When Not to Aim for the Numbers

[brcase]

<snip>

Are people actually talking about chopping to idle when abeam the numbers and riding the whole rest of downwind, base and final at idle?

Exactly, this should be a maneuver every pilot is proficient at. I think the FAA realized that many pilots were lacking in this area a few years ago and added the 180 Power off landing in to the Commercial PTS. It can be challenging in bigger aircraft over 200HP as a rule of thumb. Under 200HP they can usually easily be done and looks like a normal stablized approach if you don't insist on using full flaps.

I have seen way to many heaps of alumimum within a 1/4 mile of the end of the runway that should have been easy emergency landings. Seen the Piper in the Tree photo? I think the problem is pilots are not practicing them enough. If Rare Bare or a 737 or a 767 can be dead sticked in, then doing it in a cherokee should be a no brainer, but apparently it isn't for many pilots when presented to the the requirement to do so.

Many pilots practice these as their normal landings to ensure they are profiencent in doing them. Of course Short Feild and Soft Feild landings are not normal landings and do usually require power on approach.

Interestingly the 400ft final turn at 3/4 of mile is just about a 10 to 1 glide ratio which is close to what many small aircraft will glide at with little or no power and no flaps. However knowing how to adjust your pattern and flaps to accomadate less than perfect conditions is the point of practicing the 180 power off landing.

I did some research on base to final turns in gliders a few years ago. Since many glider carry GPS flight recorders it is pretty easy to evaluate how glider pilots fly their patterns. I found the most pilots including myself pretty consistantly turn final at 400 feet. Of course we use flaps or spoilers to lower our glide ratio to the 10:1 to get to our touch down spot.

Brian

 

[Larryo]

The FAA requires a stabilized approach even light planes on a Private Pilot practical test, as described above. However, the Stabilized VFR Approach in our context is rather different than the big jet stabilized approach, which is a much more rigorous definition.
Nobody says you have to use the same speed or configuration every time, but the FAA feels it's safer to maintain a stabilized approach on final with whatever combination of speed, configuration, and glide path is appropriate to the situation.
While I agree with the latter, the former isn't true. Power-off approaches are part of the Private and Commercial single engine airplane PTS's, although they are considered a demonstration of your emergency engine-out skills, not a "normal" landing.

Ron,

"Maintains a stabilized approach and recommended airspeed, or in
its absence, not more than 1.3 VSO, +10/-5 knots, with wind gust
factor applied."

The stabilized approach is an option, see above. Also, there is no requirement that requires pitch, power or speed to be constant.... and sometimes that's appropriate.

Also, agreed the poweroff approach "could" be used as emergency testing, but no requirement specifically for a power off approach in the pattern to a landing, like is being discussed on this thread.

 

[Larryo]

I'm having trouble understanding how you can maintain pattern altitude with 12 inches and then descend to the runway without changing power. All in all, to be able to fly as you have said in this thread, it sounds like you're a better pilot than any I've ever flown with. My hat's off to you.

Probably not hard, but depends on the plane and weight/temp.... I could do it fairly easy but need another inch or two, perhaps 14.

 

[Larryo]

Hmmm...

By the time I arrive in a IO-520 or IO-470 the MP is around 14" and the engine's had plenty of cooling in that big long descent to the airport.

Please explain how there can be "shock cooling" dropping from 14" to idle...?

Agreed, shock cooling, for the most part in our small airplanes, is a myth. However, I'm not a fan of a power off approach.

 

[poadeleted20]

"Maintains a stabilized approach and recommended airspeed, or in its absence, not more than 1.3 VSO, +10/-5 knots, with wind gust factor applied."

The stabilized approach is an option, see above.

Your 8th grade grammar teacher would be ashamed of your parsing. The "or" permits the use of 1.3 Vs0 as the target speed when there's no recommended speed; it does not make a stabilized approach itself "optional."

 

[poadeleted20]

Agreed, shock cooling, for the most part in our small airplanes, is a myth.

The "shock cooling" myth is that you can crack the cylinders by chopping power and cooling them too fast. That takes cooling rates in the 100's of degrees per second or more, and the engineering term for that is "quench cooling." That's not going to happen unless you drop your hot engine into a gigantic cold liquid bath -- even the heaviest rain won't do that. However, Lycoming says that cylinder wall/piston ring wear is accelerated by uneven contraction of the heads and pistons when cooling rates get up into the 60-80 deg/minute range or higher, which happens on a power-off approach (meaning one where you reduce power to idle at the abeam position and leave it there until clearing the runway).

 

[Ted]

However, Lycoming says that cylinder wall/piston ring wear is accelerated by uneven contraction of the heads and pistons when cooling rates get up into the 60-80 deg/minute range or higher, which happens on a power-off approach (meaning one where you reduce power to idle at the abeam position and leave it there until clearing the runway).

This poses an interesting question. Has anyone observed these cooling rates of 60-80F/minute (or more) when doing such procedures? If not, I'd venture a guess that there's nothing to be concerned about. Of course this will vary depending on what condition you're in when you start. If you're doing a more conventional approach where you've already started reducing power and slowed up some, it's probably less of an issue than if you come in at cruise power.

 

[dmccormack]

However, Lycoming says that cylinder wall/piston ring wear is accelerated by uneven contraction of the heads and pistons when cooling rates get up into the 60-80 deg/minute range or higher, which happens on a power-off approach (meaning one where you reduce power to idle at the abeam position and leave it there until clearing the runway).

No, it doesn't.

Sean Tucker goes from WOT to idle many times in his routine. Any evidence of "shock cooling" there?

Consider that every time you shut an engine down, you go from EGTs of 1200 or so and engine oil temps around 200 to ambient in an instant.

Or do you?

No -- you don't -- the engine stays warm for hours (unless it's really cold...then it stays warm for an hour... or more).

 

[Ted]

No, it doesn't.

Sean Tucker goes from WOT to idle many times in his routine. Any evidence of "shock cooling" there?

Let's look at the specifics of the condition, though. Sean Tucker, along with those other aerobatic folk, are not getting their engines to 2000 hours (or even trying). Those pilots are absolute hell on their engines. They also only run a limited amount of time. Some of them get new engines every year. That may represent a couple hundred hours, not couple thousand, and most of that time is in ferrying the plane from location to location (which is done how the rest of us fly planes - like sane people). Yes, the engines take it, but don't think that the engines don't take a toll for it.

Consider that every time you shut an engine down, you go from EGTs of 1200 or so and engine oil temps around 200 to ambient in an instant.

Or do you?

No -- you don't -- the engine stays warm for hours (unless it's really cold...then it stays warm for an hour... or more).

Well, that's not quite it if I'm reading you correctly (which I may not). Remember that when you shut the engine down, you're shutting down all flow. Engines operate in an equilibrium while they're running where heat is being produced and excess heat is being dissipated as necessary (dissipation occurs through airflow). When you shut down the engine, your cooling has stopped, and flow has stopped. Combustion has stopped so no more heat is being created, but you have zero cooling, so your natural convection, conduction, and radiation will be your heat transfer. All temperature changes will occur gradually.

Your EGTs may be going from 1200F (probably a bit cooler than that, depending on your engine) to some significantly lower value, but remember also that your exhaust gas flow is now 0, so the heat in the cylinder right around the exhaust valve and port (and exhaust pipes) will then start dissipating to cooler areas. Your oil will remain whatever temperature it was initially, but oil flow stops, so it, too, will move towards an equilibrium.

We generally recommend to keep your cooling rate below 50 deg/min. In my experience, that is not hard to do at all. My question is whether anyone has seen those greater than 50 deg/min rates while doing power-off landings as described - i.e. at an appropriate approach speed and then pull the power back abeam the numbers.

 

[dmccormack]

Let's look at the specifics of the condition, though. Sean Tucker, along with those other aerobatic folk, are not getting their engines to 2000 hours (or even trying). Those pilots are absolute hell on their engines. They also only run a limited amount of time. Some of them get new engines every year. That may represent a couple hundred hours, not couple thousand, and most of that time is in ferrying the plane from location to location (which is done how the rest of us fly planes - like sane people). Yes, the engines take it, but don't think that the engines don't take a toll for it.

Certainly -- the question is: does this activity result in shock cooling damage?

(I lean towards no).

Well, that's not quite it if I'm reading you correctly (which I may not). Remember that when you shut the engine down, you're shutting down all flow. Engines operate in an equilibrium while they're running where heat is being produced and excess heat is being dissipated as necessary (dissipation occurs through airflow). When you shut down the engine, your cooling has stopped, and flow has stopped. Combustion has stopped so no more heat is being created, but you have zero cooling, so your natural convection, conduction, and radiation will be your heat transfer. All temperature changes will occur gradually.

Your EGTs may be going from 1200F (probably a bit cooler than that, depending on your engine) to some significantly lower value, but remember also that your exhaust gas flow is now 0, so the heat in the cylinder right around the exhaust valve and port (and exhaust pipes) will then start dissipating to cooler areas. Your oil will remain whatever temperature it was initially, but oil flow stops, so it, too, will move towards an equilibrium.

We generally recommend to keep your cooling rate below 50 deg/min. In my experience, that is not hard to do at all. My question is whether anyone has seen those greater than 50 deg/min rates while doing power-off landings as described - i.e. at an appropriate approach speed and then pull the power back abeam the numbers.

I agree that shutdown means there's a cessation in the production and redistribution of heat, but more important to the shock cooling point -- heat soaked metal sits exposed to the naked, cold, ambient air.

And there's no damage.

Most twins practice engine outs on only one side. Do you see increased wear/ shock cooling damage on that side?

Don't get me wrong -- I'm all for gradual everything (heck, I even slowly add throttle on take off).

But Ron's point was that power -to-idle abeam the numbers is bad for the engine.

I say it's not.

You're a Lyc guy -- what say you?

 

[Ted]

Certainly -- the question is: does this activity result in shock cooling damage?

(I lean towards no).

And that's your option to. I've never seen the engine monitor data in those cases, but I'd be willing to be they're seeing more than the 50 deg/min recommended.

I agree that shutdown means there's a cessation in the production and redistribution of heat, but more important to the shock cooling point -- heat soaked metal sits exposed to the naked, cold, ambient air.

And there's no damage.

I'm not following you here. When the plane is flying it's flying through that same air (probably a bit colder), but the difference there is you have it flowing. With the engine shut off, it's not flowing. Thus your heat transfer (i.e. what causes the cooling) has gone from having forced convection (the big one) to just natural convection, conduction, and radiation, which combined are WAY less than your forced convection. The concept of shock cooling involves a rate of change of cylinder temps. When you shut down your engine, I have not observed any rapid change in cylinder temps due to the lack of cooling air.

Most twins practice engine outs on only one side. Do you see increased wear/ shock cooling damage on that side?

We practiced engine-outs on both sides. I think if you only practice them on one side that you take away the learning opportunity of making the student positively identify the engine rather than get in the habit of "Oh, it's always the [whatever side] engine that fails."

That said I know a lot of people don't do that. Ric had to top one of the engines on his 310 year or two back, which I seem to recall was the engine they normally shut down for engine out training. I do try to avoid doing training in my plane.

Don't get me wrong -- I'm all for gradual everything (heck, I even slowly add throttle on take off).

But Ron's point was that power -to-idle abeam the numbers is bad for the engine.

I say it's not.

I know you're for gradual everything, and I wasn't questioning that, just trying to add some of what I've seen. I don't see why it would be damaging if you're flying a normal pattern (i.e. you're not at cruise power abeam the numbers), but I also don't think that if you're flying a normal pattern you'd see more than 50 deg/min change in CHTs.

You're a Lyc guy -- what say you?

Why, Lycoming Service Instruction 1094D (the answer to all questions), of course!

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf

At all times, caution must be taken not to shock cool the cylinders. The maximum recommended temperature change should not exceed 50°F. per minute.

 

[poadeleted20]

This poses an interesting question. Has anyone observed these cooling rates of 60-80F/minute (or more) when doing such procedures?

Pretty much every time I've done a power-off approach in a light plane with a good CHT monitoring system (e.g., EDM-700). That includes Grummans, Cessnas, Pipers, you name it. And from your own company's Key Reprints:

And finally, power-off letdowns should be avoided. This is especially applicable to cold-weather operations when shock-cooling of the cylinder heads is likely. It is recommended that cylinder head temperature change not exceed 50˚ F. per minute.

Sudden cooling is detrimental to the good health of the piston aircraft engine. Lycoming Service Instruction 1094D recommends a maximum temperature change of 50˚ F per minute to avoid shock-cooling of the cylinders.

 

[poadeleted20]

Sean Tucker goes from WOT to idle many times in his routine. Any evidence of "shock cooling" there?

IIRC, Sean overhauls his engine like every 500 hours or something like that.

Consider that every time you shut an engine down, you go from EGTs of 1200 or so and engine oil temps around 200 to ambient in an instant.

We're talking CHT, not EGT, and that's another thing entirely. In fact, CHT's generally go up a bit after landing due to reduction in cooling flow, and come down very slowly after shutdown. In fact, it's better on hot days to open up the cowl after shutdown to allow the heat out so it doesn't adversely affect hoses, etc. A friend of mine insrumented the engine compartment of his Tiger and found temps in there ran up to over 200F after shutdown unless the cowl was opened, and that's not good for a lot of things inside that compartment.

 

[poadeleted20]

If y'all want, I'll donate a bit of wear on my engine and do a power-off approach next time I fly, download the data, and post it here so you can see the cooling rates that occur. I invite anyone else with a JPI EDM or similar device to do the same. But I know for sure than every time I do a power-off approach, the alarm on my EDM (set for 60 deg/min) always goes off -- and sometimes, even with just the reduction around the "avoid descents" yellow arc from 2250 to 1850.

 

[dmccormack]

IIRC, Sean overhauls his engine like every 500 hours or something like that.
We're talking CHT, not EGT, and that's another thing entirely. In fact, CHT's generally go up a bit after landing due to reduction in cooling flow, and come down very slowly after shutdown. In fact, it's better on hot days to open up the cowl after shutdown to allow the heat out so it doesn't adversely affect hoses, etc. A friend of mine insrumented the engine compartment of his Tiger and found temps in there ran up to over 200F after shutdown unless the cowl was opened, and that's not good for a lot of things inside that compartment.

500 hours is quite a few given what his engine is subjected to -- the question remains -- is shock cooling causing premature wear / breakdown?

The last airplane I flew with a JPI was an IO-520. SOP was open the cowl flaps once clear of the runway. I did my commercial training in the airplane, and did plenty of power-off 180 spot landings. No alarms, no rapid cooling, no problems.

 

[poadeleted20]

The last airplane I flew with a JPI was an IO-520. SOP was open the cowl flaps once clear of the runway. I did my commercial training in the airplane, and did plenty of power-off 180 spot landings. No alarms, no rapid cooling, no problems.

What were the observed cooling rates, and to what was the alarm set?

 

[dmccormack]

And that's your option to. I've never seen the engine monitor data in those cases, but I'd be willing to be they're seeing more than the 50 deg/min recommended.

50 deg/min recommended. If it's like the rest of engineering, that's half the proven rate.

I'm not following you here. When the plane is flying it's flying through that same air (probably a bit colder), but the difference there is you have it flowing. With the engine shut off, it's not flowing. Thus your heat transfer (i.e. what causes the cooling) has gone from having forced convection (the big one) to just natural convection, conduction, and radiation, which combined are WAY less than your forced convection. The concept of shock cooling involves a rate of change of cylinder temps. When you shut down your engine, I have not observed any rapid change in cylinder temps due to the lack of cooling air.

Very true.

But air is flowing very nicely across the front cylinders -- not so much the rear. Those front fins are right out there in the cold air -- anyone reporting cracks or shattering of those front jugs?

We practiced engine-outs on both sides. I think if you only practice them on one side that you take away the learning opportunity of making the student positively identify the engine rather than get in the habit of "Oh, it's always the [whatever side] engine that fails."

That said I know a lot of people don't do that. Ric had to top one of the engines on his 310 year or two back, which I seem to recall was the engine they normally shut down for engine out training. I do try to avoid doing training in my plane.

It would be interesting to examine trends in schools that shut down one side only as SOP and see if there is any increase MTBF on the off side.

I know you're for gradual everything, and I wasn't questioning that, just trying to add some of what I've seen. I don't see why it would be damaging if you're flying a normal pattern (i.e. you're not at cruise power abeam the numbers), but I also don't think that if you're flying a normal pattern you'd see more than 50 deg/min change in CHTs.

Lyc doesn't support my engine so I don't like Lyc. (Where's the arms folded Im going home avatar?)



I agree that normal pattern ops (practicing power-off 180s, for example) are unlikely to produce the temperature deltas required to stress anything.

So to Ron's point that pulling power to idle abeam the numbers induces "Shock cooling" -- I don't agree.

 

[dmccormack]

What were the observed cooling rates, and to what was the alarm set?

IIRC default was 60pm. Every so often it would flash -- momentarily.

 

[poadeleted20]

So to Ron's point that pulling power to idle abeam the numbers induces "Shock cooling" -- I don't agree.

If by "shock cooling" you mean that cooling which "cracks or shattering of those front jugs," I never said that and I don't believe it, either. In fact, I specifically discussed that issue. OTOH, if you mean unnecessary wear of the rings and walls due to excessively uneven cooling causing uneven shrinkage, Lycoming seems to think that will occur with cooling rates of over 50 deg/min, and I've seen that enough times in enough different airplanes to know that it does occur despite your observation of one airplane with a Continental engine, and since it did flash, it was occurring even on that engine.

 

[el con]

I went to the Ed Fred /Bob Hoover flight school.
I usually come in hot ,power off, do a full loop to a touch down at the numbers,while pouring a glass of lemonade without spilling it, coast up to the cheering crowd as I step out and wave my straw hat.
My goodness , IMHO ,do what ever works for you before you crash !
I like to stay high so I make the field, use what I call a high idle, aim for the numbers, with full flaps in when I know I can make the field,drop it in going 70-80 mph, flare and slowly pull back. Worked for the last 487 landings.
If you practice and master short field landings ,you can ALWAYS make the long ones, whens the last time you heard a pilot won't attemt to land there because the field is too long
Then again results may vary on local conditions.

 

[dmccormack]

If by "shock cooling" you mean that cooling which "cracks or shattering of those front jugs," I never said that and I don't believe it, either. In fact, I specifically discussed that issue. OTOH, if you mean unnecessary wear of the rings and walls due to excessively uneven cooling causing uneven shrinkage, Lycoming seems to think that will occur with cooling rates of over 50 deg/min, and I've seen that enough times in enough different airplanes to know that it does occur despite your observation of one airplane with a Continental engine, and since it did flash, it was occurring even on that engine.

I own the original Lycoming airplane engine and pull power to idle once I have the field made, trim for 60 and enjoy the glide -- as do many other owners of these antiques. My O-145 is 70 years old and still ticking.



Now, there is no evidence or proof that an indication of some arbitrary cooling rate per minute (in this case, 50 or 60d/m) makes one whit of difference in any engine, Continental or Lycoming.

Lycoming has provided an advisory in a publication which has a whiff of CYA. It's interesting that Lyc has not issued a Service Bulletin or POH addendum based on real world data and evidence.

If such exists, I'm all ears.

 

[Ted]

50 deg/min recommended. If it's like the rest of engineering, that's half the proven rate.

I was doing a lesson yesterday, and we pulled out the POH for the 172 we were flying. It gave the g-load limits on the plane (standard for normal and utility categories), and then stated that the design limits were 1.5 times those limits. So are you going to go out and purposely fly the plane in a +6.6 g maneuver?

This is where the old engineer cues his long sigh with the smoker's cough and says "And you wonder why things break on your plane," then shakes his head and walks away. Operating at design limits is a bad idea for longevity.

But air is flowing very nicely across the front cylinders -- not so much the rear. Those front fins are right out there in the cold air -- anyone reporting cracks or shattering of those front jugs?

Do you think that the entire cylinder is uniform, the same temperature? Of course it's not. Have you ever seen an instrumented cylinder? Not likely. It has thermocouples all over it in all sorts of different locations. We establish correlations, and the CHT limits are really based on those correlations, that if the location of the probe is whatever the limit is, that means the rest of the head is within the limit as well.

A temperature differential across the cylinder at different points is how it works. That's nothing unexpected. Remember again, when the engine is running it's in an equilibrium. If your CHTs are staying still, that means that you heat dissipated is equal to the heat created. When you're out of equilibrium your temperatures will be rising or dropping. It is not the same as what you state above with different parts of the cylinder being different temperatures.

 

[dmccormack]

I was doing a lesson yesterday, and we pulled out the POH for the 172 we were flying. It gave the g-load limits on the plane (standard for normal and utility categories), and then stated that the design limits were 1.5 times those limits. So are you going to go out and purposely fly the plane in a +6.6 g maneuver?

Of course not -- but it's good to know the airplane won't break at 1.52x design limits, isn't it?

What if the cooling rate is 49F/m?

Do you think that the entire cylinder is uniform, the same temperature? Of course it's not. Have you ever seen an instrumented cylinder? Not likely. It has thermocouples all over it in all sorts of different locations. We establish correlations, and the CHT limits are really based on those correlations, that if the location of the probe is whatever the limit is, that means the rest of the head is within the limit as well.

A temperature differential across the cylinder at different points is how it works. That's nothing unexpected. Remember again, when the engine is running it's in an equilibrium. If your CHTs are staying still, that means that you heat dissipated is equal to the heat created. When you're out of equilibrium your temperatures will be rising or dropping. It is not the same as what you state above with different parts of the cylinder being different temperatures.

OK, then we agree -- there is a variance in temperatures across the entire engine. That's what you awesome engineers designed it to do, that's why its made of various metals that expand and contract and known rates without rapid deformation.

But to argue that an engine -- which is already hotter in one part than another, etc etc -- is going to be damaged by reducing MP from 15" to 12" or RPM from 2100 to 1400 (idle) just isn't supported by facts, data, or even experience.

How many C152 O-200s go to TBO and beyond being run hard, put away wet, doing hundreds of patterns? just about all of them?

 

[gismo]

The "shock cooling" myth is that you can crack the cylinders by chopping power and cooling them too fast. That takes cooling rates in the 100's of degrees per second or more, and the engineering term for that is "quench cooling." That's not going to happen unless you drop your hot engine into a gigantic cold liquid bath -- even the heaviest rain won't do that. However, Lycoming says that cylinder wall/piston ring wear is accelerated by uneven contraction of the heads and pistons when cooling rates get up into the 60-80 deg/minute range or higher, which happens on a power-off approach (meaning one where you reduce power to idle at the abeam position and leave it there until clearing the runway).

IME you won't come close to exceeding 60°F/minute when making a typical power off approach from pattern altitude, mostly because your airspeed is too low. Where those cooling rates are most likely to occur is when you chop the power at altitude and descend at a high indicated airspeed with little power.

 

[Ted]

Of course not -- but it's good to know the airplane won't break at 1.52x design limits, isn't it?

What if the cooling rate is 49F/m?

I don't think anyone said that it will break at greather than 50 deg/min. It's a matter of long term wear, not "this will break immediately." If you looked at it in metric it would be some round number in C instead of in F. Of course there's some slop in it. Where do you think these numbers come from, God himself? They come from engineers who base them on good engineering judgement and experience. Too many people think in terms of hard limits, and that's not how the physics works out in this case. Limits still exist to provide you guidelines to work with. If you choose to ignore them, that's your decision.

OK, then we agree -- there is a variance in temperatures across the entire engine. That's what you awesome engineers designed it to do, that's why its made of various metals that expand and contract and known rates without rapid deformation.

What has me baffled is that you're equating this difference in temperature across the engine with a change in temperature. The two are completely different, as I've tried to explain twice.

But to argue that an engine -- which is already hotter in one part than another, etc etc

That is irrelevant.

-- is going to be damaged by reducing MP from 15" to 12" or RPM from 2100 to 1400 (idle) just isn't supported by facts, data, or even experience.

When did I say it was? I didn't.

 

[PittsDriver]

I'm gonna go out on a limb here and say that Sean Tucker's engines rarely make 250 hours and the reason they don't has very little to do with shock cooling. You take an AEIO-540, pump it up with high compression pistons to something like 340+ HP or more at really high RPM levels, then throw the kind of gyroscopic forces on the crank that he puts on it every time he flies an air show and you think a little rapid cooling on it will make much difference?

My reaction to the Lycoming advice that some level of rapid cooling hurts the engine is - so what? Starting it hurts it. Running it hurts it. Not running it for a while hurts it even more. Going out when it's cold and turning the prop around before you start it hurts it. Letting the oil go to 30 hours instead of 25 hurts it. Unless someone wants to actually quantify how much a power off descent to landing practice hurts my engine, my answer is still - so what. Not being well practiced at engine out emergencies and crashing my engine into a fence instead of making the field is going to hurt it and me a lot more than any worry about 60 degrees per minute crap. I've got 350 hours on what was a new AEIO-540/260 HP engine that gets a constant battering of full throttle going up with no air over it transitioning to idle throttle going down with lots of wind over it - hour after hour after hour. My compressions haven't changed 1 psi in 300 hours and there's never been a hint of metal in the screen.

This whole argument reminds me of the time, as a teenager, I told my dad that the reason I was downshifting and letting the engine slow the car down was to save the brakes. He whacked me upside the head and said "You idiot, brake pads are $8 and a new clutch is going to cost you every thing you made this summer. You just like to hear the engine rev..."

 

[gismo]

I was doing a lesson yesterday, and we pulled out the POH for the 172 we were flying. It gave the g-load limits on the plane (standard for normal and utility categories), and then stated that the design limits were 1.5 times those limits. So are you going to go out and purposely fly the plane in a +6.6 g maneuver?
This is where the old engineer cues his long sigh with the smoker's cough and says "And you wonder why things break on your plane," then shakes his head and walks away. Operating at design limits is a bad idea for longevity.

Actually that's where the old engineer says there's a misunderstanding that needs addressing here. The category based g limits are the design elastic limits. The 1.5x factor is for the design ultimate limits. If you exceed the elastic limits something will bend and stay bent i.e. you will cause permanent deformation (e.g. wrinkled skins). If you exceed the ultimate limits something will break, e.g. a wing will depart the airplane. So unless you don't care if the plane ends up unairworthy you must stay within the design elastic limits.

 

[Larryo]

IME you won't come close to exceeding 60°F/minute when making a typical power off approach from pattern altitude, mostly because your airspeed is too low. Where those cooling rates are most likely to occur is when you chop the power at altitude and descend at a high indicated airspeed with little power.

Agreed... while pulling the power in the pattern may not be the best technique.... it's very unlikely to cause shock cooling.

 

[Ted]

Actually that's where the old engineer says there's a misunderstanding that needs addressing here. The category based g limits are the design elastic limits. The 1.5x factor is for the design ultimate limits. If you exceed the elastic limits something will bend and stay bent i.e. you will cause permanent deformation (e.g. wrinkled skins). If you exceed the ultimate limits something will break, e.g. a wing will depart the airplane. So unless you don't care if the plane ends up unairworthy you must stay within the design elastic limits.

Ahh, now thanks for clarifying that.

And in case it wasn't clear (probably not), I was referring to a particular old engineer I work with who will do that after trying to explain something for a while.

 

[Larryo]

Exactly, this should be a maneuver every pilot is proficient at. I think the FAA realized that many pilots were lacking in this area a few years ago and added the 180 Power off landing in to the Commercial PTS. It can be challenging in bigger aircraft over 200HP as a rule of thumb. Under 200HP they can usually easily be done and looks like a normal stablized approach if you don't insist on using full flaps.
.......
If Rare Bare or a 737 or a 767 can be dead sticked in, then doing it in a cherokee should be a no brainer, but apparently it isn't for many pilots when presented to the the requirement to do so.

Many pilots practice these as their normal landings to ensure they are profiencent in doing them. Of course Short Feild and Soft Feild landings are not normal landings and do usually require power on approach.

......

Brian

Brian,
In fact, it's probably harder to do one in a Cherokee than a 737 or 767. But what's the point? I don't see any more point in doing a power off landing from 10,000 feet than in the pattern. Practice if, if you want, as an emergency maneuver, but not SOP.

We occasionally did them in the jets from 370, and Baron from 6K.... but it was a lesson in energy management for the most part, and fun..... The problem with the jets was that any traffic ahead would mess us up... but occasionally we could do one all the way to 500ft. when we were required the dreaded "stabilized" approach.

 

[Larryo]

Your 8th grade grammar teacher would be ashamed of your parsing. The "or" permits the use of 1.3 Vs0 as the target speed when there's no recommended speed; it does not make a stabilized approach itself "optional."

Ron,

Read the requirement again... it says "in the absence of" .... and the FAAs definition is not very precise for GA, and the "OR" options may or may not be stabilized.

 

[dmccormack]

What has me baffled is that you're equating this difference in temperature across the engine with a change in temperature. The two are completely different, as I've tried to explain twice.

I suppose it proves the engine's materials can withstand high delta in temperature across the surface and still operate without deformation.

That's pretty neat, IMHO.

 

[poadeleted20]

IME you won't come close to exceeding 60°F/minute when making a typical power off approach from pattern altitude, mostly because your airspeed is too low.

Unless the engine analyzers I've seen show that were all lying to me, I've seen enough actual data to know otherwise.

Where those cooling rates are most likely to occur is when you chop the power at altitude and descend at a high indicated airspeed with little power.

That gets you well over 100 deg/min.

 

[poadeleted20]

I don't think anyone said that it will break at greather than 50 deg/min. It's a matter of long term wear, not "this will break immediately."

Which is what I've been saying all along. Glad to know that an engine engineer from Lycoming agrees.

 

[poadeleted20]

Read the requirement again... it says "in the absence of" .... and the FAAs definition is not very precise for GA, and the "OR" options may or may not be stabilized.

Not in this case. Ask any examiner if you doubt me. The only option is what speed you choose to fly the approach, and even then, it's not really an option -- use recommended if there is one, or 1.3 Vs0 if there isn't.

 

[dmccormack]

Which is what I've been saying all along. Glad to know that an engine engineer from Lycoming agrees.

It's not really what you've been "saying all along."

This all started because you argued that a power-off approach could not be a "stabilized" approach and that:

However, Lycoming says that cylinder wall/piston ring wear is accelerated by uneven contraction of the heads and pistons when cooling rates get up into the 60-80 deg/minute range or higher, which happens on a power-off approach (meaning one where you reduce power to idle at the abeam position and leave it there until clearing the runway).

Which is simply not the case. There is no evidence that power to idle abeam the numbers causes undue wear unless you're in some odd matrix of conditions (super hot engine, super high speed descent in super cold air) -- and that is outside the scope of "normal operations."

 

[dmccormack]

Ron,

Read the requirement again... it says "in the absence of" .... and the FAAs definition is not very precise for GA, and the "OR" options may or may not be stabilized.

I don't think the language supports the option of an "unstabilized approach."

 

[poadeleted20]

Which is simply not the case. There is no evidence that power to idle abeam the numbers causes undue wear unless you're in some odd matrix of conditions (super hot engine, super high speed descent in super cold air) -- and that is outside the scope of "normal operations."

Well, Lycoming says don't exceed 50 deg/min, and I've seen numerous airplanes show 60-80 deg/min during power-off approaches. No, I haven't torn those engines down to examine wear, so there's no "proof beyond a reasonable doubt," but I've seen enough evidence to say that power-off approaches create conditions Lycoming says to avoid due to the potential for accelerated wear.

You want to keep arguing, go ahead -- it's your engine. But as an instructor, I'd think you'd want to give your trainees the benefit of this information.

 

[dmccormack]

Well, Lycoming says don't exceed 50 deg/min, and I've seen numerous airplanes show 60-80 deg/min during power-off approaches. No, I haven't torn those engines down to examine wear, so there's no "proof beyond a reasonable doubt," but I've seen enough evidence to say that power-off approaches create conditions Lycoming says to avoid due to the potential for accelerated wear.

You want to keep arguing, go ahead -- it's your engine. But as an instructor, I'd think you'd want to give your trainees the benefit of this information.

As an instructor, I should be willing to separate fact from opinion and opinion from mere advice.

I teach and practice smoothness in all aspects of flight -- so WOT to idle isn't in my playbook.

But limiting power-off approaches or worse, avoiding them due to a vague warning of increased wear that has not been substantiated by evidence (again, check out the O-200s at flight schools across the country) seems the ultimate shortchange.

Don't you think every pilot should be able to put his/her aircraft down successfully without power?

If they rarely see the glide angle required to maintain the target airspeed, and rarely practice landing the airplane all the way to touchdown and rollout, then how would you expect a real engine out event to unfold?

 

[Ted]

Which is what I've been saying all along. Glad to know that an engine engineer from Lycoming agrees.

I've been referencing the Lycoming material saying as such (which nobody believes unless it coincides with what they want to believe, IME).

As an instructor, I should be willing to separate fact from opinion and opinion from mere advice.

But limiting power-off approaches or worse, avoiding them due to a vague warning of increased wear that has not been substantiated by evidence (again, check out the O-200s at flight schools across the country) seems the ultimate shortchange.

Who ever said anything about limiting or avoiding power-off approaches during training or for recurrent emergency procedure practice? I don't think anyone was suggesting that. I went flying with a friend of mine on Sunday for a confidence builder flight. I picked an inopportune moment to kill the engine on him, chopping it from a climb condition to idle. Yeah, that about surely created the conditions that I personally avoid in my plane (or any plane I'm flying), but that's a matter of training. My instructor certainly didn't hesitate to kill the engines in my Aztec on me (both sides) during training. We wouldn't be doing our jobs as instructors if we did otherwise and deprived our students of training.

Teaching students to be kind to their engines and what the manufacturer recommends for operation and treatment, regardless of whether or not you are of the opinion that it makes a difference, is also a good practice as an instructor.