Is carb heat really necessary when 80F?

kicktireslightfires

kicktireslightfires

Pre-takeoff checklist
Joined
Jun 11, 2020
Messages
354
Display Name
Display name:
kicktireslightfires
The POH for my Cessna 162 says to always pull carb heat to ON position during landing. But if it’s 80F at 1,000 feet in the traffic pattern (and even hotter down at ground level), is there really any need to put carb heat on for landing when abeam the numbers? If so, why? There’s zero chance of carb ice at that temp, right? I understand the need for putting carb heat on if it’s in the 60sF and you’re dropping 1,000 feet rapidly for landing, but I’m struggling to see how carb ice could even remotely be a possibility at 70 or 80F, even with 100% humidity. Would appreciate insight on this if I’m missing something!
 
kicktireslightfires said:
The POH for my Cessna 162 says to always pull carb heat to ON position during landing. But if it’s 80F at 1,000 feet in the traffic pattern (and even hotter down at ground level), is there really any need to put carb heat on for landing when abeam the numbers? If so, why? There’s zero chance of carb ice at that temp, right? I understand the need for putting carb heat on if it’s in the 60sF and you’re dropping 1,000 feet rapidly for landing, but I’m struggling to see how carb ice could even remotely be a possibility at 70 or 80F, even with 100% humidity. Would appreciate insight on this if I’m missing something!
Click to expand...

Your carb venturi acts like a refrigerator.

Ever vent your air compressor tank on a hot day and it spits ice out the drain valve?

Here's a generic chart. Depends on carb model and throttle (butterfly) position, but it's easy, predictable high school science with the right chart or tools.

carb_ice.gif
 
Yes, warm air can hold lots more moisture than cold air....and it will gum up a carb in short order.
 
I have 100 hrs in a 162 and typically never ran carb heat. It has a carb temp gauge and is slightly different than your grandaddy’s 150. YMMV. Trust the POH by all means though. Heatstroke in the cockpit is an more important issue.
 
It’s one of those things that has little downside and lots of potential upside so we do it (use carb heat when at low power settings in flight). Risk management 101.
 
texasclouds said:
I have 100 hrs in a 162 and typically never ran carb heat. It has a carb temp gauge and is slightly different than your grandaddy’s 150. YMMV. Trust the POH by all means though. Heatstroke in the cockpit is an more important issue.
Click to expand...
I have a lot more than 100 hours and have repeatedly run into carb ice, even in O-200s. No, especially in O-200s. And even in the supposedly ice-resistant Lycomings, on nice summer days.

It's a fact of physics. And it's a fact that few student pilots are getting enough training on it, and it's also a sad fact that carb ice is responsible for more engine failures than anything else.

Those carb heat gauges are notorious for being inaccurate. They rely on small wires that get shaken and beat up, and on connectors that get oil and dirt in them.

So one needs to think. A savvy pilot will look at the METARs before going flying. They will tell you the likelihood of carb ice. Check the temperature and dewpoint; if they're fairly close, be aware that ice might happen if you are complacent. You can pick up a big load of it while taxiing to the takeoff point, and then have a major deficiency in performance, maybe an accident. If the temp and dewpoint are really close you might get ice in cruise.

Once again, that image from a few posts back:

carb_ice.gif


Where is the worst ice risk? Where the temp and dewpoint are close together. Look at the limit for ambient temperature: about 107°F.

That chart is from some POH and is specific to that airplane. Other charts vary somewhat, but they all show risk when the temp and dewpoint are close. And they all show possible ice at 80°F.
 
Salty said:
A great thread detailing a carb ice forced landing in 162.


https://www.pilotsofamerica.com/community/threads/the-man-who-fell-to-the-earth.115606/

the biggest thing I learned from this thread is that it can happen with no warning and carb heat might not do much when the engine isn’t running.
Click to expand...
Yup, and even if one has a working and accurate carb temp gauge, if it's not being scanned it's useless.

Training. It's all about training and understanding, at least until we have fully-automated engine control stuff like cars do. That is a long way off.
 
OP - all of the above is correct. Your instructor did you a disservice if he or she did not impress upon you the seriousness of carb ice, especially in the little continentals. The warmer the air is, the greater its ability to carry moisture. When that moisture rich air is rapidly cooled, ice will form. When it does happen, it will likely manifest itself as a slow, probably unnoticeable drop in engine RPM at first. You'll lose more and more RPM until it quits. By the time the firetrucks and EMS get to you in the field you landed in, the ice has melted and the engine runs perfectly.
 
Arbiter419 has it exactly right.

My landing with an O300 on a 95 degree day was on a very nice long asphalt runway.

When we un cowled it, water was dripping out of the induction system. Dew point that day was very high, we had been sweating heavily waiting our turn to take off after refueling. Probably made a lot of that ice before departure, but the haze layer went way up, so near saturated air in the climb.
 
I had carb icing during takeoff at 300-500 feet AGL at full power in the 162. Was really odd but put the carb heat in and engine came back.
 
I fly a 182 with a continental 0-470 and that bad boy can make some ice. I have had a lot of suspected carb icing snd have little threshold for pulling heat. Those warm evenings can be perfect for ice making. I’ve had some ?icing in lower power cruising.
what can spook ya is pulling carb heat on those hot days in an approach in lower power setting can sometimes get mixture a bit took rich snd I’ll hear what sounds like a miss.
 
texasclouds said:
I have 100 hrs in a 162 and typically never ran carb heat. It has a carb temp gauge and is slightly different than your grandaddy’s 150. YMMV. Trust the POH by all means though. Heatstroke in the cockpit is an more important issue.
Click to expand...

carb ice often can lead to heatstroke…

that prop up front isnt just there for pulling the plane, it keeps the pilot cool. If you disagree watch it stop sometime and you will see a pilot sweat! Carb ice can do that too! Well it’ll probably be windmilling- but a windmilling prop does t move enough air to cool the pilot!

I fly a c85 that’s just like the o-200 in its fridgidaire abilities… tell me my cylinders will wear out faster but my “POH” calls for carb heat on right after ya check oil pressure on start up and goes off to test it in run up and then off for departure. I also put it on cruise now n again- and yes for a solid bit of time- 20/30 seconds. I don’t want that crap sneaking up on me!

I read almost all the Cessna 120/140 accident reports and there were far far far too many whom had power failures and post crash she ran like a top…
 
Go to a mechanic on a warm day and have them do a good run up on any carb engine. After they shut down, look on the outside of the carb and see how much condensation is on the carb. Puts that temp difference with a carb into perspective.
 
Dan Thomas said:
until we have fully-automated engine control stuff like cars do. That is a long way off.
Click to expand...

You mean like FADEC stuff? It's here, it's just not retrofitted to our 1930s era engines.
 
When I am flying I like as many things to be as "automatic" as possible. When I pull the power back (out of the green) my hand just goes to the the carb heat, every time. If conditions warrant or there is an indication of possible carb ice I pull the carb heat, sometimes if conditions are right I pull it every once in awhile just to be sure.
 
I experienced "impossible" carb ice on the other end of the temperature spectrum: at -18c, I started slowly losing power immediately after takeoff. Nothing fit the normal carb-ice profile: at -18c there is virtually no moisture in the air; carb icing is extremely rare in a Piper PA-28-161 with a Lycoming O-320-D3G (even the Lycoming operator's manual says so); wide-open throttle is the least-likely position for carb ice, etc. But even on a cold day when I'd usually see over 1,000 ft/min even with my little 160 hp engine, I could barely climb, and it was getting worse.

It took a few seconds for my brain to register what was happening, but then I just turned on the carb heat anyway, even though there was no reason to think it would help. My engine coughed and sputtered for 30-60 seconds, then smoothly returned to normal power.

The plane was just out of maintenance, and our best hypothesis is that there was some moisture captured in the engine frozen from when it came out of the heated hangar, and through incredibly bad luck it had thawed when the engine warmed up, made it to the venturi and decided to freeze up there.

Moral: forget about what's supposed to happen, and focus on dealing with what does happen.
 
kicktireslightfires said:
The POH for my Cessna 162 says to always pull carb heat to ON position during landing. But if it’s 80F at 1,000 feet in the traffic pattern (and even hotter down at ground level), is there really any need to put carb heat on for landing when abeam the numbers? If so, why? There’s zero chance of carb ice at that temp, right? I understand the need for putting carb heat on if it’s in the 60sF and you’re dropping 1,000 feet rapidly for landing, but I’m struggling to see how carb ice could even remotely be a possibility at 70 or 80F, even with 100% humidity. Would appreciate insight on this if I’m missing something!
Click to expand...

The manufacturer established the procedures for your aircraft, use them. It’s that simple.
 
David Megginson said:
I experienced "impossible" carb ice on the other end of the temperature spectrum: at -18c, I started slowly losing power immediately after takeoff. Nothing fit the normal carb-ice profile: at -18c there is virtually no moisture in the air;
Click to expand...
There is still moisture even at -18°C. It can also exist as a liquid down to -20°C, meaning it can still clog things up, or give you airframe ice if it's visible. The small amount in the air at those temps means it takes a little longer to condense much of it from vapor, and the carb is still reducing temperature to below those ambient temps to achieve it.

Using carb heat below -20°C can cause problems if there is visible moisture, particularly in the form of ice crystals or snow. That stuff is already frozen. Pulling carb heat bypasses the air filter and lets those little rascals get into the heat muff where they turn to liquid and vapor, enter the carb and refreeze. Uh-oh. Cold weather ops have a whole new set of risks. Not many rec pilots fly at those temps, though. I did a little, while instructing. It was no fun at all. All I could think of was the hot coffee back at the hangar. The guys that fly, for a living, little airplanes daily in those conditions earn their wages.
 
And take the table with a grain of salt. We got severe carb icing in a pressure carb on my gopher engine in a 70 degree day after prolonged low power operation.
 
Maybe. Maybe not. My carb ice emergency was at 70F. But there is another advantage to following the Cessna recommendation to turn it on when the RPM drops below a certain number. The SOP association creates a habit making it less likely you will forget or delay it until too late it if it happens unexpectedly, even at altitude.
 
Fun experiment: Take a can of soda out of the cooler and walk directly to the ramp and set it it down. Wait 30 seconds. The amount of water that what you thought was dry air that forms on its surface will scare you to death.
 
Dan Thomas said:
There is still moisture even at -18°C. It can also exist as a liquid down to -20°C, meaning it can still clog things up, or give you airframe ice if it's visible. The small amount in the air at those temps means it takes a little longer to condense much of it from vapor, and the carb is still reducing temperature to below those ambient temps to achieve it.

Using carb heat below -20°C can cause problems if there is visible moisture, particularly in the form of ice crystals or snow. That stuff is already frozen. Pulling carb heat bypasses the air filter and lets those little rascals get into the heat muff where they turn to liquid and vapor, enter the carb and refreeze. Uh-oh. Cold weather ops have a whole new set of risks. Not many rec pilots fly at those temps, though. I did a little, while instructing. It was no fun at all. All I could think of was the hot coffee back at the hangar. The guys that fly, for a living, little airplanes daily in those conditions earn their wages.
Click to expand...
According to Lycoming's O-320 owner's manual, the cutoff is 14F (-10C), so -18c is considerably below the temperature where Lycoming recommends using carb heat. Nevertheless, it worked. That's why I suspect the moisture source wasn't atmospheric in my case.

upload_2021-6-24_18-37-5.png
 
David Megginson said:
According to Lycoming's O-320 owner's manual, the cutoff is 14F (-10C), so -18c is considerably below the temperature where Lycoming recommends using carb heat. Nevertheless, it worked. That's why I suspect the moisture source wasn't atmospheric in my case.
Click to expand...
Lycoming is probably right, since the vapor will be minimal, and Lycoming's carburetors are also mounted to the oil sump, where the heat from the oil flows into the carb body and warms it and make ice much less likely than in any Continental except the O-300, which has its carb on the sump as well.

This chart shows water vapor existing down to below -50°C.

Vbdf1.png


It doesn't show (or at least doesn't specify) that liquid water can exists as droplets down to -20°C. That's seems not well known outside IFR groundschool. https://www.cfinotebook.net/notebook/weather-and-atmosphere/icing-in-the-aviation-environment
 
Dan Thomas said:
Lycoming is probably right, since the vapor will be minimal, and Lycoming's carburetors are also mounted to the oil sump, where the heat from the oil flows into the carb body and warms it and make ice much less likely than in any Continental except the O-300, which has its carb on the sump as well.

This chart shows water vapor existing down to below -50°C.

Vbdf1.png


It doesn't show (or at least doesn't specify) that liquid water can exists as droplets down to -20°C. That's seems not well known outside IFR groundschool. https://www.cfinotebook.net/notebook/weather-and-atmosphere/icing-in-the-aviation-environment
Click to expand...
True about the droplets, but generally speaking, you need some serious lifting action to get SLD at -20c. In my experience, you won't see anything busting through a 2,000 ft SC layer at -15c, and nothing but a trace of light rime at -10c. Into a cumulus cloud, or anything ahead of a winter warm front, agreed I wouldn't feel safe even at -20c OAT.
 
I have to agree with the OP. If the outside air temp is 80 degF or higher, there is little chance of getting carb icing, since a 70 degF drop from 80 is still above freezing. Would there likely to be condensation and water into the carb? Yes.

But his question is about icing.

Do I agree to not engage carb heat under those conditions? Perhaps not only because I don't like to make unnecesary exceptions.
 
My experimental uses a Marvel Schebler with fairly long intake runners. Procedure is to pull carb heat when reducing power.

During my last flight review I pulled the carb heat on for the first landing approach on a really warm day. The instructor said, "you probably don't need that but it can't hurt."

Just need to remember to turn it off on very short final in case of the need to go around ...
 
How much does pulling the carb heat really hurt on every descent...like the POH says? How much time does it take? Should be reading a checklist so you shouldn't forget. It should be second nature. Like getting in and buckling up. If you don't fully grasp the how or why that's fine, just defer to the checklist. The air compressor analogy was one that resonated with me. Dump all the air through a tiny orifice at high pressure and it will ice up. I work well with real world application stuff over the theoretical. Makes me think less when Ive seen it happen and now I understand the why it happened.
 
You must log in or register to reply here.
Back
Top