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Engine clearing??

J

jwilbur

Well Known Member
In my experience, when flying in a 172 with instructors and they pull the power to simulate an engine out situation, they will often "clear the engine" by adding in a little power every few minutes. I never understood why they did this and never thought to ask.

Today I was doing constant airspeed tests to determine Vx and Vy and V_best_glide. I set the autopilot to hold whatever speed and went full power until reaching some altitude (I used 11,000' which worked out to 12,500 density). On descent, I would just slowly pull back the power and let the AP hold the same speed for a descent. (Download data and analyze later).

But I remembered all those instructors always "clearing the engine" and so I "cleared the engine" too every few minutes. Put in some power for 5 or 10 seconds and pull it back out.

So I ask here to this group. Is "clearing the engine" necessary and if so, why?

Thanks,
 
I would try to ?clear the engine? every thousand feet or so on simulated engine failures.

My motivation was solely to make sure the engine was still running, and that the simulated emergency had not morphed into a real one.
 
And if your engine is carbureted, that's a good way to check for possible carb ice, as well.
 
You do not want to shock cool the engine. Giving it short burst of power keeps the engine from cooling too much.
 
I ended up in field because I didn't clear the engine! I did a long high speed decent to an airport and didn't clear the engine. I was a little low on a close base, tried to add power and got nothing. I was at about 200 ft and didn't make the airport. Luckily I found a field and stuck in in without too much damage. We don't get many carb ice days here but this one of the worst. (NON-RV)

Brian
 
Well...

Use carb heat all the time when you pull the power back. I analyzed 551 RV-series accidents from the NTSB database, and there were a number of carb ice events, even though Lycomings aren't supposed to get carb ice. I had carb ice recently in the -9A in a cumulus cloud, and the carb heat worked right away. However, the RPM drop in the -9A with carb heat is slow and small, but on the -8, the RPM drop is non-existent. I'm not sure if the carb heat is worth anything on the -8.

Some engines, like the C-85, really do like to have the engine cleared on descent. Don't know the underlying mechanism there. A good friend, CFII/A&P, told me of having the engine almost stop in his Cessna 140 when the engine wasn't cleared.

Clearing the engine to prevent shock cooling is questionable, at best. Shock cooling is usually a threat to turbocharged engines that run really hot at high altitude cruise and are quickly cooled (thermal shock) by a low power, high speed descent. I try to be nice to my engines by reducing power a few inches at a time in a descent, even though I don't think that shock cooling would be much of an issue.

Here's a funny: a big aviation organization has an online course on engines and propellers, and they discuss shock cooling. The exemplar airplane they use on that slide is a (pause...) Luscombe. The inappropriateness of that airplane was pointed out to them ten years ago.
 
Ed_Wischmeyer said:
Well...
but on the -8, the RPM drop is non-existent. I'm not sure if the carb heat is worth anything on the -8.
Click to expand...

When I was building, I rode around with folks that have this..
http://vansaircraft.com/cgi-bin/sto...-132-609&browse=engines&product=carbheat-muff

The RPM drop was 0. Nobody could really verify if it worked or not.

Other suggested using this for carb heat..
http://www.vansaircraft.com/cgi-bin...30-15&browse=heatvent&product=cabin-heat-muff

Thats what we use. The results are a bit more like what you would expect ina cessna or piper setup. ie... you actually get an rpm drop and chts change as you would expect them to.
 
I can't speak to Ed's airplane, but my C85-powered airplane produces no RPM drop at run-up. I suspect this is the case for two reasons - one is that it's really not yet hot enough under the cowl to produce a large enough temperature rise in the incoming airflow, and the other is I don't think the engine is sucking enough air at run-up RPM to accurately reflect the effects of carb heat.

I've seen the same symptomology in our new O-360-equipped aircraft. So far only limited ground runs have been conducted, with CHT's being kept at very conservative levels in accordance with Mahlon Russell's engine break-in guidance. The lack of RPM drop in this new aircraft/engine seems to back up the two theoretical causes mentioned above.

As for engine clearing, I have two small anecdotes.
1) student pilot cleared an O-200 in a C-150, only to have it backfire and blow the carb off the engine. Besides the obvious need for a change of shorts after the very successful off-field landing, there were a lot of "how the heck did THAT happen?" conversations!

2) student pilot in O-320-powered C-172 failed to clear the engine on a simulated engine failure on downwind leg. It was one of those really muggy days when most of us would never think of carb ice. The engine quit when he found himself too low to make the field and tried to add a shot of power. "Airport Road" was where we found him, white as a sheet and shaking like a leaf, but otherwise none the worse for wear!
 
I have an O-360 RV-8, standard airbox per the plans, which uses ambient (warmer) cowling air for carb heat.

The Dynon carb air sensor only shows a gain of 1-2 degrees F when the carb heat is applied, which is why I'm here on this thread!

It's a really dry climate here in Utah and I only fly on nice days so I'm not too concerned, but I would like to find a better solution. I'm thinking of fab'ing a hose from the cabin air inlet to the airbox, or making a Y fitting from the cabin heat hose to the airbox.

TBD...
 
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