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Avoiding Shock Cooling in Quick Descent



 
 
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  #1  
Old January 7th 04, 07:03 AM
O. Sami Saydjari
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Default Avoiding Shock Cooling in Quick Descent

My engine operating manual (for my Piper Turbo Arrow III) strongly
discourages pulling the power back and doing a quick descent -- it warns
of engine-killing shock cooling. Sounds reasonable to me...but it (and
my airplane manual) does not really seem to say how best to do a fast
descent when you have to.

I inferred that the right thing to do might be to lower the prop speed
to a minimum and ease back power as slowly as you can. Does that sound
about right? How quickly can one expect to pull the throttle back and
not risk shock cooling? If one must get down (say, for air traffic
control reasons, or perhaps because one is trying to take advantage of
favorable winds as long as possible), what is the best procedure. What
about slipping it down? Does that risk the engine or the airframe at
all? I've never done slips at cruise speeds (just on approach), so
please forgive me if this is a naive question.

-Sami
N2057M
Piper Turbo Arrow III

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  #2  
Old January 7th 04, 09:30 AM
Jeff
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Default

I read an article that said that if you dont go below 20' MP pressure you
should be ok.
Personally, I reduce power enough where I can do a 500 fpm decent and stay
under the yellow, usually this puts me at about 140 kts IAS, but only if its
smooth air, if I need to stay below Va or if they are trying to slam dunk me
then I drop the landing gear, that and about 25' MP and 2300 rom will give
you around a 800 fpm decent, sometimes faster if you let it. But I try my
best to only do 500 fpm decents.

I was taught to pull power off at about 1' MP per minute.

BTW how is that new T-arrow of yours doing.

"O. Sami Saydjari" wrote:

My engine operating manual (for my Piper Turbo Arrow III) strongly
discourages pulling the power back and doing a quick descent -- it warns
of engine-killing shock cooling. Sounds reasonable to me...but it (and
my airplane manual) does not really seem to say how best to do a fast
descent when you have to.

I inferred that the right thing to do might be to lower the prop speed
to a minimum and ease back power as slowly as you can. Does that sound
about right? How quickly can one expect to pull the throttle back and
not risk shock cooling? If one must get down (say, for air traffic
control reasons, or perhaps because one is trying to take advantage of
favorable winds as long as possible), what is the best procedure. What
about slipping it down? Does that risk the engine or the airframe at
all? I've never done slips at cruise speeds (just on approach), so
please forgive me if this is a naive question.

-Sami
N2057M
Piper Turbo Arrow III


  #3  
Old January 7th 04, 01:24 PM
Dan Luke
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Default

"O. Sami Saydjari" wrote:
I inferred that the right thing to do might be to lower the prop speed
to a minimum and ease back power as slowly as you can. Does that

sound
about right? How quickly can one expect to pull the throttle back and
not risk shock cooling? If one must get down (say, for air traffic
control reasons, or perhaps because one is trying to take advantage of
favorable winds as long as possible), what is the best procedure.

What
about slipping it down? Does that risk the engine or the airframe at
all? I've never done slips at cruise speeds (just on approach), so
please forgive me if this is a naive question.


What is the V-le for your airplane? If it's high enough, drop the gear
and use them as speed brakes to get down while leaving some power on to
keep the engine warm. Slipping is fine.

There is considerable debate about the danger of shock cooling. Google
these groups or see http://www.avweb.com/news/columns/183094-1.html for
more discussion.
--
Dan
C172RG at BFM
(remove pants to reply by email)


  #4  
Old January 7th 04, 01:37 PM
Thomas Borchert
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Default

O.,

it warns
of engine-killing shock cooling. Sounds reasonable to me


it does? Hmm. How about shock heating on take-off?

Why not lean to max EGT on descents?

--
Thomas Borchert (EDDH)

  #5  
Old January 7th 04, 02:22 PM
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O. Sami Saydjari wrote:
: My engine operating manual (for my Piper Turbo Arrow III) strongly
: discourages pulling the power back and doing a quick descent -- it warns
: of engine-killing shock cooling. Sounds reasonable to me...but it (and
: my airplane manual) does not really seem to say how best to do a fast
: descent when you have to.

: I inferred that the right thing to do might be to lower the prop speed
: to a minimum and ease back power as slowly as you can. Does that sound
: about right? How quickly can one expect to pull the throttle back and
: not risk shock cooling?

From all I've read on it, the shock-cooling thing is probably mostly a bunch
of crap. Unless you do radical configuration changes (e.g. 8000' climb at Vy then kill
the engine and glide back down), you probably won't exceed the Lycoming recommended CHT
change rate of 50 degrees per minute. EGT only affects CHT as a secondary effect....
most of the CHT results from the combination of power produced and cooling.
Something akin to (MP x RPM)/IAS.

I pretty much figure from a cruise, you can either point the nose down and
speed up, or reduce the engine power, but shouldn't do a lot of both. For a relatively
rapid cruise descent of 500 fpm, this seems to keep the CHT from moving more than 50
degrees per minute. Either pull 4-5" MP, nose over another 10-20 kt, or maybe a bit of
both (2-3" and 5-10 kt).

The big one (I believe) is keeping the mixture at cruise lean. Since the power
is reduced, you can't hurt the engine with it. Keep cruise lean until it coughs on the
way down, then fatten as necessary.

Now with my flame-suit properly donned, what does everyone else think?

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

  #6  
Old January 7th 04, 03:28 PM
Tom Sixkiller
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Default


"O. Sami Saydjari" wrote in message
...
My engine operating manual (for my Piper Turbo Arrow III) strongly
discourages pulling the power back and doing a quick descent -- it warns
of engine-killing shock cooling. Sounds reasonable to me...but it (and
my airplane manual) does not really seem to say how best to do a fast
descent when you have to.

I inferred that the right thing to do might be to lower the prop speed
to a minimum and ease back power as slowly as you can. Does that sound
about right? How quickly can one expect to pull the throttle back and
not risk shock cooling? If one must get down (say, for air traffic
control reasons, or perhaps because one is trying to take advantage of
favorable winds as long as possible), what is the best procedure. What
about slipping it down? Does that risk the engine or the airframe at
all? I've never done slips at cruise speeds (just on approach), so
please forgive me if this is a naive question.

-Sami
N2057M
Piper Turbo Arrow III


Pretty much a myth or OWT: http://www.avweb.com/news/columns/182107-1.html


  #8  
Old January 7th 04, 08:27 PM
Jeff
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Default

Dale
does your decent method work well for not shock cooling the engine?

Dale wrote:

In article ,



I fly turbo-charged 206 hauling skydivers so I make lots of descents.
The drill we use is to reach jump altitude at least a minute before the
jumpers exit to give us time to ease the power back and start cooling
the engine. We're at 13000 and pulling 30 inches with 2600RPM. I
reduce MAP to 27 inches (about 1" every 20-30 seconds or so), then
slowly reduce RPM (which will also cause a reduction in MP) until
getting 2100 RPM set. Then it's a further throttle reduction to 17-18"
MP where I lean for 6-8 GPH (setting up for the descent). If I timed it
right it's time to open the door, reduce power to 10-11" MP and lets the
folks out..as soon as they're gone close the door, power back up to 18"
and maintain the 18" throughout the descent, monitoring fuel-flow to
keep EGT up while maintaining top of the green or even into the yellow.
This gives descent rates of 2500-3500fpm...sometimes a little more if
you want to work at it.

We've been flying the turbo 206 a couple of years now with no engine
problems...our other 206 has been operated for about 6000 hours as a
jump plane and the engine goes to TBO or beyond.

Your idea of reducing RPM is the way to go if you need a rapid descent.
It allows you to keep the engine working to maintain some heat. Your
descents will be made from cruise flight and the engine will not be as
hot as after just making a best rate climb to 13K so there will be less
change in engine temp during the descent. For a cruise descent I'd just
push the nose over for the descent needed and reduce power to maintain a
safe airspeed. ALL power changes should be made smoothly.

Thermal cycles are certainly stressful for an engine but frankly, IMO
shock-cooling is BS. Much more damage is done by not allowing the
engine to come up to temp before applying power.....thermal stress
happens both ways. G

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html


  #9  
Old January 7th 04, 09:17 PM
Dale
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Default

In article ,
Jeff wrote:

Dale
does your decent method work well for not shock cooling the engine?


G Uh, yeah. That's why we get TBO or better. Of course I guess it
depends on your definition of shock-cooling.

--
Dale L. Falk

There is nothing - absolutely nothing - half so much worth doing
as simply messing around with airplanes.

http://home.gci.net/~sncdfalk/flying.html
 




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