Carb Icing Cessna 150

[bobkiksass]

Hello, Im new to the forum.

I am student pilot with my own '76 cessna 150 which I was flying with my instructor from Santa Barbara, CA to Santa Monica today. On our return flight, we were at 4,500 ft cruising when we suddenly noticed engine roughness. My aircraft is equipped with a carb/outside are temp gauge. When the roughness started it was 94 in the carb, 78 outside. The relative humidity in Santa Monica says it was 78%. At the time, we were about 3,000 ft over a fog bank that was moving on shore.

We quickly applied carb heat, which seemed to cause the motor to go back to running smooth within a few seconds. My instructor is fairly confident that it was carb icing, but he said he has only ever experienced that once before, and has over 7,000 hrs. I am concerned that there may have been something else causing the motor to run rough.

Do the described conditions sound like they could be responsible for carb icing?

My aircraft has a penn yann motor with only 300 hrs, and the annual was done 15hrs ago. I am fairly confident with the motor, but this situation was definitely a bit hair raising.

Also, what type of oil temperatures do Cessna 150 pilots usually get when cruising. The oil temp was in the green for our whole flight, but it seemed to be hovering pretty close to the red line. Id say it was about 85% of its range , getting close to the red.

Thanks for your advice.

 

[Skylane81E]

Sounds like carb ice, get used to it on a 150!

 

[COFlyBoy]

Looks like the conditions were conducive to carb ice.

 

[Skip Miller]

I found a web site that calculates the dewpoint at various temps and RHs. It said your dewpoint was 71*, and that is very wet air. This makes the engine stumble almost certainly due to icing.

Also, attached is a Carb Ice Probability chart, which will vary somewhat from plane to plane and engine to engine. It says that under your conditions, serious icing at glide power could be expected. Particularly in a plane that is known to make ice.

...and Welcome to POA!

-Skip

 

[Pilawt]

Don't be bashful in a C-150 about pulling on the carb heat to check for ice, even if you just have a hunch that you think there might be a possibility of a suspicion of ice. :wink2: If after the initial rpm reduction from pulling the knob, the rpm starts creeping up again a little, then there was probably some in there.

I've had carb ice in a 150 both in the pattern and at high-speed (for a 150 ) cruise. Don't rule it out in any case.

 

[bobkiksass]

Thanks for all the responses. This is good information to know, applying carb heat proactively is a good idea. Any advice on oil temperatures you are used to seeing in Cessna 150's , is it normal to have the temperature up high in the gauge (85% of the green arc) during a cruise ?

 

[Let'sgoflying!]

I was in humid conditions under a low deck, rain all around on a warm day going into Osh one year and had to use it continuously for the last ~25 miles to keep it running! That, and the red knob. The O-200's main function is as an ice-maker, the propeller turning thing is secondary.

 

[Pilawt]

Any advice on oil temperatures you are used to seeing in Cessna 150's , is it normal to have the temperature up high in the gauge (85% of the green arc) during a cruise ?

78F at 4500' is about 35F above standard -- I wouldn't be surprised to see oil in the high green, especially at a higher cruise power setting.

Those gauges aren't terribly accurate anyway. Your mechanic can test it to see if it's reading too high.

 

[Dan Thomas]

It was most likely carb ice, but that carb temp gauge is pooched. 94 in the carb is way too high and ice would be absolutely impossible at that temp, of course. The temp in the carb should be considerably lower than the OAT, not higher. The pressure drop in the venturi and the vaporizing fuel will both suck heat from the incoming air and can lower it as much as 70°F.

Dan

 

[keithlamb]

I guess I'll jump into this even though many good comments have been made.
The O200, O300 and GO300 are all very susceptible to carburetor icing. But what is not said is how we can accidentally induce carburetor icing. Contrary to popular belief the O200 gets less carb ice in very cold temperatures. When it is around 60 to 70 outside, you can get lots of ice. So what's the deal. Ice forms when it is around 0 to minus 10 degrees that's when it forms on the wings and makes sense, but what about carb ice? It also forms at 0 to minus 10 but that's the temperature inside the Venturi area of the carb. The air cools up to 70 degrees as fuel is added and the cooling effect of the Venturi. Any hotter outside and it usually doesn't cool enough to form carb ice. If its very cold outside the moisture in the air is already in ice crystal form and passes harmlessly through the carburetor. Now if we add carb heat, we melt the ice crystals in the carb heat box only to refreeze them in the Venturi.
As a rule of thumb, if the engine starts into rough mode, put the carb heat on. Watch the RPM, if it is rising you had carb ice. If it is not rising within a minute or two, or getting worse at all, get the carb heat off. You may be creating carb ice. As a rule I use carb heat just before takeoff during the runup and use it during the approach and get it off immediately on the ground. This is because you are getting unfiltered air and I use a dirt runway a lot. But after I created an engine failure years ago, I am very judicious about watching the RPM when carb heat is applied.

 

[Skylane81E]

Just don't forget that if there is carb ice the engine will run worse for a period as it ingests the water

 

[Tom-D]

I guess I'll jump into this even though many good comments have been made.
The O200, O300 and GO300 are all very susceptible to carburetor icing.

The 0-200 has a remote mounted carb, the 0-300-/GO-300 mounts the carb on the oil sump and doesn't make as as well as the 0-200.