Shock Cooling

[SkyHawk-N]

Bit of a techi question I'm afraid.

Since having my EDM-700 fitted I've been spending a fair bit of time looking at shock cooling warnings when descending into the circuit. Even while keeping some power on during the descent it is very difficult to maintain a less than recommended 50 degree CLD (it's a Lycoming engine), usually a figure of around 60 or 70 is the norm. My question is how long does it take shock cooling to damage an engine? Is 30 seconds long enough to have an affect or is this something which is only important during lengthy descents greater than a few thousand feet?

I think this has been discussed before during a thread on a different subject but would like peoples views on this specific subject.

[IO540]

Shock cooling has been done to death in every pilot forum, airport bar, aviation magazine, Usenet, engine manufacturer's literature... :)

And nobody is any wiser.

This is because nobody has put a load of engines on test rigs, shock cooled them repeatedly, and then checked them for cylinder cracks. Then repeated the experiment for different engines, different CHTs, different cylinders, etc etc etc

The best data I have seen to date is an article written by somebody who was dropping paras or pulling gilders or something like that, who used to get through a lot of cylinders, and then changed procedures to avoid reducing power until the CHT was below a certain figure. I have the article.

And this is probably the answer. If your CHT is 500F (which is itself way OTT) and then you cool by 100F, that is far worse structurally than the same temperature drop if you are starting with a CHT of 300F.

And on a typical landing approach your CHTs are low by the time you are shutting off.

The trap is on an IFR arrival when they have held you fast and high and then force you into a -2000fpm descent. It can still be managed if you know what you are doing, and think ahead a bit.

[SkyHawk-N]

Shock cooling has been done to death in every pilot forum, airport bar, aviation magazine, Usenet, engine manufacturer's literature
Oh ok, I won't ask then ;)

I've been reading up on this for a while now but, as you say, I can't find any definative information. I just want to know the best action to carry out when the EDM starts flashing at me. Ignore it? keep power on to help stem the cooling? use a shallower descent?

Where did Lycoming get the -50 figure from? Why does the EDM display the CLD so prominently? Is it a gimic? Too many questions.

With regard to glider tugs, I believe our pilots carry out a high power descent and approach to be kinder to the pistons.

[stickandrudderman]

A semi-educated guess on my part, and forgive me if I'm talking twaddle, but if you use crossed controls to increase the ROD whilst minimising airspeed, does this help? :confused:
(targeted to the general reader)

[mm_flynn]

A range of things you could do

1 You may want to start your descent earlier so you don't have to pull as much power off.
2 try leaning to peak after you pull power back to keep things warm.
3 make sure your CHT are reasonably low (less than 350) before you start down
4 remember going down is where you get to make up the time you lost going up (if its smooth come down in the yellow arc with some power on)

Finally, as long as you are running cool there doesn't seem to be any conclusive evidence that shock cooling exists (or doesn't).

[IO540]

I would say that if you are on final and the EDM is flashing then all you can do is ignore it. You've got to land somehow...

The EDM is looking for a specific CHT drop rate. It doesn't care whether you are dropping from 450F to 400F, or from 350F to 300F. But in terms of structural strength of aluminium there is a big difference between those two.

As to how one can manage the engine, that's a separate issue.

One can enrich the mixture gradually before the anticipated descent, to cool the engine, and when you start the plummet you reduce power (to avoid busting Vne, to stay below Va if in IMC, whatever) and lean to peak EGT and that reduces the cooling rate.

Or one can just think ahead and reduce power before the descent to say 18"; after a couple of minutes that gives you a low enough CHT to not worry, and you can descend at that power setting, gradually reducing power further as you go down, 1" of MP every 20 secs or so.

You can play with this next time you go up. It's quite easy - I rarely get the warnings myself. Just thinking ahead and reducing power nice and slow will do the job.

Lycoming may have data on all this but if they have (unlikely) they won't be revealing it because this whole business is wrapped up in liability and litigation. Loads of people get cracked cylinders, in many cases for no apparent reason (not stupid pilots, know about engine management, etc) and loads of them now have EDMs and keep the data all the way back. Lyco can and do wash their hands of warranty as things stand but could easily make a rod for their own back by coming out with clear guidelines, because many people could prove they never exceeded them. Also if they came out with anything, people would force them to reveal when this data was discovered and why didn't it come out earlier, etc etc. Lycoming do get sued very frequently, and if you actually take legal action they do settle fast (I've known people who got whole new engines, and even on SB569 people have got Lyco to pay for engine removal) but they get you to sign an NDA so you can't talk about it. This is how things are done in aviation - you stonewall and whitewash, and then settle fast if the punter does what he threatens (most don't).

[moonym20]

The key is effective energy management, shock cooling an engine will not pose a risk if you plan ahead with effective energy management. If i remember rightly in most Colombia aircraft the POH reccomends that you retard the throttle at no more then 2"MP per minute to prevent shock cooling. that to me seems like a long time to retard your throttle from the curise setting to a decent setting. The trick is to simply plan ahead and be smooth with your throttle :ok: adjusting your mixture to suit your decent will also help to keep the engine nice and warm. You probably already know that with engines fitted with an MP gauge you really want to keep the MP in the green band at all times unless on finals to keep things warm

[SkyHawk-N]

That's if you have MP.

IO540 Thanks for the article. The worrying bit was the sentence which mentions the greatest damage is done during the first few seconds of shock cooling.

It seems the only thing to be done is to read ahead, descend earlier and more gradually and use leaning to keep the engine warm. Otherwise do the best you can and ignore the flashing EDM.

[mm_flynn]

It would be interesting to know a bit more of what flight profile is causing the alarm. My experience flying properly instrumented aircraft is limited to only two types so don't have general knowledge. However, in the two machines in question, with a bit of engine management you can get them to drop like a brick and still not get the shock cooling alert.

Something details like
Type -
Typical cruise - CHT, %power or rpm, EGT vs peak (or fuel flow), Speed
Descent - %power or RPM, EGT vs peak (or fuel flow), speed, descent rate, CHT cooling rate

[NutLoose]

see


http://www.lycoming.textron.com/cgi-bin/search.pl?query=shock+cooling