Engine out practice

"Matt Whiting" wrote

The issue with shock cooling isn't the rate of cooling per se, but rather
stress induced by differential cooling. Most engines see far higher
temperature differentials during start-up than they do during cooldown.
Jay, have you timed your engine heat up rate? It would be interesting to
watch how fast your engine heats up from say a 50 degree cold start and
then compare that to the cool-down rate when you pull the throttle for
engine out practice. I'm assuming this would be fairly trivial with your
engine analyzer.

I'm pretty sure that the rate of heating of the metal of the head is not
the big issue, according to the shock cooling proponents.

Instead, it is the heads (and cylinders) cooling more rapidly than the
pistons, and the hot pistons (not able to be cooled as rapidly) against the
cooler cylinders (the cylinders do not have heat instrumentation, so the
closest thing they can do is to measure is the head temperatures) causing a
reduction in the tolerances between the piston and the cylinder walls, thus
causing possible scuffing and abnormal wear.

At least that is my take on what they say.
--
Jim in NC

"Morgans" wrote

At least that is my take on what they say.

I should have added that the cylinders heating up faster, on start up,
than the pistons would increase the tolerances, and thus cause no scuffing.
--
Jim in NC

Morgans wrote:
"Morgans" wrote

At least that is my take on what they say.

I should have added that the cylinders heating up faster, on start up,
than the pistons would increase the tolerances, and thus cause no scuffing.

Why would the cylinders heat up faster? Only the very top of the
cylinder is in constant contact with the combustion heat whereas the top
of the piston is entirely in contact. As you go down the cylinder away
from the head, the cylinder spends less and less time in contact with
the combustion heat and thus will be cooler.

Matt

"Matt Whiting" wrote

Why would the cylinders heat up faster? Only the very top of the cylinder
is in constant contact with the combustion heat whereas the top of the
piston is entirely in contact. As you go down the cylinder away from the
head, the cylinder spends less and less time in contact with the
combustion heat and thus will be cooler.

Perhaps they would not. I was going simply on the converse.

My thought is that the cold oil shooting on the piston, and a relatively
weak combustion would keep the piston cooler.
--
Jim in NC

Morgans wrote:
"Matt Whiting" wrote

The issue with shock cooling isn't the rate of cooling per se, but rather
stress induced by differential cooling. Most engines see far higher
temperature differentials during start-up than they do during cooldown.
Jay, have you timed your engine heat up rate? It would be interesting to
watch how fast your engine heats up from say a 50 degree cold start and
then compare that to the cool-down rate when you pull the throttle for
engine out practice. I'm assuming this would be fairly trivial with your
engine analyzer.

I'm pretty sure that the rate of heating of the metal of the head is not
the big issue, according to the shock cooling proponents.

Instead, it is the heads (and cylinders) cooling more rapidly than the
pistons, and the hot pistons (not able to be cooled as rapidly) against the
cooler cylinders (the cylinders do not have heat instrumentation, so the
closest thing they can do is to measure is the head temperatures) causing a
reduction in the tolerances between the piston and the cylinder walls, thus
causing possible scuffing and abnormal wear.

At least that is my take on what they say.

It seems to me that upon engine start the pistons would heat up much
faster than the cylinders causing the same net affect as cooling down
the cylinders faster once hot. Either way the pistons are hotter than
the cylinders.


Matt

On Oct 13, 8:55 pm, Matt Whiting wrote:

It seems to me that upon engine start the pistons would heat up much
faster than the cylinders causing the same net affect as cooling down
the cylinders faster once hot. Either way the pistons are hotter than
the cylinders.

Matt

At idle or low power settings there is little heat generated. So
little, in fact, that it can take forever to get the CHT warm enough
to carry out the runup when the temps here are -15 or 20°C. The
cylinder has plenty of time to warm up. It's the sudden removal of the
heat source when the atmosphere is really cold that problems might
arise. In Canada we have to think about it a little more than the
pilot in Arizona. Pistons are aluminum and expand at twice the rate of
the steel cylinders, clearances get small during operational temps,
and shrinking a cylinder quickly around a hot piston is asking for
scuffing or seizure.
We run six Lycs in flight training ops. They usually reach TBO
in good condition. They get a lot of rapid throttle movement, even
though I constantly make noises about not abusing the engines. In my
opinion, opening the throttle too fast can do more damage than closing
it too quickly. Cylinder pressures can get high enough with rapid
throttle movement to cause detonation, however briefly, and cracking
of various parts might occur. A pilot who bangs the throttle open is
applying high manifold pressures to an engine at very low RPM, the
definitive extreme oversquare situation.
Closing it quickly in flight will cause afterfiring (lean
mixtures that often don't fire in the cylinder, igniting instead in
the hot muffler). Cracking of exhaust components is a risk there, and
we find that often enough.
Our students get plenty of forced-approach practice. The engine
is throttled back in two or three or four seconds. Transport Canada
tells us that some practice forced landings (PFLs) end in the real
thing when the carb ices up during the glide. The syllabus calls for
an application of power for a few seconds every 1000' of altitude loss
to clear the engine, but since the exhaust system is cool in the
glide, it can take much more than a few seconds to clear any ice
accretion and the engine might not respond when necessary.
For those lucky ones with injection, carb ice is not a problem,
but most of us are stuck with carbs and need to be thinking, when we
check the weather before the flight, about what the atmosphere is up
to. We wouldn't dive into unknown waters without making sure there
weren't hidden rocks or sharks around, and we shouldn't launch without
knowing the temp and dewpoint spread, right?

Dan

On 2007-10-13 06:57:56 -0700, Jay Honeck said:

As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.

Most flight schools practice engine out emergencies frequently, even on
high performance aircraft. The engines typically make it to TBO.

The things that seem to shorten engine life have little to do with
shock cooling. The biggest factor seems to be how much the engine is
used.

If the engine is flown in accordance with the manual, you should not
have any problems with shock cooling.



--
Waddling Eagle
World Famous Flight Instructor

On Oct 13, 8:57 am, Jay Honeck wrote:
As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.

I eventually agreed that gradual power changes would not unduly harm
an air-cooled engine, and vowed that I would endeavor to practice this
most-important skill on our next flight. And we did.

We were on a flight back from Galesburg, IL when I started the
procedure, and very gradually began a power reduction whilst in cruise
flight at 3500 feet. I took a full minute to reduce the power to
idle, watching our (newly reinstalled) JPI EDM-700 engine analyzer for
signs of stress.

As RPMs dropped below 1000, the "shock-cooling alarm" suddenly went
off, flashing its dire warnings that EGTs had dropped beyond (and
faster) than recommended limits. (I can't remember what the threshold
is for that alarm -- it's preset.)

This despite my most careful power reduction, which (obviously) wasn't
slow enough.

Since the damage (so to speak) had already been done, I continued the
descent toward an Illinois corn field. With the harvest under way, I
had my choice of:

- Freshly harvested corn stubble, not plowed
- Freshly harvested crops, plowed dirt
- Unharvested corn or winter wheat

I opted for the corn stubble, as the stalks would hold the soil
together firmly and not present as much "flip force" to the landing
gear as the plowed or unharvested field. I took it down to 200 AGL
before applying power and heading home, satisfied that we would have
survived and giving the farmer a nice show.

For you aircraft owners who do this regularly, how slowly do you
retard the throttle to prevent shock cooling? (I know -- does shock
cooling really exist? For purposes of this discussion, I'll pretend
that it does.)

Given that the power reduction must be incredibly gradual, do you feel
that this exercise is realistic? There really is no chance to
simulate how you must "suddenly" find best glide speed (after your
engine has presumably just crapped out), since you're gradually
reducing your speed along with your power. Or do you put the plane
into a shallow dive as you reduce power, so as not to lose airspeed?

It's funny -- as renters we practiced this all the time. Now, after 9
years of ownership, we haven't practiced it in ages -- and didn't even
realize this lack until Shirl's comments in Paul's thread. Another
good thing about "belonging" to this newsgroup...

Thoughts?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993www.AlexisParkInn.com
"Your Aviation Destination"

I'm not an expert, but it seems to me that the important thing is to
reduce airspeed immediately when reducing power. It's the wind
whistling through an idling engine at 140 knots that's going to do
some serious shock cooling.
--
Gene Seibel
Tales of Flight - http://pad39a.com/gene/tales.html
Because I fly, I envy no one.

Gene Seibel wrote:

I'm not an expert, but it seems to me that the important thing is to
reduce airspeed immediately when reducing power. It's the wind
whistling through an idling engine at 140 knots that's going to do
some serious shock cooling.
--
Gene Seibel
Tales of Flight - http://pad39a.com/gene/tales.html
Because I fly, I envy no one.

Which is what you are supposed to do anyway in an engine out; immediately
reduce airspeed to best glide.

--
Jim Pennino

Remove .spam.sux to reply.

A slow reduction in power setting certainly isn't realistic. I've had a few
power failures in my time (all in twins) and in every case the loss was
fairly sudden. I've never run out of fuel, but I have run a tank almost dry
on purpose, and the associated coughing and sputtering gave me plenty of
warning.

Bob Gardner

"Jay Honeck" wrote in message
ups.com...
As previously noted (in the thread about Paul's wife getting scared),
Mary and I had virtually stopped doing this kind of flying for fear of
harming our (very expensive) engine. A lively debate ensued as to
whether or not repeated high-to-low-to-high power applications would
wear out your engine any faster than would normal operations.

I eventually agreed that gradual power changes would not unduly harm
an air-cooled engine, and vowed that I would endeavor to practice this
most-important skill on our next flight. And we did.

We were on a flight back from Galesburg, IL when I started the
procedure, and very gradually began a power reduction whilst in cruise
flight at 3500 feet. I took a full minute to reduce the power to
idle, watching our (newly reinstalled) JPI EDM-700 engine analyzer for
signs of stress.

As RPMs dropped below 1000, the "shock-cooling alarm" suddenly went
off, flashing its dire warnings that EGTs had dropped beyond (and
faster) than recommended limits. (I can't remember what the threshold
is for that alarm -- it's preset.)

This despite my most careful power reduction, which (obviously) wasn't
slow enough.

Since the damage (so to speak) had already been done, I continued the
descent toward an Illinois corn field. With the harvest under way, I
had my choice of:

- Freshly harvested corn stubble, not plowed
- Freshly harvested crops, plowed dirt
- Unharvested corn or winter wheat

I opted for the corn stubble, as the stalks would hold the soil
together firmly and not present as much "flip force" to the landing
gear as the plowed or unharvested field. I took it down to 200 AGL
before applying power and heading home, satisfied that we would have
survived and giving the farmer a nice show.

For you aircraft owners who do this regularly, how slowly do you
retard the throttle to prevent shock cooling? (I know -- does shock
cooling really exist? For purposes of this discussion, I'll pretend
that it does.)

Given that the power reduction must be incredibly gradual, do you feel
that this exercise is realistic? There really is no chance to
simulate how you must "suddenly" find best glide speed (after your
engine has presumably just crapped out), since you're gradually
reducing your speed along with your power. Or do you put the plane
into a shallow dive as you reduce power, so as not to lose airspeed?

It's funny -- as renters we practiced this all the time. Now, after 9
years of ownership, we haven't practiced it in ages -- and didn't even
realize this lack until Shirl's comments in Paul's thread. Another
good thing about "belonging" to this newsgroup...

Thoughts?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"