Full Rich for landing?

[Wagondriver]

That's unusual. Do you have a MP gauge? It sounds like the carb throat isn't big enough for the engine, so at full throttle the restriction is creating too much vacuum. An MP gauge would show how much pressure drop you have at full throttle.

MP pressure gauge, uhhh, yes. Its a 520.

Not actually unusual at all. Very many 470's and 520's build ice in those situations.

 

[Stewartb]

I have had carb ice on downwind once, and numerous times shortly after takeoff. I also have a JPI 830 with carb temp. It is interesting to watch the temperature, with the throttle closed all the way, carb temp goes way up, like up to 60 or 70 degrees on days with ambient in the 20's or so. With carb heat on as suggested, with closed throttle and a warm engine, carb temp will get over 100, I have seen 140. Full throttle on a zero degree day with just about any humidity will create ice after takeoff. Full throttle dumps so much fuel, enough it seems to really help cool off and create ice. This last winter was the first winter after having my carb flow checked and a larger jet installed, it nearly shut off on me twice just after takeoff. I use carb heat on takeoff in those situations now, and also have tried using only about 3/4 throttle.
I pretty much never run carb heat, UNLESS, there is high humidity or visible moisture AND carb temp is less than 38. I may pull some carb heat now and then on decent, just to see if there is much exhaust heat to work with.

There’s almost no humidity in the air at 0°. There’s likely to be moisture in the fuel, though. I always have isopropyl in my fuel in winter. Not for carb ice, but to assure the fuel screens flow and don’t get restricted by slush. Going from a warm hangar to cold outside air makes things worse, especially until the fuel in the wings chills to ambient.

 

[MRC01]

MP pressure gauge, uhhh, yes. Its a 520.
Not actually unusual at all. Very many 470's and 520's build ice in those situations.

What's a typical difference between MP and ambient pressure at full throttle in these engines? Is it a greater difference than other engines that are less likely to make ice at full throttle?

 

[Stewartb]

The difference is the volume of air feeding 520 cubic inches. I have the same engine. Carb ice occurred one time on a low power descent to get below clouds on a scuzzy day coming out of a mountain pass. Ice was no surprise in that scenario. Once in 25 years ain’t bad. I know guys with similar airplanes that manage carb heat to prevent ice on most flights.

 

[MRC01]

The difference is the volume of air feeding 520 cubic inches. I have the same engine. Carb ice occurred one time on a low power descent to get below clouds on a scuzzy day coming out of a mountain pass. Ice was no surprise in that scenario. Once in 25 years ain’t bad. I know guys with similar airplanes that manage carb heat to prevent ice on most flights.

Of course, that makes sense. To clarify my question, here's an example:
Engine A at WOT has MP that is 1" less than ambient.
Engine B at WOT has MP that is 3" less than ambient.
All else equal, engine B is more prone to carb icing at full throttle. Its full throttle is less full, more restricted, which means more vacuum and lower temps.

 

[Stewartb]

Makes sense. I’ve never had carb ice in any airplane at takeoff or initial climb. Only low power descents.

 

[MRC01]

Makes sense. I’ve never had carb ice in any airplane at takeoff or initial climb. Only low power descents.

Neither have I, except you can ice up during ground ops too, especially if mixture is full rich. Yet I've spoken with pilots who have encountered carb ice at full throttle.

After reading the Lycoming operating manuals for the common O-320 and O-360 engines, I have a simple rule for engine operation: never use full rich except when engine is above 70% power. If you are below 70% power, lean it. No matter whether you are on the ground or in the air, no matter your altitude. Using full rich mixtures below 70% power is unnecessary and only wastes fuel, increases risk of carb ice, fouling plugs, and gunking up the valve stems.

YMMV: different engines, different airframes, different procedures.

 

[Wagondriver]

Of course, that makes sense. To clarify my question, here's an example:
Engine A at WOT has MP that is 1" less than ambient.
Engine B at WOT has MP that is 3" less than ambient.
All else equal, engine B is more prone to carb icing at full throttle. Its full throttle is less full, more restricted, which means more vacuum and lower temps.

I've never noticed a significant difference between what the MP gauge says, and what atmospheric conditions would indicate.

It is NOT uncommon on 470's and 520's to make ice on take off, even in fairly "dry" climates.

 

[MRC01]

... It is NOT uncommon on 470's and 520's to make ice on take off, even in fairly "dry" climates.

There must be reasons why and I'm curious what they are. Is it intake restriction? Routing? Carbs set extra-rich? Something about the manifold design?

 

[Dan Thomas]

There must be reasons why and I'm curious what they are. Is it intake restriction? Routing? Carbs set extra-rich? Something about the manifold design?

The carbs are not mounted to the engine itself as Lycoming does. They're on the induction system and therefore mechanically insulated from that hot engine. Heat isn't conducted into them so much.

And it's only the carbed engines. The injected engines very rarely get any ice. There is no evaporative cooling in the throttle body, and no venturi in the Continental injection system.

 

[Stewartb]

There must be reasons why and I'm curious what they are. Is it intake restriction? Routing? Carbs set extra-rich? Something about the manifold design?

My question is why some planes make ice and other identical planes don’t. My 0-520 doesn’t make ice. My old Cub’s 0-320 didn’t, either. I know guys with similar airplanes to both who report ice is common, and in the same climate.

I’ve flown my own C-180 with three different engines. No ice issues. A guy I know with the same year, same engine history has ice all the time. The only difference we can identify is the pilot.

 

[Magman]

Since the focus seems to have shifted to Carb Heat I’ll throw in a few comments.

Carb Heat provides air that is less dense. This has the effect of richening the mixture and can be useful in diagnosis.

Carb Heat and Alternate Alternate Air is taken from a Sheltered Source. Thus; it should

not be subject to Impact Icing. Many aircraft have the Inlet well inside the cowling .

The Cessna 150 though, has the Inlet forward of #3 cylinder, which is the right side of the

engine . The Inlet is on a flat baffle and is protected by a “Lip” which is the

Lower Cowl Nosebowl. Adjacent to it is the Cabin Heat Inlet which has a Scoop for

maximum air intake. They are often found with ducting swapped which would be

dangerous if ice is encountered.


Since Carb Heat does not have a Filter it’s obvious to leave OFF in dirty environments.

However; some aircraft ( C-150 in particular) will accumulate crud even when parked.

It’s worthwhile cleaning the airbox and possibly the ducting periodically. They are your

cylinders though.

Low points in Inlet Ducting tend to accumulate water if exposed to the elements.

This can cause the wire inside the ducting to rust and fail.

This can become apparent at higher engine speeds as when activating Carb Heat on

downwind. The water turns to steam and the collapsing duct causes a Large drop in RPM.

Gets your attention! Piercing a small hole in the Inlet Ducting low point will prevent accumulation.

 

[MRC01]

My question is why some planes make ice and other identical planes don’t.
...
I’ve flown my own C-180 with three different engines. No ice issues. A guy I know with the same year, same engine history has ice all the time. The only difference we can identify is the pilot.

As mentioned above, how prone the engine is to carb ice depends on both its design, and operating procedures.
Some designs are more prone to ice, mainly due to the location of intake and manifolds, how warm they get, etc.
Some operating procedures are more likely to ice up, for example full rich mixtures at part throttle (where full rich is never needed).
So, it makes sense that some pilots encounter carb ice more often than others, even with the same airplane.

 

[MRC01]

... Carb Heat provides air that is less dense. This has the effect of richening the mixture and can be useful in diagnosis. ...

Another tip: after leaning the engine for cruise flight, apply the carb heat for a moment and see how much RPM drop you get. If the engine is at or rich of peak power/RPM/MP, you'll get a bigger drop than if the engine is lean of peak. Some engines (particularly on Lycomings which have closer to equal length intake manifold runners) can run lean enough in cruise that you get little or no RPM drop when applying carb heat.