Shock cooling

[Cap'n Jack]

would you rather own an aircraft that is at 2000 TT with a new engine or one with 10,000 TT and a run out 2500 engine.....FBO rental.

the answer to why is what I am pointing out.

stuff wears out metal fatigue sets in faster when the aircraft is worked hard, it is no different with trucks and equipment.

I'm not sure what your point is here. I'm reading that stuff that is used more gets worn out faster. I'm not reading that gross weight takeoffs are bad.

Maybe your point is that consistently flying a plane at gross weight is bad. I would understand your point better if you compared two otherwise identical aircraft at the same total time and same engine time except one was consistently loaded to maximum weight, and the other was not. What would you see that differentiates these planes during an inspection?

 

[Trapper John]

Why do you believe that Grumman and some other aircraft have a time life? Cessna didn't do that, does that mean fatigue does effect the aircraft in the same way?

What do Grummans have to do with it? I was asking about 172s.

Most defiantly, I cam spot an old aircraft with out seeing the logs. and I'm not alone.

Sure, a 172A looks different from a new one. But you didn't answer any of my questions, either. But you did specifically mention fatigue. Can you look at two wing ribs and tell me which one has more fatigue? Can you tell me which rib has been operating at gross weight and which one hasn't?

 

[Jaybird180]

Can Eddy Current testing detect fatigue?

 

[Trapper John]

Can Eddy Current testing detect fatigue?

Non-destructive test methods like eddy current and ultrasound only detect cracks caused by fatigue, not fatigue itself. In other words, there isn't any practical way to see where a part in service lies on the S-N curve.

 

[ChrisS]

Non-destructive test methods like eddy current and ultrasound only detect cracks caused by fatigue, not fatigue itself. In other words, there isn't any practical way to see where a part in service lies on the S-N curve.

I appreciate this may be semantics but cracks are fatigue. Fatigue is the failure of a material (in metal/aluminum this will be cracking) at a level of stress below the nominal strength of the material. When fatigue occurs there will be cracks, although initial fatigue may not be visible to the human eye, as it progresses these fatigue cracks become larger and combine into bigger cracks that do become visible. You can't say that ultrasound and eddy current will always detect fatigue at an early stage but they can detect fatigue before absolute failure in many cases.

 

[JohnAJohnson]

I'd be curious to know how non aircraft air-cooled engines fare with rapid cooling. Based on 40+ years of riding air-cooled street bikes at highway speeds into downpours, or off road air-cooled two, three and four wheelers hard, then through creeks that almost cover cylinders, I haven't had a failure yet, or noticed issues. Do air boat guys have higher failure rates than us (because they certainly don't slow down gradually)?

 

[gismo]

I'd be curious to know how non aircraft air-cooled engines fare with rapid cooling. Based on 40+ years of riding air-cooled street bikes at highway speeds into downpours, or off road air-cooled two, three and four wheelers hard, then through creeks that almost cover cylinders, I haven't had a failure yet, or noticed issues. Do air boat guys have higher failure rates than us (because they certainly don't slow down gradually)?

While I'm not a "shock cooling" believer, at least to the extent that closing the throttle in the air and descending with it closed will cause cylinders to crack, there is a big difference in the heat extraction in that situation vs an airboat gliding on the water with the throttle closed or a motorcycle coasting with a closed throttle. In the airplane you can maintain or increase the cooling airflow for an extended period but the motorcycle or airboat engine isn't going to see high "airspeed" for anywhere near as long unless the motorcycle is descending from Pike's peak or the airboat went over a very big waterfall. As to the effects of rain on cylinder temps I've never seen my CHTs drop noticeably in heavy rain, anyone else notice that?

 

[fgcason]

While I'm not a "shock cooling" believer, at least to the extent that closing the throttle in the air and descending with it closed will cause cylinders to crack, there is a big difference in the heat extraction in that situation vs an airboat gliding on the water with the throttle closed or a motorcycle coasting with a closed throttle.

There's a HUGE difference between very delicately cooling an engine at 50F per minute vs taking a hard run very hot offroad air or water cooled motorcycle engine and very abruptly dunking it into 45F mountain river water up over the rocker covers and injesting a bucket full of cold water into the carburetor intake thus having to pull the spark plugs to get the water out of the cylinders. The cooling rate to ambient is under a minute somewhere in the 400F+ per minute range. And that doesn't happen just once, it happens over and over and over and over.
If that doesn't crack a cylinder or piston, which it doesn't, not a lot will.

 

[Arbiter419]

I'd be curious to know how non aircraft air-cooled engines fare with rapid cooling. Based on 40+ years of riding air-cooled street bikes at highway speeds into downpours, or off road air-cooled two, three and four wheelers hard, then through creeks that almost cover cylinders, I haven't had a failure yet, or noticed issues. Do air boat guys have higher failure rates than us (because they certainly don't slow down gradually)?

My thoughts exactly. I've dunked my trusty Honda ATC into water (very cold mountain stream water too) above the valve cover more than once with no complaints from it. All original hardware too, that thing will never die.

 

[Tom-D]

I'm not sure what your point is here. I'm reading that stuff that is used more gets worn out faster. I'm not reading that gross weight takeoffs are bad.

Maybe your point is that consistently flying a plane at gross weight is bad. I would understand your point better if you compared two otherwise identical aircraft at the same total time and same engine time except one was consistently loaded to maximum weight, and the other was not. What would you see that differentiates these planes during an inspection?

Two identical aircraft is a myth, they do not exist. so how can you compare ?

Heavily loaded aircraft will be subjected to more stress in flight and will show signs of wear much quicker than one that is not.

If you don't believe me ask any part 135 operator.

The California Highway patrol had a fleet of 185s that after 25,000 hours became too expensive to operate, If the state can't afford the maintenance, do you believe you can?

 

[Tom-D]

Can Eddy Current testing detect fatigue?

Eddy Current works on the principal of an echo, there must be a change of distance to get a different echo. a subsurface flaw will not show any difference.

 

[Tom-D]

You might be able to say that a 10,000 hr 172 is "more worn out" than a 2,000 hour 172, but what is the useful life of a 172 airframe? Does Cessna publish a life limit? Do they require special inspections at a certain number of hours?

If you took a 2,000 hour 172 and a 10,000 hour 172 of the same vintage and did side-by-side annual inspections, what differences would you see? Could you tell the difference if you didn't know the number of hours on each before you started inspecting?

What if you inspected two 10,000 hour 172s, an A-model that racked up 200 hr/year for 50 years, and one that racked up 2,500 hr/yr for 4 years at a big flight school? What differences would you see? Which one is "more worn out"?

The one with the most time.

 

[gismo]

There's a HUGE difference between very delicately cooling an engine at 50F per minute vs taking a hard run very hot offroad air or water cooled motorcycle engine and very abruptly dunking it into 45F mountain river water up over the rocker covers and injesting a bucket full of cold water into the carburetor intake thus having to pull the spark plugs to get the water out of the cylinders. The cooling rate to ambient is under a minute somewhere in the 400F+ per minute range. And that doesn't happen just once, it happens over and over and over and over.
If that doesn't crack a cylinder or piston, which it doesn't, not a lot will.

Dunking does sound a lot more abusive than chopping the throttle in an airplane, but do these motorcycle engines run 1500-2000 hrs between overhauls? Not sure if this matters but if the engines don't last all that long, thermally induced fatigue cracking might not be much of a factor. Only other difference I can think of: Casting of aircraft engine cylinder heads might not be as robust especially given the desire for light weight and the likely cruder methods used for the aircraft components which might be made in much smaller quantities.

Personally, I believe that the head cracking issue has more to do with poor casting technique (voids and variations in fin area are common) that seems to affect aircraft cylinder heads coupled with the once common practice of reusing cylinders without paying any attention to the total hours on them. Also I suspect that motorcycle cylinders don't see anywhere near the same peak cylinder pressures that aircraft cylinders cope with due to the much higher RPM at high power for the motorcycle engines.

 

[Tom-D]

The question was aircraft with the same hours but flown with different passenger weights since you seemed to imply that flying a C150 at gross was bad.

Which aircraft will exert more stress load in turb, a light one or a heavy one.

Look at a old 207 in 135 service, and one that is not see which one has more wrinkles.

 

[Tom-D]

What do Grummans have to do with it? I was pointing out that Grumman was smart enough to place a life limit on their aircraft as a CYA thing for the company.

I was asking about 172s.
Does Fatigue act on cessna's metal any different? Just because Cessna didn't place a CYA thing in the type certificate does not mean their aircraft won't collect fatigue and fail one day.

Sure, a 172A looks different from a new one. But you didn't answer any of my questions, either. But you did specifically mention fatigue. Can you look at two wing ribs and tell me which one has more fatigue?

You betcha, look for the smoking rivets it'll have them
Can you tell me which rib has been operating at gross weight and which one hasn't?

compare the TT to the number of wrinkles, you will note a big difference from the same aircraft with the same TT that has been run light most of its life.

 

[flyingcheesehead]

That's you, most of us think different and want a bit more safety factor.

I have no problem flying an airplane at max gross. That's what it was designed to do.

And the engine was designed for full power on takeoff. Hell, Deakin's articles even talk about some "redlines" that are meant to be at redline on takeoff but are frequently misadjusted by mechanics because they erroneously think that if a redline shouldn't be crossed, that farther away from the redline is better.

Really? Because most of the pilots that I know end up doing pretty similar when there's a need. We also use full power on takeoff.

Bingo. And the "safety factor" is something you get by flying with full power on takeoff, regardless of weight, if you're in a small airplane.

 

[Trapper John]

The one with the most time.

6 questions, 1 answer.

 

[Trapper John]

Does Fatigue act on cessna's metal any different?

Of course it does, compared to a Grumman. They are different parts subject to different loadings. You can't compare apples and bananas.

You betcha, look for the smoking rivets it'll have them

A smoking rivet only tells you that it's loose.

 

[ChrisS]

Eddy Current works on the principal of an echo, there must be a change of distance to get a different echo. a subsurface flaw will not show any difference.

Eddy Current *can* be used to detect subsurface cracks and flaws. It works on electromagnetic induction, anything that interupts it's flow can be detected and measured. You get a higher resolution when testing for surface flaws but it can also find subsurface flaws.

 

[ChrisS]

A smoking rivet only tells you that it's loose.

Yep, caused by fretting corrosion and not fatigue. Although if left alone they may cause/probably will cause fatigue.

 

[Trapper John]

Cessna has something called the Continuing Airworthiness Program (CAP) that applies to their singles. It's a program that most people have never heard of and even Transport Canada inspectors here had never seen it. There's a long list of items that they want checked at hourly or calendar intervals, most of them never appearing on Cessna's inspection sheets, and we've found it wise to take a look at them. Further, Cessna's Service Bulletins are words to the wise and usually have some merit. Cessna will eventually come out with an Aging Aircraft program that will demand removal of high-time airplanes from the fleet, and governments will probably endorse it.

That's good to know, thanks.

So you can see, a good-looking 10,000-hour airplane might have a $10,000 paint job, while the one next to it has ugly original paint but that $10,000 was spent on maintenance of the above. Which one will sell the easiest?

Dan

People like pretty.

 

[TMetzinger]

Yes -- to demonstrate high DA conditions, especially in over-powered birds.

+1. A good idea for demos with students.

 

[Tom-D]

That's good to know, thanks.

People like pretty.

Pilots always buy paint and radios.

 

[gismo]

Which aircraft will exert more stress load in turb, a light one or a heavy one.

Look at a old 207 in 135 service, and one that is not see which one has more wrinkles.

Depends on where you're measuring the "stress load". If it's wing structure or or wing attach structure loading, lighter weight will mean less stress in identical air at the same airspeed but if it's ancillary structure (engine mounts, battery box, seat attachments etc.) the airplane with more weight will stress those areas less.

 

[Fearless Tower]

The California Highway patrol had a fleet of 185s that after 25,000 hours became too expensive to operate, If the state can't afford the maintenance, do you believe you can?

That's actually a bad analogy Tom.....the state of CA can't afford ANYTHING!

 

[Ted]

That's actually a bad analogy Tom.....the state of CA can't afford ANYTHING!

Nevermind the fact that 25,000 hours is way more than the 10,000 that we usually see as "high time."

If 25,000 hours is the limit, then my Aztec should survive another 60 years.

 

[fgcason]

do these motorcycle engines run 1500-2000 hrs between overhauls?

I'm not sure about all of them however quite a few run for decades and multiple thousands of miles which has got to rack up the hours.

 

[Trapper John]

I'm not sure about all of them however quite a few run for decades and multiple thousands of miles which has got to rack up the hours.

I'm not a high-miler rider, but I do know people with air-head BMWs that have gone over 150,000 miles without significant engine work. That would be 2,100+ hours at interstate speeds.

 

[Cap'n Jack]

Two identical aircraft is a myth, they do not exist. so how can you compare ?

Tom- I'm not giving sarcastic remarks nor picking on you. I'm sure you know perfectly well what I was asking. If you don't know, I won't think less of you if you say so.

Heavily loaded aircraft will be subjected to more stress in flight and will show signs of wear much quicker than one that is not.

You've said this already. Several times.

What wear do you see in a more heavily loaded aircraft compared to one that was treated more "gently"?

You mentioned smoking rivets and wrinkles, presumably in the skin, but spread this information over at least two responses.

If you don't believe me ask any part 135 operator.

I didn't say that I didn't believe you. Is this a valid comparison anyway? Part 135 aircraft are larger and strength doesn't scale with size AFAIK- you or a structural engineer could straighten me out on that.

The California Highway patrol had a fleet of 185s that after 25,000 hours became too expensive to operate, If the state can't afford the maintenance, do you believe you can?

Were they flying them at gross weight? If not, how is this pertinent to my question?

 

[OffCenter]

I'm not a high-miler rider, but I do know people with air-head BMWs that have gone over 150,000 miles without significant engine work. That would be 2,100+ hours at interstate speeds.

I have a 1974 BMW R90 that I bought new. It has 172,000 miles on it.
Except for points and plugs, the engine is completely original.
Never been opened.

 

[gismo]

I'm not a high-miler rider, but I do know people with air-head BMWs that have gone over 150,000 miles without significant engine work. That would be 2,100+ hours at interstate speeds.

Chances are 2100 hrs at interstate speeds wouldn't include much time under water. I was asking if the kind of abuse described by Frank was conducive to long engine life.

 

[JohnAJohnson]

Chances are 2100 hrs at interstate speeds wouldn't include much time under water. I was asking if the kind of abuse described by Frank was conducive to long engine life.

I had an 83 Honda XR500R (off road only, air cooled enduro) that I rode in the Utah desert at pretty fast speeds (70+), maybe averaging 2-3 hours a week. After 15 years, it broke a cam chain and I decided to overhaul the engine while I was in there. The piston, rings, bore, rod bearings (plain), etc., all miked within serviceable limits. This would equate to around 2,000 hours. During the same period, I also rode the bike in the mountains, crossing many creeks and riding on lots of wet trails (water from the front tire splashes right on the front of the cylinder). Not sure if this is typical, but I've found most air cooled engines, as long as they aren't overheated or subjected to preignition/detonation, will last a very long time with lots of reliability.