one FACT remains. Gradually power reductions, certainly can do no harm. So, while the jury is still out, best policy might just be to continue doing things as we are now.

When possible.

[Disco Stu]

STU, you flying it at Perimeter? I loved EQK - the little yellow corvette, before it got its paint job. I miss that harley sound those stacks throw out. If I was to buy a twinn piston, it would be the barron.

You're talking about my plane. No more yellow paint, but still has the what is probably the original 1970 seat that would have matched the old paint job.

[Arrow]

How to operate a piston engine, the age old question? When I was flying pistons I read everything I could on this, and never had an engine problem. On descent I pulled back the power 1" per minute until I got down to 18"-19" then I'd go back to 14"-16". Figured by the time I was back to 19" the engine had been cooled slowly. Of course this was in summer, in winter I was very very gentle with power reductions. On takeoff the piston engine has a fuel enrichment feature at takeoff power which helps cool the engine. Those who advocate reduced power takeoffs in a piston do not understand that rather than helping the engine they are hurting it. Having said all that, one of my best friends has about 6,000 hours in a 185, and he routinely pulls the power from 24" to 16" all at once, and his engines always make TBO.

[Butterz]

The 1" per minute and 2"/2 min. is also used by our company's piston twin pilots to prevent turbochargers from warping as well as cracked jugs. A couple other procedures we have in place is keeping the descent to around 500fpm (adds to passenger comfort) and allowing around three min. of cool down for the engines after landing.

[co-joe]

...Given that the above argument applies, shouldn't one take off using only 1" of MP per minute? If so, what runways are you guys using? The trans-canada highway?...

Funny.

My understanding was that you could heat them jugs up as fast as you want to (once the oil is warm) without a problem, it's the cooling that causes problems. I learned that as a dish pig at Phils Pancake House.

[TopRudder]

Hello

You guys got me thinking.

Wouldnt bringing back the power 1 inch per min in the decent actually cool the engine more. THink about it. you reduce the power at a high altidude... lower the nose...increase airspeed .... there you have it folks SHOCK COOLED BANGERS!!!

Reducing power at alt + lowering the nose = alot cooler air flowing over the jugs.

I think anyways. Take it from someone who doesnt buy it. turbo or no just be gentle and they will be fine.

TR

[Always Moving]

That is because you do not descent by reducing the power....(if you want maximum speed of course)
Lower nose stabilize airspeed at required Vertical velocity THEN when the temperature stabilize, reduce power (at that time increase mixture, It depends before or after, but I am not going to teach you how to fly) and then the one or two inch at a time game.
What about the cowl flaps.....we live that for another time....

Sorry if I sound very Flight Instructorish but I think it is pretty basic and it will take a few pages to explain.
I am done
PM me if it is serious

[ahramin]

Just like leaning, engine cooling can only be accomplished in an efficient manner if you have a good engine analyzer looking at all cylinders.

I use a simple rule to prevent shock cooling: Δt° ≤ 50°F / minute. In english that means do not cool any cylinder faster than fifty degrees (fahrenheit) per minute. Any faster and, depending on the goodness of your cylinder, you may strain it.

What this means for us in the real world is that unless you have paid beaucoup for a multi-port engine analyzer, you have to guess. This is where 1" per minute and like rules become useful. This is a way of ensuring that you do not cool the cylinders too fast without actually knowing how fast they are cooling. These rules may be very conservative, and are not portable between aircraft. Cooling depends both on the engine and on the airflow. Airflow is affected by speed and the shape of the cowl intakes, baffles, plenum, ad nauseum. A turboed engine requires careful handling because it generates more heat, has a greater range of speed, and generally is closely cowled. A 185 on the other hand has a big warm engine, lots of cooling air at all times and probably goes the same speed with a 24" cruise as with a 16" descent.

When i was towing gliders many tow pilots were very concerned with my power reductions on the pawnees because it did not fit the rules they had used for other towplanes. A pawnee has huge air intakes, a big derated O-540, good baffling, and a big hole at the bottom to let the air out (Side Note: Exit holes for cooling air should be at least 110% the size of the intake holes). And you can go from 80 mph at full throttle to 100 mph, reduce power to 15" in one to two minutes, and come screaming down without getting close to a shock cooling regime.

Power reduction rules all depend on the engine and the airframe, but are all trying to accomplish the same feat. Never cooling your cylinders faster than fifty degrees per minute.

As for turbines, over time it is the high temperature spikes which cause undue wear. Pouring on the power on the outside engine to make that quick turn at the end of the runway brings your engine up to the same temperature that a takeoff does.

[KAG]

Disco, man I had some fun times on 82/83. The power line approach into I falls, river running enroute to YQT, and that gimpy deice system. Good times, good times. and ya gotta love month end!!!! I came back one day completely balked out. I mean I had stuff seat belted into the seat next to me, the back packed to the ceiling. I shut down and the damn thing almost fell on her tail. The look on my face must have been priceless. The one thing that you can say about the barron, and few other planes, If you can put it in, it will fly.


Cheers.