Carb Temp Increase?

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buzzard86

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Jim R
My 150 likes to make carb ice (particularly at cruise in ‘serious’ carb ice conditions) and I’ve had several instances where it’s been difficult/impossible to shed. I just installed a carb temp probe and it looks like I’m only getting about a +22F rise with full heat in cruise. In the pattern with the plane slowed and power back, I’m seeing about a +70F rise.

Does this sound normal? My mechanic has examined the system and can’t find any obvious issues - RPM drop during runup is about 100, scat tubing and engine baffling is all brand new and the carb heat box passes a visual inspection. In the absence of any issues, I guess I will utilize the temp gauge to keep the carb outside the icing range, but I feel like that +22 won’t do much if I need to melt any accumulated ice.

Trying to figure out if this is just par for the course with a C150/O200 or if something is amiss. I posted the same in the C150 club but not many members have temp gauges, so I’m posting here for a broader sample.

Thanks!
 
I would expect the heat rise to be less at higher throttle settings, since the same amount of heat has to heat more air. But that seems a bit weak.

FAR 23.1093 says, in part:
(a) Reciprocating engines. Each reciprocating engine air induction system must have means to prevent and eliminate icing. Unless this is done by other means, it must be shown that, in air free of visible moisture at a temperature of 30 °F—

(1) Each airplane with sea level engines using conventional venturi carburetors has a preheater that can provide a heat rise of 90 °F. with the engines at 75 percent of maximum continuous power;
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What model of 150? I would be consulting the 150 service manual for that model and seeing what the heat plumbing is supposed to look like. Cessna changed a few things along the way, and if someone has modified something, or left something off, I could see where you'd be having issues. For instance, some 150s had both mufflers supplying heat to the cabin but only one supplying heat to the carb. If the plumbing was messed up you could have the engine getting cooler air back from the cabin mixed with the carb heat air. The new baffles thing raises those concerns.

And was the airbox inspected internally to see that the carb heat control rigging is right and the flapper is moving through its full travel so that full heat actually gets full heat? Misrigged controls are all too common. The mechanic needs to remove the air filter and look in there with a flashlight while someone actuates the control.

The problem will be a simple one. Just have to look harder.

Go to https://www.redskyventures.org/doc/cessna-maintenance-manuals /Cessna_150_1969-1976_MM_D971-3-13.pdf
...and download the pdf. Go to figure 14-1 (page 174 of the pdf) and look at the top left corner of the page. If your airplane has the winterization kit, the plumbing is more complicated.
 
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Do you fly in temperatures below 11F?
 
Confirm everything's hooked up right and if so? Ask your mechanic to find a better heat source, like directly from the muffler shroud.
 
Thanks, guys. So is the consensus here that my heat rise is unusually low, even for a 150? Aside from the reputation that 150/O200s have for carb ice, I don't have any measurable comparison.

A little more info based on the discussion above. It's a 150M, currently breaking in a fresh engine overhaul. Had a hard time clearing carb ice before the OH but I was optimistic that the new baffling and scat tubing might help. No improvement. I added the carb temp probe when the engine went back on which is why I now have real temp info. My mechanic has looked over everything. Together we looked at the carb heat box as it was installed, could see in the side and see the flapper moving through what appears to be a full range of travel.

He surmises that I might not be getting good flow through the carb heat box and suggested that we try to add a small scoop to direct more air. Think this would help? Any other ideas? Stewartb, doesn't the shroud already provide the heat? And Salty, yes I'm based outside in the NE and don't object to flying in the bitter cold.
 
buzzard86 said:
Thanks, guys. So is the consensus here that my heat rise is unusually low, even for a 150?
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Seems pretty low, but it could also be your carb temp gauge that's bad. That wouldn't explain the much lower temp rise in cruise compared to the circuit at a lower power setting. Such symptoms would imply an airspeed or angle-of-attack differential that's in play here. That's why I suggested the messed-up cabin/carb heat plumbing if the winterization kit is there. Airspeed and AoA can both affect cabin pressures.
 
Dan Thomas said:
Seems pretty low, but it could also be your carb temp gauge that's bad. That wouldn't explain the much lower temp rise in cruise compared to the circuit at a lower power setting. Such symptoms would imply an airspeed or angle-of-attack differential that's in play here. That's why I suggested the messed-up cabin/carb heat plumbing if the winterization kit is there. Airspeed and AoA can both affect cabin pressures.
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I'm having a hard time clearing the carb once I make ice, so I suspect that the temp is probably accurate. Flew this morning, was 20F at altitude and full carb heat at cruise only rose the temp to about 38F-40F.

I don't have a winterization kit and don't think that I'm plumbed for such, although admittedly your post was the first time I learned that there were changes to the piping in this configuration (I thought that the winterization kit was just the metal insert to restrict airflow). Going to take another look at the tubing configuration just to be sure.

Thanks for taking the time to share your insight.
 
buzzard86 said:
I do. Why?
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11°F plus your 22°F carb heat takes the carb air to one degree above freezing. Not good enough. I wouldn't be flying it until the trouble is located and fixed.
 
Dan Thomas said:
11°F plus your 22°F carb heat takes the carb air to one degree above freezing. Not good enough. I wouldn't be flying it until the trouble is located and fixed.
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Ah, makes sense. I think most carb ice gauges have a yellow arc that ends around -10C, which is close to 11F - thought that may have been related. Totally over thinking it. :)

So I'm totally stumped here. Scat tubing all looks good and I don't have the funky winterization plumbing that shares the carb heat with the cabin heater. Starting to suspect that it may be a bad box. Regardless, will keep investigating with my mechanic.
 
I fly a Cessna 195 which is very prone to carb ice. I have a carb temp gage that reads in the yellow arc all the time...even in warm weather. However, I do have enough carb heat to raise the carb temp to over 100 degrees f almost all the time. I would think you should be able to raise the carb temp more than what you are seeing. Before starting the engine, you should see ambient temp on the carb temp gage....check that out. Check your sending unit and gage.
 
Doug Reid said:
Before starting the engine, you should see ambient temp on the carb temp gage....check that out. Check your sending unit and gage.
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Yesterday morning with an overnight low in the 20s and a 12-hour preheater soak on the tie down, carb temp was indicating about 58F just before startup. I would think that this is about right. I’m still inclined to think that the temps I’m seeing are accurate because I don’t seem to be able to clear the ice well, which would support the weak carb heat that the temps are indicating.
 
At 1000 rpm on the ground, carb heat took the temp from about 45 to 90.
At 2450 in cruise, carb heat took the temp from about 30 to 50.
 
GeorgeC said:
At 1000 rpm on the ground, carb heat took the temp from about 45 to 90.
At 2450 in cruise, carb heat took the temp from about 30 to 50.
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Still too weak. Must be getting cold air into the system somewhere. Did you have a look inside the air box (take the filter off) to see that the flapper valve is going all the way to the warm position? "Almost" isn't good enough. And the muffler shroud needs to be in good condition, too. No splits or tears in the SCAT hose.
 
GeorgeC said:
At 1000 rpm on the ground, carb heat took the temp from about 45 to 90.
At 2450 in cruise, carb heat took the temp from about 30 to 50.
Click to expand...

Thanks, George. This is about what I'm seeing too. Plane is in the shop now - mechanic is going to go over both the carb heat and fuel systems to see if there are any anomalies.
 
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