Avoiding Shock Cooling in Quick Descent

once the gear comes down, your going to slow down in a few seconds. your also going
to start decending so you need to trim the airplane to how fast your wanting to
decend. If you have taken to much power off to slow down to gear speed then you
will need to be ready to add power once it comes down also.

You will get the hang of it after a few times.
BTW I also keep the auto extend off, most people I have talked to if theirs hasnt
been disabled, put it on manual so you can get the gear up at a slower speed on
take off and it dont fall out if you slow down to much. You will want to ask if
your auto extend has been disabled or if it still works, There is a service
bulletin on it I think.


"O. Sami Saydjari" wrote:

I read somewhere recently that making a habit of doing things like
dropping gear and flaps right at the edge of the allowed speed puts too
much stress on them...so I was trying to be conservative. Since this is
probably not a typical maneuver, I guess it makes sense to drop it right
at 129.

-Sami

Jeff wrote:
if you wait till you slow down to 115 to drop the gear, you better start way
way the heck out, its hard to get it slowed down that much with the gear up.
Especially if your up high.

If your to fast, the gear wont come down, your red unsafe light will come on.

once I hit 129 kts I pop the gear .. then you turn into a rock.

"O. Sami Saydjari" wrote:


VLE is 129 KIAS...Cruise is around 140-150 KIAS. To avoid stess, I
would probably not drop gear until I was about 115 KIAS. Still, it is a
good suggestion once I slow to that speed. Thanks.

Sami

Dan Luke wrote:

"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, Thanks for the pointer. Even more appropriate, since we are
talking about a Turbo aircraft, is the AVWeb article.
http://www.avweb.com/cgi-bin/udt/im....ry.id=1821 07

Very interesting article. One interesting thing about it is that the
guy actually presents hard data to back up his claims (at least for his
aircraft).

-Sami

Dan Luke wrote:
"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.

O.,

all the engine management columns by John Deakin are a must read. There
are many more at Avweb.

--
Thomas Borchert (EDDH)

Thomas,

Thanks for the pointers. The articles are indeed quite good, but there
is something I do not understand. It seems the whole theory on LOP
operation is conditioned on having GAMI injectors, because the article
implies that you can not do LOP operations on "normal" engines because
of uneven fuel distribution causes roughness due to different cylinders
outputing different powers when you start leaning past peak. Am I
missing something here? What percentage of planes actually have GAMI
injectors?

-Sami

Thomas Borchert wrote:
O.,

all the engine management columns by John Deakin are a must read. There
are many more at Avweb.

"O. Sami Saydjari" wrote:
Thanks for the pointers. The articles are indeed quite good, but there
is something I do not understand. It seems the whole theory on LOP
operation is conditioned on having GAMI injectors, because the article
implies that you can not do LOP operations on "normal" engines because
of uneven fuel distribution causes roughness due to different cylinders
outputing different powers when you start leaning past peak. Am I
missing something here? What percentage of planes actually have GAMI
injectors?

The 1997 Cessna 182S that is new to the club I am in flies LOP quite
easily. To the best of my knowledge, this is a stock engine (not GAMI
equipped). It is only advantagous to fly this way if you are flying LONG
legs. The corresponding drop in cruise airspeed is not suited to short,
fast trips.

"O. Sami Saydjari" wrote in message
...
Thomas,

Thanks for the pointers. The articles are indeed quite good, but there
is something I do not understand. It seems the whole theory on LOP
operation is conditioned on having GAMI injectors, because the article
implies that you can not do LOP operations on "normal" engines because
of uneven fuel distribution causes roughness due to different cylinders
outputing different powers when you start leaning past peak. Am I
missing something here?

No, you're not. You're seeing a good example of just how sloppy the typical
FI systems are.

What percentage of planes actually have GAMI
injectors?

GAMI has sold about 7,000 sets of their GAMIjectors, so figure that against
the entire fleet of FI engines; probably 5%. They cost less than $1000 a
set, but most pilots will spend much more for other toys, never realizing
the fuel cost savings they're missing and the damage they do to their
engines (note how many planes need a top overhaul well before TBO).

http://www.gami.com/gamijectors_order_form.html

I'll be buying an F33A in the next few weeks, and damn sure it will have
GAMI's installed before it goes anywhere.

http://www.gami.com/gamibrochure.html

(Also, I'm itching for a PRISM ignition system.)

http://www.gami.com/prism.html

HTH!!

"O. Sami Saydjari" wrote in news:100jq2b8dks3j44
@corp.supernews.com:

Thanks for the pointers. The articles are indeed quite good, but there
is something I do not understand. It seems the whole theory on LOP
operation is conditioned on having GAMI injectors, because the article
implies that you can not do LOP operations on "normal" engines because
of uneven fuel distribution causes roughness due to different cylinders
outputing different powers when you start leaning past peak. Am I
missing something here?

Not a lot. The article is not restricted to planes with GAMIjectors, but
it *is* relevant to engines that can operate with acceptable smoothness
LOP. That rules out most every carb'd engine (although sometimes you get
lucky), and most every big-bore TCM engine. Smaller engines and Lycoming
engines are a "sometimes can and sometimes can't" proposition.

Try it at 65% power and see what happens, especially if you have all-cyl.
monitoring. [I did once have an O-320 engine that would run absolutely
smooth all the way to idle-cutoff. Just a fluke.]



-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------

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)

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$) *
************************************************** ***********************

In a previous article, said:
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

Glider tows and jumper planes do that all day every day, and they don't
seem to get shortened TBO out of it.


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