Flying an engine past TBO...

[Dan Thomas]

I've seen pictures similar to the one you shared about the dangers of cold starts (<-5°c) without preheating, where the clearances between the piston and cylinder wall are inadequate to allow lubrication.

I don't have the personal expertise to argue this, but I will cite someone who does, https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/

Note that he does talk about CHT rather than the actual cylinder temperature (as does Lycoming, for that matter, with the 50°F/minute restriction). Presumably, since we can't measure the cylinder directly, the CHT is the closest proxy we can use, so Lycoming chose the 50°F/minute CHT number because of whatever actual temperatures the cylinder and piston would experience. Also acknowledging that the "shock cooling" Durden mentions after shutdown — where the engine cools at 100°F/minute — wouldn't result in the scuffing you mention, because the engine isn't running, and that "shock heating" when advancing the throttle would cause the cylinder to expand faster than the piston, so again, no scuffing. What about shutting down and restarting when the engine is still hot, though?

And finally, this subthread started by talking about how flight-school planes often go past TBO (and don't go through cylinders at an accelerated pace). What is your theory on why "shock cooling" doesn't damage them?

There is no shock cooling on shutdown, because there is no high-velocity blast of cold air going through the cooling fins. So starting a hot engine is no big deal.

No preheating means that the entire engine is cold, cylinders and all. Since that aluminum piston shrinks much more than the steel cylinder, there is more clearance, not less. Scuffing when cold is more a result of that thick oil not being pumped to critical areas soon enough.

Shock heating? I've read that leaning the mixture too fast can cause that. The ultralight guys with their two-strokes know they have to warm their engines properly before takeoff, or they can seize suddenly shortly after takeoff.

We ran our flight school airplanes down to -25°C (-13°F). I forbade any spin practice at those temperatures, since that always results in a power-off dive in the recovery. Touch-and-goes don't cause shock-cooling, either, since we're not at idle at high airspeed for any extended periods. Glider tugs and jump planes don't fly in the cold winter air either, so shock-cooling is far less likely. When I flew glider tug the ambient temperatures were 70-100°F. Even then we sometimes read of jump planes needing cylinder work more often than the typical cross-country flyer. They climb to much higher altitudes where the air is a lot colder.

You can find any number of articles to support your opinions one way or another. I speak from my experience as a machinist and aircraft mechanic and pilot. A lot of articles are written by pilots who have read opinions by other people, just like today's media which is full of journalists who are suddenly virologists or economists or scientists or politicians depending on the topic at hand. They have absolutely no education or experience in the field they're writing about, and it often shows. They're just parroting someone else, or are drawing inaccurate conclusions because they don't understand the terminology, technology and history.

Here's another opinion:
https://www.aopa.org/news-and-media/all-news/1996/november/pilot/shock-therapy

Bottom line:

1. An aircraft engine costs a lot of money.
2. If an aircraft engine quits, things can get mighty unpleasant.
3. Engines that are handled roughly, aggressively, outside POH limitations, receive inadequate inspections and maintenance, are flown too infrequently or just run on the ground, are more likely to quit or suffer significant power loss or shortened life long before TBO.
4. If Lycoming and Continental are concerned about shock cooling, who is qualified to dispute that other than another engine designer and manufacturer?
5. So why wouldn't we just try to treat that engine with respect?

 

[Checkout_my_Six]

Regarding shock cooling...how are jump planes operated in the decent? Any shock cooling?

 

[Kenny Phillips]

No preheating means that the entire engine is cold, cylinders and all. Since that aluminum piston shrinks much more than the steel cylinder, there is more clearance, not less. Scuffing when cold is more a result of that thick oil not being pumped to critical areas soon enough.

Actually, if you warm the piston on a tightly clearanced cold engine quickly enough, it will eat up the clearance before the cylinder gets warm enough to accommodate. Probably not a concern for most aircraft, but we've seen it in motorcycles that were started up and hammered. The term was "cold seizing" although it was caused by heat. But we often ran clearances of 0.0008" skirt to cylinder wall (pistons are tapered.) At five times that there is no issue.

 

[Dan Thomas]

Actually, if you warm the piston on a tightly clearanced cold engine quickly enough, it will eat up the clearance before the cylinder gets warm enough to accommodate. Probably not a concern for most aircraft, but we've seen it in motorcycles that were started up and hammered. The term was "cold seizing" although it was caused by heat. But we often ran clearances of 0.0008" skirt to cylinder wall (pistons are tapered.) At five times that there is no issue.

That's the thing the ultralight guys have to watch out for. Still, I wouldn't take off soon after startup on a cold Lyc/Cont engine, especially in cold weather. I've seen renters do it, trying to save money.

 

[Daleandee]

Regarding shock cooling...

I wonder the same about the times when you suddenly find yourself in a rain shower ...

 

[Dan Thomas]

I wonder the same about the times when you suddenly find yourself in a rain shower ...

We discussed that back in the spraying-water-into-a-hot-engine thread. The amount of water in a cubic foot of air during rain is tiny. It's not a firehose. In that thread, Gismo said this: A back of the napkin estimate of the amount of water encountered per minute when flying at 200 mph through heavy rain comes to around 1-2 gallons per minute for each square foot of exposure.

That's at 200 MPH, and the cowl inlets probably don't come near one square foot between them.

Heavy rain is defined as more than 0.3 inches per hour, or 0.52 gallons per square foot. Not much water, really, falling through a cubic foot of air at any given time.

 

[Matthew Rogers]

We discussed that back in the spraying-water-into-a-hot-engine thread. The amount of water in a cubic foot of air during rain is tiny. It's not a firehose. In that thread, Gismo said this: A back of the napkin estimate of the amount of water encountered per minute when flying at 200 mph through heavy rain comes to around 1-2 gallons per minute for each square foot of exposure.

That's at 200 MPH, and the cowl inlets probably don't come near one square foot between them.

Heavy rain is defined as more than 0.3 inches per hour, or 0.52 gallons per square foot. Not much water, really, falling through a cubic foot of air at any given time.

I did some really rough calculations and by my estimate, with a small engine (O-200), the amount of heat capacity (0C to 500C) of the air going through the engine at idle in a dive (2000 rpm) for one minute is about half the heat capacity required to boil 1 gallon of water. Boiling a gallon of water takes a ton of energy. Think about how much propane it takes a stove to completely boil away a gallon pot of water - it could be an hour or more of constant propane fire to get it all turned to steam.

 

[JBolton]

Heat stress is still the highest wear factor in air cooled engines. With the engine at low power settings, you aren't able to cause nearly as much thermal damage as you can at higher power settings. Don't worry about starting and taxiing wear on the engine if you can do it at less than 50% power settings.

 

[MooneyDriver78]

Regarding shock cooling...how are jump planes operated in the decent? Any shock cooling?

I would only worry about it with turbos, at high altitudes you have extremely cold temperatures and turbos allow running the engine hard, so pull the power, at -20°C or lower, maybe a little supercooled moisture...
The problem is the cylinders are made from 2 different metals. So imagine the steel cylinder is hot (expanded) and the aluminum head is quickly cooled (contracted), I can see where this could cause the head to crack.

 

[Checkout_my_Six]

Meh....not likely.

I would only worry about it with turbos, at high altitudes you have extremely cold temperatures and turbos allow running the engine hard, so pull the power, at -20°C or lower, maybe a little supercooled moisture...
The problem is the cylinders are made from 2 different metals. So imagine the steel cylinder is hot (expanded) and the aluminum head is quickly cooled (contracted), I can see where this could cause the head to crack.

 

[Hiflyr1]

There is a great article on this subject in this months AOPA magazine. If the engine is flown regularly their opinion is keep running it but oil analysts and keeping a good eye on the health of the engine.

 

[NordicDave]

Shock cooling for flat cylinder engines, as John Deakins’ describes it, “an OWT - Old wive’s tale’.

Like many OWT’s they come from the early day’s of radial engines.

Prudent approach is best, add or remove power incrementally; but chopping or sudden application of power is OK as needed.

Lots of good articles on the subject: https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/

John Deakins’ Pelican Perch articles are excellent reading: https://www.avweb.com/features/avweb-classics/pelicans-perch/pelicans-perch-index/

 

[Checkout_my_Six]

now is that tale or ....tail?

 

[JBolton]

This particularly opinionated article has glaring errors on the subject: https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/
Read with caution, then proceed for education on aircraft engines elsewhere....

Some simple source criticism:
The author is a pilot, and as such, does not impress with knowledge of heat transfer or engines.
He conflates CHT with every other temperature inside the engine, making any comparison of thermal stress meaningless.
He also conflates long term damage fatigue with immediate danger of catastrophic failures, so that isn't helpful at all either.
Shock cooling doesn't have to fail the engine the first time it happens in order to be avoided.

It's unfortunate the author wasn't taking a more informational approach as this would benefit the readers more than putting off a ton of opinionated but wrong statements.

The facts are:
Heat weakens metals.
Heat accelerates oxidation of metals.
Heat creates thermal gradients that cause fatigue inside metals.
Heating two different metals causes additional stress and fatigue.
Put it all together, heat-weakened metals experiencing higher corrosion and stress will fatigue and wear faster. Shock cooling is one aspect of this that pilots would be wise to understand.

 

[Country Flier]

I agree with all the suggestions so far, so I'll take this in a slightly different direction. I change my oil often, about every 20 hours. I don't change the filter unless there is 60 hours on it. I usually fly less than 50 hours a year so I let my AI change the oil and filter during the annual. Part of the oil's job is to hold dirt in suspension. Full flow oil filters only catch the larger particles but the fine particles remain in suspension and as time builds on the oil those particles can act like a lapping compound, thus increasing engine wear. Draining the oil and replacing it with new clean oil gets rid of the fine dirt and reduces wear on the engine. This also reduces the work load on the oil filter theoretically allowing it to last longer.

This may not be perfect but it works for me.

You most likely are doing more harm than good. There is no evidence that your oil is not doing its job after 20 hours. There are plenty of documented cases of maintenance induced failures.

I remember reading an article (albeit about car engines, not airplane engines), where a small oil sample was taken every 500 miles for thousands of miles, on a car using synthetic oil. They found the most wear occurred immediately after the oil change. The author then ensured he was using the cleanest methods possible to change the oil, speculating that they were introducing foreign particles during the oil change, yet the wear stayed highest immediately after the oil change. What they later believed, after talking with the oil manufacturer, was that oil needed a "break-in" period (I'm paraphrasing), I seem to recall something about long molecule chains don't lube as well as shorter, broken down ones, or something to that effect.
20 hours is ridiculous. Of course, it sounds more like its not the 20 hours, but 4 months, which isn't as ridiculous.

 

[NordicDave]

This particularly opinionated article has glaring errors on the subject: https://www.avweb.com/ownership/shock-cooling-time-to-kill-the-myth/
Read with caution, then proceed for education on aircraft engines elsewhere....

Some simple source criticism:
The author is a pilot, and as such, does not impress with knowledge of heat transfer or engines.
He conflates CHT with every other temperature inside the engine, making any comparison of thermal stress meaningless.
He also conflates long term damage fatigue with immediate danger of catastrophic failures, so that isn't helpful at all either.
Shock cooling doesn't have to fail the engine the first time it happens in order to be avoided.

It's unfortunate the author wasn't taking a more informational approach as this would benefit the readers more than putting off a ton of opinionated but wrong statements.

The facts are:
Heat weakens metals.
Heat accelerates oxidation of metals.
Heat creates thermal gradients that cause fatigue inside metals.
Heating two different metals causes additional stress and fatigue.
Put it all together, heat-weakened metals experiencing higher corrosion and stress will fatigue and wear faster. Shock cooling is one aspect of this that pilots would be wise to understand.

Maybe you’re not aware of the engineering work from Braley, Deakins, and Atkinson disproving old theories about LOP is bad and peak EGT above 70% power is good.

NO WHERE does John Deakins state High CHT is OK. He advocated CHT’s remain warm enough to burn off lead and cool enough to preserve engine life - usually around 380º.

He says to use EGT to set mixture to effect a desirable CHT value.

 

[tspear]

You most likely are doing more harm than good. There is no evidence that your oil is not doing its job after 20 hours. There are plenty of documented cases of maintenance induced failures.

I remember reading an article (albeit about car engines, not airplane engines), where a small oil sample was taken every 500 miles for thousands of miles, on a car using synthetic oil. They found the most wear occurred immediately after the oil change. The author then ensured he was using the cleanest methods possible to change the oil, speculating that they were introducing foreign particles during the oil change, yet the wear stayed highest immediately after the oil change. What they later believed, after talking with the oil manufacturer, was that oil needed a "break-in" period (I'm paraphrasing), I seem to recall something about long molecule chains don't lube as well as shorter, broken down ones, or something to that effect.
20 hours is ridiculous. Of course, it sounds more like its not the 20 hours, but 4 months, which isn't as ridiculous.

I have read from multiple chemist, formulation experts and material engineers that what you state is absolutely wrong for aviation engines. I will attempt to give my very limited understanding a descent explanation, but do not shot me if I miss something.

TEL reacts in specific ways, plus the greater tolerance in the engines means our aviation engines have significantly more combustion byproducts. The nature of these by products is that many react negativity with metals in the engine. Additives in the oil or such as Camguard are designed to both suspend these byproducts away from the metal surfaces and sacrifice themselves to neutralize the byproducts. However, not all byproducts can be 100% neutralized, in addition oil when exposed to air over time becomes acidic. This is why oil changes are calendar based and mileage (car) or time (planes/boats) based.

Tim

Sent from my HD1907 using Tapatalk

 

[Country Flier]

I have read from multiple chemist, formulation experts and material engineers that what you state is absolutely wrong for aviation engines. I will attempt to give my very limited understanding a descent explanation, but do not shot me if I miss something.

TEL reacts in specific ways, plus the greater tolerance in the engines means our aviation engines have significantly more combustion byproducts. The nature of these by products is that many react negativity with metals in the engine. Additives in the oil or such as Camguard are designed to both suspend these byproducts away from the metal surfaces and sacrifice themselves to neutralize the byproducts. However, not all byproducts can be 100% neutralized, in addition oil when exposed to air over time becomes acidic. This is why oil changes are calendar based and mileage (car) or time (planes/boats) based.

Tim

Sent from my HD1907 using Tapatalk

So change your oil after every flight, and see where that gets you...not "absolutely wrong" as you say, or we would change after every flight.

 

[Dan Thomas]

I remember reading an article (albeit about car engines, not airplane engines), where a small oil sample was taken every 500 miles for thousands of miles, on a car using synthetic oil. They found the most wear occurred immediately after the oil change. The author then ensured he was using the cleanest methods possible to change the oil, speculating that they were introducing foreign particles during the oil change, yet the wear stayed highest immediately after the oil change. What they later believed, after talking with the oil manufacturer, was that oil needed a "break-in" period (I'm paraphrasing), I seem to recall something about long molecule chains don't lube as well as shorter, broken down ones, or something to that effect.

Fresh oil cleans a lot better than old oil. The new oil would scrub the engine for a few hours, raising the analysis numbers.

 

[Dan Thomas]

So change your oil after every flight, and see where that gets you...not "absolutely wrong" as you say, or we would change after every flight.

Your engine is going to wear out. That's a given. It will cost money to overhaul or replace it. That, too, is a given. It also costs money to change the oil and filter, so the manufacturer recommends an oil change at some time, based on their research, to get the most life for the least money. They are not interested in you having your engine wear out at 1000 hours any more than you are; it does their reputation no good. So most recommend 50 hours with a full-flow filter, and 25 for a screen-only system, and some calendar interval to catch the oil before it oxidizes and its performance suffers.

If we changed the oil at 100 hours, the carbon (and maybe water) content of the oil would get high enough that the lifters would get sludged up real quick. The additive package would suffer, too. The filter can only catch the big stuff; there are contaminants in there--acids and so forth--that go right through filters. Those cause corrosion.

 

[pfarber]

Start the engine at the lowest possible throttle setting. I can't count the number of times I've heard people start their engines at or near full throttle. There is no oil in the cylinders for a while until the oil pressure builds up.

It will take a few days for the oil film to completely drain off/dry out. The film is not the same as the oil bearing of a pressurized engine, but for a few days there will be an oil coating to reduce wear.

I don't know of any POH that states 'open throttle fully, engage starter'. People can break things if operated improperly.

If you really wanted to save your motor a pressurized pre-oiler would be a cheap and easy way to reduce start up friction to almost nill.

 

[pfarber]

Think about how much propane it takes a stove to completely boil away a gallon pot of water - it could be an hour or more of constant propane fire to get it all turned to steam.

Propane has 91,500 BTUs per gallon
Gasoline (automotive) has 120,000 BTUs per gallon

That's a pretty large difference to heat that gallon of water.